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Braided rivers can provide substantial recharge to regional aquifers, with flow exchange between surface water and groundwater occurring at a range of spatial and temporal scales. However, the difficulty in measuring and modelling these complex and dynamic river systems has hampered process understanding and the upscaling necessary to quantify these fluxes. This is due to an incomplete understanding of the hydrogeological structures that control river–groundwater exchange. In this paper, we present a new conceptualisation of subsurface processes in braided rivers based on observations of the main losing reaches of three braided rivers in Aotearoa / New Zealand. The conceptual model is based on a range of data, including lidar, bathymetry, coring, particle size distribution, groundwater level and temperature monitoring, radon-222, electrical-resistivity tomography and fibre-optic cables. The combined results indicate that sediments within the recently active river braidplain are distinctive, with sediments that are poorly consolidated and better sorted compared with adjacent deposits from the historical braidplain that become successively consolidated and intermixed with flood silt deposits due to overbank flow. A distinct sedimentary unconformity, combined with the presence of geomorphologically distinct lateral boundaries, suggests that a "braidplain aquifer" forms within the active river braidplain through the process of sediment mobilisation during flood events. This braidplain aquifer concept introduces a shallow storage reservoir to the river system, which is distinct from the regional aquifer system, and mediates the exchange of flow between individual river channels and the regional aquifer. The implication of the new concept is that surface water–groundwater exchange occurs at two spatial scales: the first is hyporheic and parafluvial exchange between the river and braidplain aquifer; the second is exchange between the braidplain aquifer and regional aquifer system. Exchange at both scales is influenced by the state of hydraulic connection between the respective water bodies. This conceptualisation acknowledges braided rivers as whole "river systems", consisting of channels and a gravel aquifer reservoir. This work has important implications for understanding how changes in river management (e.g. surface water extraction, bank training and gravel extraction) and morphology may impact groundwater recharge (and potentially flow, temperature attenuation and ecological resilience) under dry conditions. [ABSTRACT FROM AUTHOR]

This paper highlights a disjunct between geomorphic concepts of braided riverbed lateral boundaries and legal definitions used to decide these boundaries in New Zealand—a country that hosts over 150 gravel‐bed braided rivers. These powerful morphodynamic systems are prone to recurrent bar reworking and channel shift. When parts of the riverbed are temporarily abandoned by active channels, they are vulnerable to land use intensification. Associated flood protection measures that often follow intensification constrict the rivers' capacity to adjust to ever‐changing flows of water and sediment. Despite the rivers' vulnerability and constriction, New Zealand law defines braided rivers in a way that limits local councils' authority to manage land use within the braidplain. This paper explores the relationship between the law and science of braided rivers, demonstrating how legislative reforms underway in 2023 express the ways in which particular social processes play out in the landscape. [ABSTRACT FROM AUTHOR]

Braided rivers can provide substantial recharge to regional aquifers, with flow exchange between surface water and groundwater occurring at a range of spatial and temporal scales. However, the difficulty of measuring and modelling these complex and dynamic river systems has hampered process understanding and the upscaling necessary to quantify these fluxes. This is due to an incomplete understanding of the hydrogeological structures which control river-groundwater exchange. In this paper, we present a new conceptualisation of subsurface processes in braided rivers based on observations of the main losing reaches of three braided rivers in New Zealand. The conceptual model is based on a range of data including: lidar, bathymetry, coring, particle size distribution, groundwater, temperature monitoring, radon-222, electrical resistivity tomography, and fibre optic cables. The combined results indicate that sediments within the recently active river braidplain are distinctive, with sediments that are poorly consolidated and better sorted compared to adjacent deposits from the historical braidplain, which become successively consolidated and intermixed with flood silt deposits due to overbank flow. A distinct sedimentary unconformity, combined with the presence of geomorphologically distinct lateral boundaries, suggests that a "braidplain aquifer" forms within the active river braidplain through the process of sediment mobilisation during flood events. This braidplain aquifer concept introduces a shallow storage reservoir to the river system, which is distinct from the regional aquifer system, and mediates the exchange of flow between individual river channels and the regional aquifer. The implication of the new concept is that surface water-groundwater exchange occurs at two spatial scales. The first is hyporheic and parafluvial exchange between the river and braidplain aquifer. The second is exchange between the braidplain aquifer and regional aquifer system. Exchange at both scales is influenced by the state of hydraulic connection between the respective water bodies. This conceptualisation acknowledges braided rivers as whole "river systems", consisting of channels, and gravel aquifer. This work has important implications for understanding how changes in river management (e.g., surface water extraction, bank modification and gravel extraction) and morphology may impact groundwater recharge, and potentially river flow, temperature attenuation, and ecological resilience during dry conditions. [ABSTRACT FROM AUTHOR]

We use two‐dimensional physics‐based numerical modeling to study multi‐decadal evolution of vegetation and morphology under different flow regimes in real‐world gravel‐bed braided rivers. To assess model realism, we focus on two rivers in Canterbury (New Zealand) that, despite having been subjected to the introduction of similar invasive vegetation species in the last ∼100 years, show very different trajectories of vegetation presence due to their different flow regimes: the Lower Waitaki River and the Waimakariri River. The former, featuring a naturally damped flow regime—and having experienced further artificial flow damping due to hydropower generation from the 1930s, experienced vegetation encroachment; while the latter, featuring a flashy flow regime, retains an unvegetated braided planform. We propose an innovative calibration and validation procedure to determine an optimal setup of vegetation parameters that allows the model to robustly reproduce the trajectories of both rivers, thereby proving that the model responds sensibly to different hydrological conditions. Then, we isolate the impact of hydropower‐related flow regime modifications on vegetation encroachment in the Lower Waitaki by running the calibrated model with a natural flow regime that does not feature the effect of hydropower generation, and find that vegetation encroachment would have happened even without flow alteration, albeit to a milder degree. Finally, we apply to simulation results a conceptual framework based on a synthetic parameter that compares the relative strength of hydrological and vegetation controls, and discuss the use of this parameter as a predictor of vegetation presence across flow regimes. Plain Language Summary: We study how numerical models can predict the amount of vegetation that establishes on the gravelly bed of wide rivers. We focus on two rivers in New Zealand: the Waimakariri River, having frequent and intense floods, and the Lower Waitaki River, with less frequent and intense floods. This milder character of the Waitaki relates to the presence in the river catchment of three natural lakes that store flood flows and release them slowly, and to the additional storage of artificial hydropower reservoirs. Both rivers have been exposed to invasive vegetation that tends to spread on their beds; however, vegetation spread was significantly more pronounced in the Waitaki than in the Waimakariri. First, we find the parameter values that allow the model to reproduce the vegetation changes seen in both rivers. Then, we simulate the vegetation changes that would have happened in the Waitaki had the hydropower reservoirs not been built, finding there would still have been significant vegetation spread. Finally, we interpret the intensity of vegetation spread as depending on the frequency of floods and on the speed of vegetation growth: a higher vegetation presence is found if vegetation grows faster and if the floods are more spaced in time. Key Points: Calibration and validation on two rivers with same vegetation species but different hydrology ensures model's sensible response and realismNumerical modeling allows quantification of the impact of a hydropower scheme on vegetation encroachment in a braided riverThe ratio between inter‐flood period duration and vegetation growth timescale is a key control and predictor of vegetation presence [ABSTRACT FROM AUTHOR]

A novel aerial tracer particle distribution system has been developed. This system is mounted on an unmanned aerial vehicle (UAV) and flown upstream from where surface velocimetry measurements are conducted. Tracer particles can be rapidly deployed across the full channel width and distributed in a crescent shape (for larger channels), so that they reach the measurement cross section at a similar time. This enables surface velocimetry techniques to be applied in rivers and channels lacking sufficient natural tracer particles or surface features. Lack of tracers is a common problem during low flows, in lowland rivers, or in artificial channels. Techniques for colouring tracer particles with biodegradable dye have also been developed, along with methods for extracting them from Red Green Blue (RGB) imagery in the Hue Saturation Value (HSV) colour space. The use of coloured tracer particles enables flow measurements in situations where sunglint, surface waves, moving shadows or dappled lighting on riverbeds can interfere with and corrupt results using surface velocimetry techniques. These developments further expand the situations where surface velocimetry can be applied, as well as improving the accuracy of the results. The tracer particle distribution system has been successfully deployed in multiple rivers and canals in New Zealand and has enabled surface velocimetry measurements where they were not previously possible due to a lack of natural tracers. [ABSTRACT FROM AUTHOR]

Trophic cascades are important features of aquatic ecosystems, but much variation in their strength remains unexplained. Disturbance could restrict cascade occurrence by limiting predator distribution, and increase cascade strength by lowering defended herbivore abundance. These possibilities were investigated in 20 New Zealand rivers varying both in flooding disturbance (measured by bed movement) and presence of non‐native predatory trout. Trout were often, but not always, associated with indirectly increasing algal abundance compared to similar rivers with native galaxiid fishes. Floods explained variation in cascades by (1) resetting algal abundance such that algal minima were related to bed movement; (2) preventing occurrence by restricting trout abundance in the most disturbed rivers; and (3) increasing potential cascade strength by removing defended herbivores. These observations indicate strong nonlinear influences of disturbance on both trophic cascade occurrence and maximum strength which will be important to consider as climate change alters disturbance regimes. [ABSTRACT FROM AUTHOR]

Galaxias maculatus is a declining amphidromous fish that supports New Zealand's culturally important whitebait fisheries targeting the migratory juvenile stage. Spawning ground protection and rehabilitation is required to reverse historical degradation and improve fisheries prospects alongside conservation goals. Although spawning habitat has been characterised in tidal rivers, there has been no previous study of spawning in non-tidal rivermouths that are open to the sea. We assessed seven non-tidal rivers over 4 months using census surveys to quantify spawning activity, identify environmental cues, and characterise fundamental aspects of the biogeography of spawning grounds. Results include the identification of compact spawning reaches near the rivermouths. Spawning events were triggered by periods of elevated water levels that were often of very short duration, suggesting that potential lunar cues were less important, and that rapid fish movements had likely occurred within the catchment prior to spawning events. Spawning grounds exhibited consistent vertical structuring above typical low-flow levels, with associated horizontal translation away from the river channel leading to increased exposure to anthropogenic stressors and associated management implications for protecting the areas concerned. These consistent patterns provide a sound basis for advancing protective management at non-tidal rivermouths. Attention to flood management, vegetation control, and bankside recreational activities is needed and may be assisted by elucidating the biogeography of spawning grounds. The identification of rapid responses to environmental cues deserves further research to assess floodplain connectivity aspects that enable fish movements in ephemeral flowpaths, and as a confounding factor in commonly used fish survey techniques. This paper provides a first assessment of the biogeography of Galaxias maculatus spawning grounds and amphidromous spawning behaviour at non-tidal rivermouths. Consistent patterns in catchment position, vertical structuring, and environmental cues were identified that provide a sound basis for protective management to assist the conservation of this culturally important fish, which is currently in decline. [ABSTRACT FROM AUTHOR]

Monitoring for lahars on volcanoes can be challenging due to the ever-changing landscape which can drastically transform the properties and dynamics of the flow. These changes to the flows require the need for detection strategies and risk assessment that are tailored not only between different volcanoes, but at different distances along flow paths as well. Being able to understand how a flow event may transform in time and space along the channel is of utmost importance for hazard management. While visual observations and simple measuring devices in the past have shown how lahars transform along the flow path, these same features for the most part have not been described using seismological methods. On 18 March 2007 Mt. Ruapehu produced the biggest lahar in New Zealand in over 100 years. At 23:18 UTC the tephra dam holding the Crater Lake water back collapsed causing 1.3 x 106 m3 of water to flow out and rush down the Whangaehu channel. We describe here the seismic signature of a lake-breakout lahar over the course of 85 km along the Whangaehu river system using three 3-component broadband seismometers installed < 10 m from the channel at 7.4, 28, and 83 km from the crater lake source. Examination of 3-componennt seismic amplitudes, peak frequency content, and directionality combined with video imagery and sediment concentration data were used. The seismic data shows the evolution of the lahar as it transformed from a highly turbulent out-burst flood (high peak frequency throughout), to a fully bulked up multi-phase hyperconcentrated flow (varying frequency patterns depending on the lahar phase) to a slurry flow (bedload dominant). Estimated directionality ratios show the elongation of the lahar with distance down channel, where each recording station shows a similar pattern, but for differing lengths of time. Furthermore, using directionality ratios shows extraordinary promise for lahar monitoring and detection systems where streamflow is present in the channel. [ABSTRACT FROM AUTHOR]

In the last 30 years, work on large wood (LW) has expanded and matured considerably, and river scientists, managers and practitioners now have a better appreciation of the role of LW in maintaining ecosystems, forming or stabilizing riverine landforms, and interacting with river morphodynamics. We have gained a better understanding of the hazards posed by the recruitment and transport of LW in the river channel and associated infrastructure. While LW dynamics have traditionally been studied in the natural river environment, innovations in laboratory techniques have enabled important advances in understanding LW process dynamics, using physical scale models, new sensors, scanners and sophisticated model boundary conditions. Current trends in LW laboratory research focus on (1) mobilization and transport of logs, (2) trapping and deposition of sediment in the presence of LW and (3) LW contribution to hydraulic flow resistance. Ultimately, a combined process understanding is needed to assess impacts upon infrastructure with erodible boundaries, such as bridge piers and LW retention racks. In this review, we present a critical analysis of emerging experimental work on LW obtained through physical modelling studies. We put recent experimental work in context with global LW management challenges. In particular, we set our work in context with the present environmental and engineering issues that confront catchment and natural resource managers in Switzerland and New Zealand. We show how improved physical models incorporating LW transport, accumulation and scouring processes are needed to contribute to more reliable hazard and risk assessment and improved river management in LW‐prone systems. [ABSTRACT FROM AUTHOR]

From midnight of 26 March 2020, New Zealand became one of the first countries to enter a strict lockdown to combat the spread of COVID‐19. The lockdown banned all non‐essential services and travel both on land and sea. Overnight, the country's busiest coastal waterway, the Hauraki Gulf Marine Park, became devoid of almost all recreational and non‐essential commercial vessels. An almost instant change in the marine soundscape ensued, with ambient sound levels in busy channels dropping nearly threefold the first 12 h. This sudden drop led fish and dolphins to experience an immediate increase in their communication ranges by up to an estimated 65%. Very low vessel activity during the lockdown (indicated by the presence of vessel noise over the day) revealed new insights into cumulative noise effects from vessels on auditory masking. For example, at sites nearer Auckland City, communication ranges increased approximately 18 m (22%) or 50 m (11%) for every 10% decrease in vessel activity for fish and dolphins, respectively. However, further from the city and in deeper water, these communication ranges were increased by approximately 13 m (31%) or 510 m (20%). These new data demonstrate how noise from small vessels can impact underwater soundscapes and how marine animals will have to adapt to ever‐growing noise pollution. [ABSTRACT FROM AUTHOR]

Sediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse‐grained, hard sediment on the geometry of bedrock channels from the Rangitikei River, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l min−1) under different Qs (between 0 and 20 g l−1) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope and width‐to‐depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterized by simple in‐channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterized by dynamic channels with multiple active threads and a non‐uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single‐thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

Wood additions to streams can slow water velocities and provide depositional areas for bacteria and fine particles (e.g., particulate organic carbon and nutrients sorbed to fine sediment), therefore increasing solute and particle residence times. Thus, wood additions are thought to create biogeochemical hotspots in streams. Added wood is expected to enhance in‐stream heterogeneity, result in more complex flow paths, increase natural retention of fine particles and alter the geomorphic characteristics of the stream reach. Our aim was to directly measure the impact of wood additions on fine particle transport and retention processes. We conducted conservative solute and fluorescent fine particle tracer injection studies in a small agricultural stream in the Whatawhata catchment, North Island of New Zealand in two reaches—a control reach and a reach restored 1‐year earlier by means of wood additions. Fine particles were quantified in surface water to assess reach‐scale (channel thalweg) and habitat‐scale (near wood) transport and retention. Following the injection, habitat‐scale measurements were taken in biofilms on cobbles and by stirring streambed sediment to measure fine particles available for resuspension. Tracer injection results showed that fine particle retention was greater in the restored compared to the control reach, with increased habitat‐scale particle counts and reach‐scale particle retention. Particle deposition was positively correlated with cobble biofilm biomass. We also found that the addition of wood enhanced hydraulic complexity and increased the retention of solute and fine particles near the wood, especially near a channel spanning log. Furthermore, particles were more easily remobilized from the control reach. The mean particle size remobilized after stirring the sediments was ~5 μm, a similar size to both fine particulate organic matter and many microorganisms. These results demonstrate that particles in this size range are dynamic and more likely to remobilize and transport further downstream during bed mobilization events. [ABSTRACT FROM AUTHOR]

Although extreme hydrological events are a natural component of river ecosystem disturbance regimes, their frequency is predicted to increase with climate change. Anthropogenic activities have the potential to exacerbate the impact of such disturbances but there are few studies on the combined effects of both anthropogenic and extreme hydrological disturbances on stream ecosystems.We investigated the recovery of stream ecosystems over a 5‐year period following the impact of an anthropogenic (forest clear‐cut harvesting) and an extreme rainfall disturbance (estimated one‐in‐100 year average return interval) that generated debris flows in three headwater streams in New Zealand.Initially, most of the riparian vegetation was eliminated and showed little recovery 1 year later. Subsequent riparian recovery was led by wind‐borne, light‐demanding, pioneering exotic weed species, lengthening and altering the long‐term successional and recovery trajectories to a pre‐disturbance composition of indigenous shrubs.Stream shade, water temperature, and habitat had largely recovered after 5 years. However, the contribution of large wood to channel morphology and in‐stream habitat was compromised due to diminished wood supplies in the stream channel and a hiatus in up‐slope wood inputs until the riparian vegetation re‐establishes and the next crop of trees matures.After an initial decline, most indigenous fish taxa thrived in the post‐disturbance conditions, with significant increases in densities and biomass. The more sensitive fish taxa were scarce or absent, particularly those taxa that prefer pools with overhead and in‐stream cover provided by riparian vegetation and wood. Recovery of these taxa was outside the time frame of this study. Riffle dwelling fish communities were more resilient than pool dwelling fish communities.Invertebrate densities showed a similar response to fish. Post‐event invertebrate community composition differed from that typically found in post‐harvest headwater streams, comprising comparatively lower proportions of Chironomidae, Oligochaetes, and Mollusca taxa, and higher proportions of Trichoptera taxa. Progression toward pre‐event community composition was evident 5 years after the event.The compounding effect of forest removal from harvesting, along with riparian vegetation and wood removal by debris flows, lengthened the recovery of riparian vegetation and wood supplies with cascading effects on in‐stream habitat and biological communities. [ABSTRACT FROM AUTHOR]

Gravel‐bed braided rivers are highly energetic fluvial systems characterized by frequent in‐channel avulsions, which govern the morphodynamics of such rivers and are essential for them to maintain a braided planform. However, the avulsion mechanisms within natural braided rivers remain unclear due to their complicated hydraulic and morphodynamic processes. Influenced by neighbouring channels, avulsions in braided rivers may differ from those of bifurcations in single‐thread rivers, suggesting that avulsions should be studied within the context of the entire braid network. In this study, braiding evolution processes in gravel‐bed rivers were simulated using a physics‐based numerical model that considers graded bed‐load transport by dividing sediment particles into multiple size fractions and vertical sediment sorting by dividing the riverbed into several vertical layers. The numerical model successfully produced braiding processes and avulsion activities similar to those observed in a laboratory river. Results show that bend evolution of the main channel was the fundamental process controlling the occurrence of avulsions in the numerical model, with a cyclic process of channel meandering by lateral migration that transitioned to a straight channel pattern by avulsion. The radius of bend curvature for triggering avulsions in the numerical model was measured and it was found that the highest probability for a channel bend to generate an avulsion occurs when its radius of curvature is approximately 2.0–3.3 times the average anabranch width. Other types of avulsion were also observed that did not occur specifically at meander bends, but upstream meander evolution indirectly influenced such avulsions by altering channel pattern and discharge to those locations. This study explored the processes and mechanisms of several types of avulsion, and proposed factors controlling their occurrence, namely increasing channel curvature, high shear stress, tributary discharge, riverbed gradient and upstream channel pattern, with high shear stress being a direct indicator. Furthermore, avulsions in a typical gravel‐bed braided river, the Waimakariri River in New Zealand, were analysed using sequential Google Earth maps, which confirmed the conclusions derived from the numerical simulation. [ABSTRACT FROM AUTHOR]

The integrity of freshwater ecosystems worldwide is under threat from agriculture and invasive species. Past agricultural activity can have persistent effects on aquatic diversity even decades after restoration, and the spread of invasive species is increasingly difficult to prevent due to globalisation. In the South Island of New Zealand, the invasive diatom Didymosphenia geminata (Didymo) causes nuisance blooms in streams. The impact of Didymo on stream invertebrate communities in upland streams with natural flow regimes remains poorly understood. We investigated the relationships between legacy effects of agriculture, Didymo and benthic invertebrate communities at 55 stream sites in Mahu Whenua, a 530 km2 conservation area comprising four former New Zealand high-country farms. The farms were destocked of sheep 4–9 years before stream sampling started. Kick-netting was used to collect macroinvertebrates from 7–23 streams within each farm to provide a land-use legacy gradient. Moreover, samples from 16 sites with clearly visible Didymo mats covering most of the stream bed (indicating high biomass and a dominant role in the biofilm) were compared with 39 sites without such Didymo mats. Total invertebrate taxon richness and EPT richness (taxon richness of larval mayflies, stoneflies and caddisflies) were lower in the stream catchments destocked most recently. When Didymo was present, relative EPT abundance was lower than when Didymo was absent, and Deleatidium mayflies decreased whereas midges and oligochaetes increased. These results highlight the need to look at past land-use practices when restoring high-country streams after agricultural impacts. They also show that Didymo can have negative effects on invertebrate communities in upland streams with natural flow regimes, a stream type previously overlooked in studies on this invasive diatom. [ABSTRACT FROM AUTHOR]

Information on wood dynamics across a forest rotation in streams within plantation forests is sparse. Understanding temporal changes in in-stream wood in plantation forests can assist in identifying when additional targeted interventions aimed at maintaining in-stream wood supplies may be ecologically beneficial. This study measured changes in in-stream wood and riparian vegetation in 14 streams in plantation forests (predominantly Pinus radiata) in New Zealand. The study measured in-stream wood volumes pre-harvest, post-harvest, and 5, 10 and 22 years after harvest, covering the span of a typical forest rotation in a New Zealand plantation forest. Harvesting was the primary source of in-stream wood with hauling across the stream channel delivering the highest wood volume, followed by hauling back from the stream and ground-based harvesting, with stream-cleaning delivering the least wood. Total wood volumes subsequently declined over time, the influence of harvest method was still evident at 10 years after harvest, but by 22 years after harvest total wood volumes were similar to pre-harvest, with no statistical differences between the four harvest methods. Large wood (≥ 10 cm diameter) volumes showed a similar pattern over time to total wood volumes. Small wood (< 10 cm diameter) volumes declined more rapidly (within the first five years after harvest) and by 10 years after harvest there were no significant differences between the four harvest methods. Wood from thin-to-waste operations and windthrow provided additional sources of in-stream wood in the mid to latter part of the forest rotation. While indigenous vegetation was regenerating in the riparian margins by the end of the forest rotation, the time needed to reach maturity (≈ 70 years) along with current regulatory riparian setbacks of 5 m to 10 m, indicate a limited future capacity to provide large wood to streams. In addition, current forestry regulations take a risk-based approach to the management of in-stream wood from forestry activities, with a focus on its removal. These factors will limit the future supply of wood to streams across a forest rotation. There is the potential to assess the feasibility of a targeted wood regime, that balances the benefits of retaining some in-stream wood for ecological purposes against the potential risks and hazards. This would need an accompanying, supportive regulatory framework and a public educational programme on the natural ecological role of wood in forested streams. • Harvest was the main source of in-stream wood across a plantation forest rotation. • Windthrow and thinnings provided supplementary sources of in-stream wood. • Harvest method influenced in-stream wood volumes for at least 10 years after harvest. • Twenty-two years after harvest, in-stream wood volumes were similar to pre-harvest. [ABSTRACT FROM AUTHOR]

Here we examine the landscape of New Zealand's Marlborough Fault System (MFS), where the Australian and Pacific plates obliquely collide, in order to study landscape evolution and the controls on fluvial patterns at a long-lived plate boundary. We present maps of drainage anomalies and channel steepness, as well as an analysis of the plan-view orientations of rivers and faults, and we find abundant evidence of structurally controlled drainage that we relate to a history of drainage capture and rearrangement in response to mountain-building and strike-slip faulting. Despite clear evidence of recent rearrangement of the western MFS drainage network, rivers in this region still flow parallel to older faults, rather than along orthogonal traces of younger, active strike-slip faults. Such drainage patterns emphasize the importance of river entrenchment, showing that once rivers establish themselves along a structural grain, their capture or avulsion becomes difficult, even when exposed to new weakening and tectonic strain. Continued flow along older faults may also indicate that the younger faults have not yet generated a fault damage zone with the material weakening needed to focus erosion and reorient rivers. Channel steepness is highest in the eastern MFS, in a zone centered on the Kaikōura ranges, including within the low-elevation valleys of main stem rivers and at tributaries near the coast. This pattern is consistent with an increase in rock uplift rate toward a subduction front that is locked on its southern end. Based on these results and a wealth of previous geologic studies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the eastern MFS, Miocene folding above blind thrust faults generated prominent mountain peaks and formed major transverse rivers early in the plate collision history. A transition to Pliocene dextral strike-slip faulting and widespread uplift led to cycles of river channel offset, deflection and capture of tributaries draining across active faults, and headward erosion and captures by major transverse rivers within the western MFS. We predict a similar landscape will evolve south of the Hope Fault, as the locus of plate boundary deformation migrates southward into this region with time. [ABSTRACT FROM AUTHOR]

High-resolution microscale modelling of the response of the Waiho River to width reduction by stopbanks reveals that, unless the river is already aggrading in its prior state, width reduction does not on its own cause bed aggradation. This finding conflicts with and corrects findings of earlier work, due to improvements in data acquisition technology, and has prompted a review of the prehistoric and historical behaviour of the Waiho River. The river has aggraded by about 10 m at the State Highway 6 bridge cross-section since records began in 1907, and since 1980 it has aggraded by about 6 m (an average rate of about 0.17 m/year). The reason for this multi-decade aggradation is presently not well understood; other major rivers in the region are not behaving in this way. At the start of the 20th century the Waiho River was incised by about 8 m into its fanhead, but by about 1970-80 aggradation was causing overbank flows during floods, affecting infrastructure, and stopbanks were constructed to confine the river to its previous bed area. The confinement caused aggradation to continue at the previous rate, whereas without stopbanks the river would have aggraded across its entire fanhead (about three times the current bed area) at about one-third the rate, i.e. at about 0.06 m/year. If stopbanks are maintained in their present positions, about 17 m of further aggradation can be anticipated over the next century; if, by contrast, the river is allowed free access to the whole of its natural bed the short-term response would be about 2 m of degradation, followed by 6 m of aggradation after 100 years - that is, a bed-level rise of about 4 m over the next century. During this time, however, a severe aggradation episode, probably due to an earthquake, is likely to alter the river behaviour drastically. The response of the river to climate change over the next century is unclear at present. [ABSTRACT FROM AUTHOR]

Predictions of river bed substrate cover are required for various purposes including delineating management zones, linking with ecological status and assessing river rehabilitation options. Three contrasting methods were tested for predicting the proportion of river bed covered by seven different substrate categories: generalised linear models (GLMs), machine learning regression models (random forest), and a summed normal distribution model (SND) which incorporates distribution of predictors and substrate covers throughout the modelling framework. Various predictors representing climate, geomorphology, land cover and geology were derived from existing environmental databases to generate predictive models. Model performance was assessed through a cross-validated comparison with substrate samples collected from 229 river sites distributed across New Zealand. Model performance for 10-fold cross-validated predictions showed that the SND model performed best in predicting the proportions of riverbed covered by bedrock, boulder, cobble and fine gravel categories. Random forest models performed best in predicting coarse gravel, sand and mud plus vegetation proportions. Therefore, combined random forest and SND methods were used for estimating substrate cover proportions at unsampled sites across New Zealand. Texture analysis of predicted substrate cover consistently showed downstream fining of sediment size. The national predictions of substrate cover proportions are key descriptors that can be linked with a wide range of national scale applications for ecological assessment of New Zealand Rivers. The techniques developed and tested are applicable to other locations but it is notable that relatively poor performance in regional cross-validation tests shows that transferability of substrate models to locations with no calibration data is challenging. [ABSTRACT FROM AUTHOR]

Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7 m s-1 and the buoyancy frequency squared was in the range 10-5 to 10-3 s-2. Turbulence parameters were estimated using both shear microstructure and velocimeterderived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1 ? 10-6 m² s-3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact. [ABSTRACT FROM AUTHOR]

• Novel installation of fibre optic cable below riverbed to monitor SW–GW exchange. • Active distributed temperature sensing used to quantify infiltration river loss. • Spatial variability in groundwater velocity associated with river geomorphology. • Method demonstrated applicability to dynamic braided river systems. Braided river systems are continuously changing environments, comprising heterogeneous gravel-forming meandering channels and bars, and dynamic surface water–groundwater (SW–GW) interactions. In alluvial aquifers associated with braided rivers, river leakage is volumetrically the most important source of groundwater recharge, yet neither the leakage process or recharge rates are well understood. Insights into how braided river systems recharge underlying braidplain aquifers during different hydrological conditions, and how this can be measured, is of critical concern for river management practices. Innovative field techniques using heat as a tracer can be applied to spatially map and quantify the temporal changes of SW–GW interaction and the processes and mechanisms of groundwater recharge for different flow regimes. Horizontal directional drilling was used to construct two 100 m long drillholes at a depth of ∼ 5 m beneath and perpendicular to the river channel at a field site on the Waikirikiri Selwyn River, South Island, New Zealand. The two drillholes were completed with a hybrid fibre optic cable, containing four multi-mode fibres and 2x18AWG copper conductors. A Silixa XT-DTS™ distributed temperature sensor unit combined with a Silixa Heat Pulse System, was used to collect active distributed temperature sensing (A-DTS) measurements along both cables. Results from two A-DTS surveys showed distinct temperature changes across the active river channel indicating spatial variability in river loss and preferential groundwater recharge pathways to the shallow braidplain aquifer. The average groundwater velocity was 14 m d−1. The average calculated river loss at the survey sites was 21 m3 d−1 m−1 compared to 25 – 26 m3 d−1 m−1 that was measured using differential flow gauging conducted during the same time period. The horizontal installation of the fibre optic cable and the A-DTS method provided valuable insights into SW–GW exchange between the Waikirikiri Selwyn River and shallow braidplain aquifer and how the river leakage rates varied spatially across the river channel. [ABSTRACT FROM AUTHOR]

Physicochemical conditions in refugia must remain within the tolerance limits of aquatic invertebrates if they are to persist in intermittent streams when surface water is absent. We investigated shallow streambed sediments as a refuge for aquatic invertebrates and survival of invertebrates at high temperatures during the dry period in 3 headwater intermittent streams in New Zealand. Sediments from dry stream beds inundated in the laboratory contained 37 of the 53 taxa found in nearby flowing streams, a result suggesting that many taxa attempt to use the dry stream bed as a refuge during dry periods. Relative abundances of microcrustacea (copepods and ostracods), Gastropoda and Coleoptera (mostly Scirtidae) were greater in rewetted sediments than in sediments from nearby flowing streams, and several Ephemeroptera, Plecoptera, and Trichoptera taxa were present in low densities in rewetted sediments. All taxa that became active in rewetted sediments, except Hydrobiosidae, Tarapsyche olis (Trichoptera), Lymnaea (Gastropoda), Chironomini, and Cladocera, were active within a few hours of rewetting. We tested invertebrate survival in rewetted sediments at high temperatures (comparable to near-surface temperatures of dry stream beds in the field) in 2 experiments. In experiment 1, abundances of all major taxa were 11 to 94% lower in warm (air temperature = 30°C, sediment temperatures = 22-25°C for 1 h/d) than in cooler treatments. In experiment 2, abundances of most major taxa except Coleoptera and Acari were >75% lower in warm (air temperature = 37°C, sediment temperature = 32°C for 1 h/d) than in cooler treatments. Mid-summer daily maximum temperatures in the top 5 cm of the stream bed were 35 to 42°C in open pasture, 17 to 19°C under dense forest canopy, and 22 to 29°C under sparse riparian trees. Results suggest that maintaining the microclimate provided by riparian forest is important for protecting the aquatic invertebrate community in intermittent streams. [ABSTRACT FROM AUTHOR]

The Rational Method of flood estimation is used in an exploratory study to calculate values of mean annual flood in New Zealand basins. To improve applicability of the method and accuracy of the results a power law formula for time of concentration as a function of length, slope and average Manning roughness coefficient of the main channel is derived. It is calibrated employing data from 25 basins with widely differing areas, rainfall intensities and catchment characteristics. The formula explains 88% of the variance between the logarithms of measured and predicted values. A table for calculating the value of the catchment factor (ratio of runoff to rainfall) is compiled which is also designed for use in New Zealand. Rainfall intensities with twoyear return periods are determined using the HIRDS(V3) computer programme. With these new estimators for time of concentration and catchment factor, application of the Rational Method to calculate mean annual flood yields a root mean square error (RMSE) of ±27% between measured and predicted values for the 25 basins. Performance of the Rational Method is compared to that of the McKerchar-Pearson estimator for mean annual flood using data from a further 10 basins, giving RMSE values of ±34% and ±90% for the two approaches respectively. While further testing, calibration and development of the formulae is required, particularly regarding the computation of catchment factor values, application is relatively simple and the Method should result in a substantial improvement in both the accuracy of estimates of mean annual flood and the reliability of use of the index flood technique in ungauged New Zealand basins. [ABSTRACT FROM AUTHOR]

Abstract: Coarse woody debris (CWD) is an important component of fluvial systems affecting in-channel hydrology and sediment storage which in turn controls channel geometry and evolution. The role of coarse woody debris in coastal geomorphology is an emerging research field and this study investigates its role on mixed sand and gravel beach systems. A case-study approach is used whereby two field areas in Wellington, New Zealand, each with two distinct beaches with differing wood accumulations, were investigated through field surveying. It is found that on gravel beaches CWD is most important during storm events where it causes the berm crests to be 0.5–1.0m higher than on unwooded beaches, as well as making the beachface twice as steep. During calm periods the berm appears to be inactive as non-storm waves can only rework the lower beachface. A review of the literature indicates that on sandy beaches CWD is most important in the post-storm recovery of beach systems. The same wave event that causes beach erosion also deposits the CWD. During calm periods the woody material is a trap for sand being transported along the beachface and therefore aids in re-establishing foredune systems. The differing sediment composition between sandy and gravelly beaches drives their morphological response to storm events and in turn determines how CWD interacts with the beach profile. [Copyright &y& Elsevier]

This assessment of the interaction between groundwater and surface water in the braided Waimakariri River, New Zealand, between Waimakariri Gorge and Old State Highway Bridge identifies five reaches of the river bed that lose groundwater to the surrounding Holocene-age Springston Formation gravel aquifer. Steady-state groundwater outflow from the river bed in the reaches between Courtenay and Old State Highway Bridge totals 12.9 m3/s, providing the major source of groundwater for the Christchurch City area. Groundwater use in the Christchurch City area averages 1.7 m3/s and baseflow in spring-fed rivers and streams is an estimated 10.9 m3/s. Waimakariri River bed reaches are within the groundwater catchments of springfed rivers and streams. The reaches between Courtenay Road and Weedons Ross Road (groundwater outflow 3.7 m3/s) provided most of the baseflow in the Halswell, Heathcote and Avon rivers (surface water baseflow 3.6 m3/s); reaches between Weedons Ross Road and Wrights Cut (groundwater outflow 8.3 m3/s) provided baseflow (7.3 m3/s) to the Journal of Hydrology (NZ) 51 (1): 1-24 2012 © New Zealand Hydrological Society (2012) Old South Branch, Styx River and streams north of the Waimakariri River. Mean daily groundwater outflow from the five losing Waimakariri River reaches, assessed with a model that is primarily based on groundwater level observations in the Crossbank monitoring array located beside the river, was relatively uniform (standard deviation of approximately 1.0 m3/s) over a wide range in mean daily river flow (approximately 27 to 1310 m3/s) in the period 1 July 2002 to 30 June 2008. Mean daily groundwater outflow was greater at higher mean daily Waimakariri River flows, e.g., a groundwater outflow of 17.0 m3/s was estimated at river flows above 240 m3/s. However differences in groundwater outflow in two river flow ranges (27-60 m3/s and 60-120 m3/s) are not statistically significant. Groundwater outflow from the Waimakariri River was the most important control on the chemistry of groundwater in the Springston Formation aquifer and spring-fed rivers and streams. For example, low nitrate-nitrogen concentrations in the Waimakariri River (mean 75 µg/L) were reflected in low concentrations measured in Crossbank groundwater level monitoring array wells (mean 82 µg/L) and the Old South Branch stream (range 200-500 µg/L). Low nutrient (i.e., carbon, phosphorus and nitrogen) concentrations estimated in groundwater outflow from the Waimakariri River imply a low faunal density in the Springston Formation aquifer near the river. [ABSTRACT FROM AUTHOR]

Modelling groundwater and surface water is important for integrated water resources management, especially when interaction between the river and the aquifer is high. A transient groundwater and surface water flow model was built for Ruataniwha basin, New Zealand. The model covers a long-time period; starting in 1990, when water resources development in the area started, to present date. For a better resolution, the simulation period was divided into 59 stress periods, and each stress period was divided to 10 time steps. The model uses data obtained from surface water, and groundwater collected over the last 20 years. Rivers and streams were divided into 28 segments and flow and streambed data at the beginning and end of each segment was used. Parameter estimation and optimisation 'PEST' was used for automatic calibration of hydraulic conductivity, groundwater recharge and storativity; whereas riverbed conductance was manually calibrated. Model results show that the rivers gain from the aquifer considerably more than the river losses. The cumulative groundwater abstraction over the last 20 years is approximately 210 million m. This amount is very low compared to other water budget components; however, the effect of groundwater abstraction on storage is significant. Based on the results of this study, it was found that the loss of storage over the last 20 years is more than 66 million m. Results also reveal that the effect of groundwater abstraction on rivers and springs flow is significant. The rivers gain from the groundwater system, and the springs flow have been decreasing. [ABSTRACT FROM AUTHOR]

Strata and fault relationships revealed in five trenches excavated across the recent trace of the Alpine fault at the Haast, Okuru, and Tumbull Rivers, South Westland, New Zealand, record the three most recent surface-faulting events. Using back-stripping techniques to remove the three faulting events and the sedimentary units associated with the faulting restores the cross-sections to gravel-bed floodplains at the Haast and Okuru Rivers, at about A.D. 750. Horizontal and vertical offsets of stream channels and terrace risers reveal characteristic displacements of about 8-9 m dextral and up to 1 m vertical per event. Cumulative dextral displacement is 25 ± 3 m in the past three events. The most recent surface-rupture event was probably in A.D. 1717, and the next prior events were about A.D. 1230 ± 50 and about A.D.A.D.750 ± 50. The timing of these events is consistent with past large-great earthquakes on the southern section of the Alpine fault inferred from off-fault data, but there are fewer events identified in trenches. Our three-event dataset indicates the average surface-rupture recurrence interval for the South Westland section of the fault is about 480 years, much longer than the current elapsed time of 295 years. Therefore, the Alpine fault in South Westland may not be close to rupture as is often speculated. [ABSTRACT FROM AUTHOR]

Abstract: The Darfield earthquake caused widespread damage in the Canterbury region of New Zealand, with the majority of damage resulting from liquefaction and lateral spreading. One of the worst hit locations was the small town of Kaiapoi north of Christchurch, an area that has experienced liquefaction during past events and has been identified as highly susceptible to liquefaction. The low lying town sits on the banks of the Kaiapoi River, once a branch of the Waimakariri, a large braided river transporting gravelly sediment. The Waimakariri has been extensively modified both by natural and human processes, consequently many areas in and around the town were once former river channels. Using historical accounts and maps of the region, areas of land reclamation and old channels that had been cut off from the river since the beginning of European settlement in the 1850s were identified. These areas correlated well with many of the areas having significant liquefaction damage following the Darfield event. Substantial lateral spreads and sand boils developed in areas of reclamation along the current river path, causing significant damage to stopbanks and structures along the river, with fissures up to 2m deep and 1m wide. Much of the residential housing was damaged by lateral spreading, with cumulative displacement offsets up to 3m parallel to old channel beds that had aggraded over time due to river shifts. In former channel areas that were free of lateral spreading, large volumes of ejecta were present over wide areas, with depths of up to 400mm in places. Houses in these regions were damaged as a result of settlement and tilting. In all these areas underground services and roadways were severely impacted as a result of ground deformation. The severity of this damage indicates the importance of knowing the location of old channels when defining liquefaction prone regions. [Copyright &y& Elsevier]

For many gravel-bed rivers in New Zealand, there is a general channel degradation trend in the catchment, with sediments being transported down to lower reaches, where there is aggradation close to the coast. In some cases, the gravel bed material cannot be transported out to the coast, and there are natural deposition areas on alluvial plains. Repeat surveys of channel cross sections along the lower reaches allow volume balance estimates of the supply of bed material to these alluvial plain depositional zones to be determined for the inter-survey periods. Gravel-bed transport rates can be calculated from the bed material size and hydraulic parameters for given flood flows, using empirically derived formulae. If a sediment rating curve can be derived from transport rates for given flows and thresholds for the initiation of bed movement, then continuous flow records, generated at recorder sites, can be used to estimate bed material supply over the period of records. Comparing the volume balance estimates with those derived from the formula-based sediment rating curves indicates the applicability of different formulae, and allows some calibration or factoring of rating curve estimates. The results obtained from the application of volume balances and sediment rating curves are presented for a range of gravel-bed rivers in the North Island of New Zealand. The author carried out the sedimentation investigations or undertook detailed reviews of the supply estimation for the results presented. Some comment on these estimates of gravel supply in New Zealand rivers and the implications for extraction from rivers is included. [ABSTRACT FROM AUTHOR]

Morphological change in gravelly river channels and active slope environments can be used to derive lower-bound estimates of sediment flux between successive dates on which the morphology of channels or slopes was measured. This paper provides examples of such morphological budgets derived from analysis of digital elevation models built from ground survey data acquired from three sites in the upper Motueka River, and the Tarndale Fan and Gully system in the upper Waipaoa catchment. We demonstrate the utility of such an approach in providing information on processes operating in channel and slope environments at the same time as estimating volumes of sediment flux. [ABSTRACT FROM AUTHOR]

Abstract: In the headwaters of the Waipaoa catchment, fluvio-mass movement gully complexes are currently significant contributors of sediment, delivery of which is buffered by fans which have developed in the small tributaries that lead from the gully complexes. Biannual channel cross-section surveys made since 1983 show seasonal aggradation / incision cycles; channels generally incise in summer and infill during the winter months. This seasonal pattern of cutting and filling is driven by variations in sediment supply from the gully complexes. In this paper, to better understand patterns and processes of sediment delivery at the top of the sediment cascade, we examine cut and fill cycles on the ~11ha Tarndale Fan over a two and a half year period. Analysis of sequential DEMs compiled from survey data obtained between December 2004 and August 2007 indicates cutting and filling may involve up to 47,000tonnes of sediment over as little as three months. During discrete severe rainstorm and/or wet weather periods, mass movements in the gully complex are enhanced and contribute large quantities of sediment to the fan, via debris flows and landslides, which infill channels. During drier periods, mass movement activity is inhibited and runoff incises the feeder channels. [Copyright &y& Elsevier]

Short-term channel dynamics of mountain stream reaches in the southern North Island of New Zealand were assessed over two successive 3-month periods using morphological budgeting. Response to floods varies between reaches, even when the catchments were located close to each other and had similar characteristics. The reaches on the Central Volcanic Plateau experienced least morphological change, while streams with steep catchments and migrating planform in the Tararua and Ruahine Ranges showed frequent channel adjustments. Channel response is conditioned by intrinsic variables rendering reaches responsive or robust to the effects of floods, and this is likely to reflect the degree of connectivity between slopes and channels, and reaches. [ABSTRACT FROM AUTHOR]

Abstract: Effects of riparian harvesting on in-stream biota were monitored in five streams in exotic conifer plantations that differed in how logs were extracted, patterns in deposition of woody debris, and the degree to which riparian buffers were retained. Streams were sampled on three occasions in summer, once prior to harvesting and twice afterwards. Consistent effects of harvesting included an increase in the amount of woody debris in the channel, increased fine sediment and a trend towards higher algal productivity. Invertebrate communities changed dramatically in some streams. General patterns included an increase in small sized taxa and taxa with generalist diets in the first year after harvest, then a trend towards a greater representation of grazers and larger taxa the following year. Responses in particular taxa were highly variable between streams. Some of this variability was related to forestry management, the least dramatic changes occurring where a narrow riparian buffer strip was retained. Reduced impacts were also observed in streams where woody debris was piled over the channel. We suggest that management practices that maintain shading of the stream channel will moderate forestry effects, and that practices such as channel cleaning are likely to be detrimental. [Copyright &y& Elsevier]

Although the geomorphic and ecological importance of large wood in streams and rivers is well recognized, most studies consider only dead wood in channels. However, we have observed that living parts of trees are often found within active channels and that this "livewood" shares functions with both instream dead wood and live riparian trees, while also providing some functions unique to living woody material within a channel. We describe the mechanisms that produce livewood and illustrate its characteristics and influences on riparian and stream ecosystems with examples from Europe, North America, and New Zealand. We hypothesize that, compared with dead wood in channels, livewood (a) persists longer because of greater stability and greater resistance to decay, and (b) imparts greater structural complexity (with associated hydraulic roughness and retentiveness). The phenomenon of livewood implies that a broader range of tree species and sizes than previously considered may contribute functionally important wood to channels. We encourage the study of livewood in a range of forest-stream ecosystems to test our hypotheses and further our understanding of how forests interact with rivers and streams. [ABSTRACT FROM AUTHOR]

Abstract: Gully erosion is commonly associated with agricultural landscapes where vegetation clearance and farming practices increase runoff, leading to fluvial incision. However, gully erosion can also occur in forests that have undergone some form of disturbance, either natural or resulting from human impacts. This paper reports on recent gully development within areas of undisturbed indigenous forest as a result of a high magnitude rainfall event on the East Coast of New Zealand''s North Island. This region, through a combination of crushed and sheared rock types, steep topography, and tectonic and climatic setting, has high natural rates of erosion, exacerbated by European deforestation in the late 19th and early 20th centuries. Sequential air photographs, spanning a 58 year period between 1939 and 1997 were used to classify and document the growth and recovery of gully systems in the 14.1 km2 headwaters of the Mangaoporo catchment. Following a severe cyclone in 1988, with a rainfall of 535 mm, there were 21 active gully systems within the indigenous forest. On photography prior to 1988 only four gully systems were present. During this period there were 8 major rainfall events (150–250 mm). Despite further 5 rainfall events of 150–250 mm between 1988 and 1997 all gully systems showed signs of recovery, with a combined reduction in active area of 37%. The nature and location of these features is strongly influenced by lithology (orientation of jointing and bedding), and to a topographic threshold defined by catchment slope and catchment area. [Copyright &y& Elsevier]

An inventory of 846 mass movements, mainly landslides, in two alpine regions of southwest New Zealand was created to explore the geomorphic impacts of slope-failure processes on river channels and valley floors. In total, 213 (i.e. 27 per cent) of the slope failures descended to valley floors, affecting the geomorphology of trunk channels (catchment area AC > 10 km2) and valley floors in recurring patterns. A nominal classification system is introduced for characterizing (a) the physical contact nature between landslides and river channels, and (b) the resulting geomorphic consequences for drainage. Although landslide area A is useful for estimating the length of channel directly impacted by debris, it does not necessarily predict the direction of fluvial response or type of impact. Dominant persistent geomorphic imprints of bedrock landslides include channel occlusions and landslide dams in South Westland and Fiordland, respectively. Differences in size distribution and geomorphic effects on river systems between the two study regions are attributed to bedrock geology, tectonics and sediment flux. Although South Westland rivers are more frequently affected by landslides, disrupting long-term effects such as blockage are more persistent in Fiordland. Copyright 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article presents an illustrated reference of biophysical environment of Pacific Coast, New Zealand. Topics discussed include coastal belt linked classification (CBLC) , related environments and habitats; dominant catenary sequence (DCS): coastal ecological sequence (CES); and related geology, biology, geomorphology, littoral oceanography.

In rapidly uplifting regions, deep seated landslides are a prominent natural hazard that influence landscape evolution and are an important source of sediment. They pose a threat to infrastructure, and their failure can deliver sediment that overwhelms or blocks river channels. Consequently, a better understanding of why and where deep-seated landslides occur would help us reduce hillslope erosion, improve water quality, and allow us to better understand how landscapes evolve. Although the slope threshold model provides a framework for understanding how these landslides form, it has several shortcomings. Specifically, it fails to predict the spatial organization of landslides within a region, and it does not explain why landslide-prone portions of a region can exhibit relatively low slopes. To explore these issues, we have used a landslide susceptibility analysis within the Whanganui Basin of the North Island of New Zealand to explore the role that multiple susceptibility factors play in the occurrence of both translational and rotational deep-seated rock slides. Our findings contribute to a growing view of the relative importance of fluvial incision and the more subordinate role that earthquakes play in the development of large, deep-seated landslides. Additionally, our analyses identify the potential influence that widespread deforestation may have had on promoting instability, despite the deep-seated nature of the landslides studied. Most importantly however, we find that where slopes and weak geological structures align, a universal threshold slope does not exist, and many landslides develop on hillsides with below-average slope angles. This suggests that the presence of structural controls on landslide occurrence create a more complex landscape, where hillsides structurally predisposed to landslide occurrence have a much lower threshold slope angle. • We implement a deep-seated landslide susceptibility model within New Zealand. • Seismic factors were not predictive of deep-seated landslide occurrence. • Slope and bedding alignment was predictive of deep-seated landslide occurrence. • Slope and bedding alignment resulted in deviations from a universal slope threshold model. • Reducing river incision may be an effective way to prevent future failures. [ABSTRACT FROM AUTHOR]

Daily time lapse imagery and pixel tracking was used to monitor and track spatial and temporal changes in sediment-mantled hillslopes, during and immediately after glacier retreat in the Fox Glacier/Te Moeka o Tuawe valley, New Zealand. Observations from 2014 to 2018 of the Fox Glacier and surrounding hillslopes show hillslope failure is primarily coincident with, and triggered by, glacier retreat with rainfall accelerating movement of the hillslope. During glacier retreat, failure of the hillslope primarily occurred through sediment sliding, internal deformation of the sediment and occasional surficial debris falls, flows and avalanches delivering approximately 9.2 M m3 of sediment directly to the underside of the glacier over the study period with a maximum daily averaged movement of 0.4 m per day of the main sediment mass. Following debuttressing, hillslope failure became dominated by localised rainfall-induced debris flows which initiated gullying of the sediment-mantled slope. Ongoing instability of the slope and associated movement is maintained by toe erosion from the Fox River and melting dead ice, while continued rapid failure is facilitated through localised debris flows. The tracking of temporal and spatial changes of sediment-mantled hillslopes during glacier retreat has shown broad-scale hillslope response to occur quickly within days of rainfall or accelerated glacier retreat, particularly during summer. Debris flows, commonly thought to be a dominant erosion process within paraglacial environments, only occur following complete debuttressing and are supply-limited, only occurring after sediment sliding has occurred. Unlike many other case studies, sediment connectivity immediately following glacier retreat is high due to a lack of storage space and high rainfall inducing mass movements, efficiently delivering hillslope sediments to the proglacial stream channel. Attempts to quantify displaced volumes of sediment from paraglacial systems are likely underestimated due to a) a lack of focus on the early and latter stages of debuttressing b) a reliance on debris flows being a primary transport mechanism, and c) sediment being delivered sub-glacially rather than supra-glacially, further enhancing connectivity. • 4 year record of paraglacial sediment slope failure in a temperate, maritime environment • Debuttressing promotes slow sediment sliding and deformation followed by debris flows. • Previously undocumented subglacial entrainment of landslide sediment • Sediment erosion rates are ten times higher than previously reported in other studies. • Sediment connectivity in the system is high due to steep valley morphology and climate. [ABSTRACT FROM AUTHOR]

The river mouth lagoons of New Zealand's gravel-bed braided rivers are highly dynamic, responding to changes in waves and river flows at timescales of hours and days to years. Unlike other freshwater-dominated coastal wetlands and lagoons, they exist along open coasts that are typically retreating with cliffed hinterlands. These lagoons are common on New Zealand's high energy, coarse sediment coastlines and are known locally as 'hāpua'. This paper employs field observations from the Hurunui hāpua, to investigate the main processes controlling the dynamics of hāpua lagoons. Based on two years of time-lapse imagery, plus concurrent wave, river flow and sea level data an improved conceptual model of lagoon dynamics was developed. The model describes how onshore bar migration of river sourced gravel and sand drives initial constriction/offsetting of the river mouth following floods. It also explains how gravel deposition in the lagoon reduces the threshold flow required to cause primary breach of the barrier opposite the point the river enters the lagoon. Three processes are identified which affect lagoon width: wave overwashing of the gravel barrier narrows the lagoon; migration of the highly dynamic lagoon outlet channel 'resets' the barrier position to seaward; and river floods occurring while the outlet channel is offset from the river flush sediment from the lagoon system, eroding its bed and banks. Both processes affecting lagoon widening rely on lagoon outlet channel migration, indicating that outlet channel dynamics are an important control on lagoon size. • Non-estuarine river mouth lagoon dynamics are explained via a new conceptual model. • Onshore bar migration of flood bedload drives initial outlet channel constriction. • Bedload deposition in the lagoon increases likelihood of barrier breach by river. • Wave overwash, outlet channel migration and river flood erosion control lagoon width. [ABSTRACT FROM AUTHOR]

Groundwater monitoring networks are designed to provide information for environmental monitoring and water resources management which includes not only the temporal and spatial availability water resources, but potentially also other system states such as the mean travel time of water at abstraction locations or the flow at groundwater-fed springs. The aim of this work is to assess the degree of information provided by different observation types and locations for predicting different groundwater-system states. Further we investigate whether there is a common "core" set of information, i.e. a monitoring network design that provides information simultaneously for all required management purposes.A model-based data-worth analysis and experimental design framework is proposed, where pragmatic data-worth estimation methods from the suite of PEST utilities are used. These methods have been extended for discrete multilocation, multitype monitoring network design which is still pragmatic and efficient. The application of the framework requires a predictive groundwater flow model to be set up for the investigated groundwater system. Since models are inherently simplifications of reality, model predictions are subject to error and uncertainty. However, data from monitoring networks can inform models and thereby reduce predictive uncertainty. Therefore, data worth is defined here as the utility of data to reduce the predictive uncertainty of a quantity of interest. We demonstrate our methods on the application of the framework to the Wairau Plain Aquifer system which covers a small proportion of the Wairau catchment in the Marlborough District of New Zealand. The aquifer is almost exclusively recharged by surface water from the Wairau River and serves as the major resource for drinking water and irrigation in the region. A numerical flow model has been previously set up for the Wairau Plain to predict river-groundwater exchange flows, groundwater heads and storage, spring flows, aquifer storage and mean-travel times. We investigate the degree of information (data worth) of groundwater head data, hydraulic conductivity, specific yield, river-bed conductance, and hydraulic properties of the braided Wairau River bed for these predictions. Not surprisingly, our analysis revealed that data worth is purpose-specific. Different predictions are informed by different data types and monitoring locations. There is common data worth only for some predictions and there are also "hot-spots" of information location. Rather intuitive results are that specific yield is informative for aquifer storage; spring-bed conductance and groundwater head in the vicinity are informative for spring flows; and hydraulic conductivity and heads best inform mean travel times. But there are also some not so obvious results such as the impact of the hydraulic parameters of the braided-river bed of the Wairau River and of the assumption about deep groundwater losses toward the sea. Often, data worth is not independent and the best information gain can be achieved from a combination of data types and locations. This is an important aspect to be considered in the design of multi-purpose groundwater monitoring networks. [ABSTRACT FROM AUTHOR]