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Anthropogenic climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Understanding the pattern of regional streamflow trends can help to inform region-specific policies to mitigate and adapt to any negative impacts on society and the environment. We present a benchmark dataset of long, near-natural streamflow records across Aotearoa New Zealand (NZ) and the first nationwide analysis of observed spatiotemporal streamflow trends. Individual records rarely have significant trends, but when aggregated within homogenous hydrologic regions (determined through cluster analyses), significant regional trends emerge. A multitemporal approach that uses all available data for each region and considers trend significance over time reveals the influence of decadal variability in some seasons and regions, and consistent trends in others. Over the last 50+ years, winter streamflow has significantly increased in the west South Island and has significantly decreased in the north North Island; summer streamflow has significantly decreased for most of the North Island; autumn streamflow has generally dried nationwide; and spring streamflow has increased along the west coast and decreased along the east coast. Correlations between streamflow and dynamic and thermodynamic climate indices reveal the dominant drivers of hydrologic behavior across NZ. Consistencies between the observed near-natural streamflow trends and observed changes in circulation and thermodynamic processes suggest possible climate change impacts on NZ hydrology. [ABSTRACT FROM AUTHOR]

We show that climate and topography control the spatial distribution of stable isotope values on the South Island of New Zealand, based on a spatially dense (n = 193) river isotopic survey. Our data show a δ18O minimum in isotope values east of the Southern Alps that demonstrates topographically driven continentality associated with the Southern Alps, which intersect the prevailing, moisture‐laden westerlies. Our data define a South Island surface water line of δ2H = 8.17 (±0.26) × δ18O + 10.57 (±2.04), which is identical within 95% confidence intervals to the global and New Zealand meteoric water lines established from monthly precipitation samples. The observed river δ18O values are strongly correlated with annual temperature range and winter temperature. Strongest correlations are between δ18O and mean minimum winter temperatures (r > 0.7 for June, July, August), with gradients of 0.58–0.66‰ /°C. Based on a multiple regression analysis of δ18O against climate data, we present a river δ18O model and isoscape that demonstrate the control of continentality and moisture source on New Zealand surface water isotope spatial patterns. Model validation against previously published river samples shows skill in predicting river δ18O values (root‐mean‐square error = 0.83), confirming that the spatial variations in river δ18O (and δ2H) are robust to sampling period and reflect continental, precipitation source and temperature effects. Our data suggest that oxygen or hydrogen isotope paleoclimate proxies derived from rivers or open‐system lakes on the South Island should be sensitive to winter temperature. Plain Language Summary: We investigated the spatial variations in oxygen and hydrogen stable isotopes in rivers on the South Island of New Zealand and show that they are strongly controlled by the presence of the Southern Alps. Indicators of continental climates, such as seasonal temperature range and temperature of the coldest months are strongly related to measured oxygen isotope values of river waters. We show that winter temperature and continentality are the dominant control on New Zealand South Island river oxygen isotope values, and they result in minimum values in the lee of the highest portion of the Southern Alps. We also show that river deuterium excess values are controlled by Pacific Ocean versus Tasman Sea moisture sources. Key Points: Survey of spatial river δ18O values on New Zealand's South Island shows strong correlations to winter temperature and continentalityLowest river δ18O values are present in the most continental locations in the lee of the high Southern AlpsData are useful for interpreting isotopic paleoclimate proxy records related to surface waters [ABSTRACT FROM AUTHOR]

Monitoring for adaptive management (AM) involves collection of data with the aim of reducing uncertainty about links between human pressures (e.g. water abstraction from rivers), consequent stressors (e.g. low river flows) and environmental state (e.g. biodiversity). 'Surveillance monitoring' involves documenting trends in state, without the aim of understanding relationships between state, stressors, and pressures. Critics have highlighted that surveillance monitoring dominates monitoring investments but is not supporting AM. Decision-makers continue to be disappointed by monitoring data that are unsuitable for AM, yet designers of monitoring programs tend to make decisions that reinforce rather than reimagine the status quo. We argue that a structured, collaborative approach to objective-setting is required to break the status quo. We collaborated with regional management authorities to develop monitoring objectives and implementation strategies to support AM of New Zealand's rivers. Our collaborative approach discouraged 'failure fearing' and encouraged reimagining 'what could be' as opposed to 'what is.' Seventeen monitoring objectives were identified based on the AM requirements of national policy and regional authorities. Several objectives—particularly those arising from national policy—stretch the limits of what environmental science can currently provide. There were also strong trade-offs among objectives. We offer practical implementation strategies for overcoming the technical challenges of, and reducing trade-offs among, monitoring objectives. These strategies point to a monitoring program that contrasts strongly with one aimed at surveillance. Monitoring for AM is more complex than monitoring for surveillance, so strong leadership is required for successful implementation. • Environmental monitoring often fails to provide data required for adaptive management. • Monitoring design should be based on a structured, collaborative approach to objective-setting. • This approach yields challenging monitoring objectives, among which there are strong trade-offs. • Pragmatic implementation strategies are available to help manage these challenges and trade-offs. [ABSTRACT FROM AUTHOR]

The Snowy River processing site was used to extract gold for the large historic Blackwater mine, Westland, between 1906 and 1938, and then abandoned. The site is now one of the best-preserved historic gold-processing localities, and access has been enhanced for visitors. The site has some high arsenic (As) concentrations on the surface, with some spots exceeding 40 wt% As. A variety of As-bearing minerals occurs along the former processing stream, from primary sulphides (arsenopyrite, pyrite) at the upstream end to arsenolite (As2O3) and scorodite (FeAsO4.2H2O) formed by the roasting of ore materials at the downstream end. Weathering over the past century of sulphide residues and some of the arsenolite has yielded more scorodite and some Ca arsenates. Minor authigenic As sulphide precipitates have formed on rotting wood. Arsenolite is principally distributed near to the roaster site, and this is the most soluble and potentially toxic mineral on site. Despite the locally abundant arsenolite, only minor As is mobilised into nearby waters. [ABSTRACT FROM AUTHOR]

How should the success of river and floodplain restoration be assessed? What should we be restoring? We contend that the benchmark for restoration should be the river in its "natural," that is, quasi‐equilibrium condition prior to deliberate modification. The pre‐modification condition of the river represents a quasi‐equilibrium state in which the river accommodates and adjusts to catchment water and sediment fluxes, whether in a pristine or modified catchment. The resulting assemblage of river landforms (e.g., bars, channels, backwaters) is in balance with the prevailing flood and sediment regime. Furthermore, equilibrium channel forms can adjust to changes in, for example, flood magnitude and frequency via for example, channel expansion or contraction. A restored river system is one that once was anthropogenically restricted, but now has regained capacity to adjust its form. River restoration needs understanding of the history and trajectory of pre‐modification channels, so that the "right" river type can be restored. To assess the scope and success of restoration, we propose using a natural character index (NCI). The NCI is the ratio between a parameter which describes the form and function of a river now (observed) and the same parameter measured at a point or points in time past (expected). Exemplar parameters include sinuosity, floodplain width, active channel width, bar area (at a given flow) and riparian vegetation. Measurement of these parameters utilizes a combination of aerial photos, archive maps and LiDAR‐derived terrain models, the availability of which necessarily limits and determines what parameters can be used in NCI assessment. LiDAR is a useful tool providing an opportunity to comprehend pre‐modification river and floodplain character. We illustrate the NCI concept and its potential for use in assessing restoration effects using examples from New Zealand, Spain and Croatia. The result is an index of use as a "first cut" preliminary assessment for river practitioners. [ABSTRACT FROM AUTHOR]

Global change is increasing biotic homogenization globally, which modifies the functioning of ecosystems. While tendencies towards taxonomic homogenization in biological communities have been extensively studied, functional homogenization remains an understudied facet of biodiversity. Here, we tested four hypotheses related to long‐term changes (1991–2016) in the taxonomic and functional arrangement of freshwater macroinvertebrate assemblages across space and possible drivers of these changes. Using data collected annually at 64 river sites in mainland New Zealand, we related temporal changes in taxonomic and functional spatial β‐diversity, and the contribution of individual sites to β‐diversity, to a set of global, regional, catchment and reach‐scale environmental descriptors. We observed long‐term, mostly climate‐induced, temporal trends towards taxonomic homogenization but functional differentiation among macroinvertebrate assemblages. These changes were mainly driven by replacements of species and functional traits among assemblages, rather than nested species loss. In addition, there was no difference between the mean rate of change in the taxonomic and functional facets of β‐diversity. Climatic processes governed overall population and community changes in these freshwater ecosystems, but were amplified by multiple anthropogenic, topographic and biotic drivers of environmental change, acting widely across the landscape. The functional diversification of communities could potentially provide communities with greater stability, resistance and resilience capacity to environmental change, despite ongoing taxonomic homogenization. Therefore, our study highlights a need to further understand temporal trajectories in both taxonomic and functional components of species communities, which could enable a clearer picture of how biodiversity and ecosystems will respond to future global changes. [ABSTRACT FROM AUTHOR]

Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lateral variations in these submarine deposits and provide a rare opportunity to examine deposits of an arc environment within a thin slice of time. We describe pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites, all hydrothermally altered and metamorphosed to prehnite-pumpellyite facies. Trace element geochemical data indicate multiple primitive island arc to MORB-like magma sources. Two separate volcanic centres apparently tapped different mantle domains in a source region transitional between the arc and back-arc. The centres' proximity and inferred small sizes suggest they were satellite vents of a larger arc volcano. Tuff and lapilli tuff were formed as hyaloclastite during eruption of pillow lava, and redeposited by subaqueous mass flows. Peperitic contacts indicate that the lavas intruded unconsolidated volcaniclastic deposits. We infer that hydroclastic fragmentation associated with pillow emplacement produced debris that was then intruded by later pillows, with local peperite formation, as eruption persisted. The hydroclastic debris from these satellite vents was resedimented in turbidite fans with channels, and periodic turbidity currents delivered fine-grained material from the volcano's main vent. [ABSTRACT FROM AUTHOR]

Increasing attention is turning to moderating the impact of human activity on the environment, both to preserve the intrinsic value of ecosystems and species for their own sake, and to protect the benefits we derive from nature for future generations. Internationally, various regulations and policies are in place or in development to improve our stewardship of the environment and develop more sustainable and resilient management practices. However, policies formulated at national or regional scales are not always suited to enacting targeted and cost-effective approaches at the local scale due to geoclimatic, topographical, or management constraints. The direct monitoring of the local and upstream impacts of every management unit to determine their "net impact" is a costly practice, thus emphasising the need for modelling approaches to complement limited on-ground measurements. This paper describes and demonstrates tools (LUCI-EntEx v1.0) that automatically identify the fluvial and terrestrial flow of water in and out of a study area, such as a river that enters a farm that is impacted by upstream management, or terrestrial flow coming from neighbouring property. By identifying the stream entry/exit points, the "net impact" of land management within the study area can be more easily quantified based on the contribution of neighbouring and upstream areas, aiding in the decision-making process. This algorithm also facilitates the identification of inconsistencies in data such as differences between the legal/official catchment boundaries and the hydrological boundaries determined by the representation of terrain and river networks. If such inconsistencies are not resolved, they can cause further error propagation in later stages of the modelling process. Four case studies of New Zealand management units - two at the farm scale and two at the catchment scale - demonstrate the algorithm's utility in determining fluvial and terrestrial entry/exit points and highlighting potential data inconsistencies. The farm case studies also use the Land Utilisation and Capability Indicator (LUCI) framework to demonstrate how this algorithm can be embedded in other models for further value: in this case, we show its potential to improve predictions and enhance management of nutrients and sediment. [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]

Landscape evolution is governed by the interplay of uplift, climate, erosion, and the discontinuous pattern of sediment transfer from the proximal source of erosion to distal sedimentary sinks. The transfer of sediment through the catchment system is often referred to as a cascade, the pattern of which is modulated by the interaction of key network characteristics such as the distribution of transport capacity and resultant zones of sediment storage. An understanding of how sediment production is modulated through river networks with different topological structures at the associated timescales has remained elusive but presents significant implications for the knowledge of river response to disturbance events, and floodplain asset management. A multivariate method of identifying representative topological structures from a range of river networks is presented. Stream networks from 59 catchments in the South Island of New Zealand were extracted from a digital elevation model and their key topological parameters quantified. A principal component analysis was implemented to reduce these to two‐dimensional axes that represent the magnitude of network branching and the topographic structure of each catchment, respectively. An agglomerative hierarchical clustering analysis revealed five network 'types', which are examined in terms of their internal structural characteristics and relationships to potential regional‐scale controls. Implications for sediment transfer in these network 'types', and their use as representative networks for further analysis, are discussed. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

In animals with indeterminate growth, such as predatory fishes, mouth size is related to, and increases with, body mass of the organism. As fishes grow in body size, they can consume larger-bodied prey items at potentially-higher trophic levels. Therefore, body size should predict trophic position. Trophic position (TP) is useful for describing energy flow between biota, giving a more comprehensive picture of aquatic food webs, but the factors that influence the body-size–TP relationship are unclear. Additionally, stable isotope-derived TP and body-size relationships are untested for temperate freshwater fishes. Using linear mixed-effects models, we tested fish body mass as a predictor of TP across 6 species of temperate freshwater fish (n = 921 total individuals) sampled from 110 stream reaches in New Zealand, and we evaluated habitat factors (flood disturbance and stream size) likely to affect trophic structure of stream fish assemblages. Supporting ecological theory, but contrasting with some previous findings in marine and tropical environments, we found that TP was positively associated with body size across all species. Habitat factors did not affect fish TP. However, stream network spatial factors included in models as random effects explained the majority of variance in TP across species. Thus, while our study shows that New Zealand temperate freshwater fish body size is positively associated with stable isotope-derived TP, researchers will need to account for catchment-specific spatial factors to make accurate inferences about TP based on body size. [ABSTRACT FROM AUTHOR]

Degradation of aquatic ecosystems by fine sediment is a global environmental problem, and damages reflect particle properties as well as mass concentration. We studied sediment-related water quality (SRWQ) of small streams in hill-country near Whatawhata, NZ, that have been monitored since 1995. In 2001, an integrated catchment management (ICM) plan was implemented in the Mangaotama (originally sheep-beef) catchment – including conversion of steep headwaters to pine plantation, cattle exclusion, and native riparian plantings. The long-term SRWQ within the Mangaotama catchment was examined with reference to a nearby native-forested control catchment. Correlations between visual clarity, suspended particulate matter concentration and related variables were moderate to fairly strong. The ICM has not improved SRWQ as expected – plausibly because removal of cattle disturbance has been offset by decline in pasture ground cover under increasing shade of tree plantings. Worsening SRWQ is expected over the next several decades as channels widen towards a 'forest' morphology. [ABSTRACT FROM AUTHOR]

There is concern about the deteriorating nutrient status of aquatic receiving environments in New Zealand. We estimated the amount by which current nitrogen (N) concentrations and loads exceed criteria in rivers, lakes and estuaries nationally. Criteria corresponded to national 'bottom-line' (i.e. minimal) environmental objectives set by government policy. Three metrics were evaluated: (1) degree of compliance describes the current TN loads in receiving environments relative to criteria; (2) catchment N status describes the acceptability of catchment N loads compared to criteria; and (3) excess load indicates the amount by which the N load exceeds the maximum allowable load (kg yr−1). Non-compliance with N criteria was broadly distributed nationally particularly in low-elevation catchments. Catchments with unacceptable N status constituted at least 31% of New Zealand's land area, which corresponds to at least 43% of the country's agricultural land. The national excess load was estimated to be at least 19.1 Gg yr−1. We are 97.5% confident that estimated excess loads exceed zero for nine of 15 regions and for the nation as a whole. The analyses provide a strategic assessment of where reductions in N emissions are required to achieve the minimal national objectives. [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]

Water is an important component of many recreational opportunities and New Zealand invests significant resources in recreational water quality (RWQ) monitoring to assess risks to public health. However, monitoring primarily focuses on microbiological hazards and RWQ is considerably broader than this. The goal of this study was to take a comprehensive approach to evaluating RWQ while exploring the potential for greater public involvement in monitoring. A popular river swimming site was co-monitored weekly by volunteers and a council field officer. The attributes monitored–water temperature, visual clarity, benthic cyanobacteria and filamentous periphyton cover, E. coli and rubbish–were identified from a volunteer workshop. There was good concordance between volunteer and council data suggesting that volunteer measurements can be considered reliable. This highlights the potential for volunteers, with professional support, including training and quality assurance, to extend council RWQ surveillance to additional monitoring sites while improving public engagement on water management. [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]

The authors discuss a new method for using regression analysis to study nutrient concentrations in periphyton. It mentions the use of nutrient concentrations in cases of eutrophication, the study of periphyton biomass in rivers in New Zealand, and how regression models can identify biomass size.

Rivers, lakes, streams, and springs have been described as the bloodways of Papatūānuku, earth mother. On this view of the world, water is life. We are witnesses to the importance of water for all communities when the health and well‐being of our waterways are at risk. In New Zealand, governance and management systems have not been able to cope with legacy issues nor are they coping with the complex problems of diffuse pollution from intensive farming, climate change, pest species, and population growth. In seeking ways to improve freshwater management, there are increasing calls to include tikanga (indigenous laws and values) into decision‐making frameworks. Indigenous concepts such as kaitiakitanga (the responsibility to take care of natural resources) and Te Mana o te Wai (restoring and protecting the integrity of water) have gained traction. This article will explain these concepts through a distinctively indigenous lens and highlight some of the challenges of integrating tikanga into dominant western frameworks. [ABSTRACT FROM AUTHOR]

This paper examines how attitudes towards rivers in Aotearoa New Zealand have evolved since the country's settlement by Europeans, two centuries ago. For most of our postcolonial history, rivers have been viewed as something to be controlled and managed—even "improved." But today, rivers are increasingly being recognized as embodying a broad range of values from the ecological to the spiritual—not simply as a "channel of water" that can be exploited for human ends. Although much of this evolving understanding stems from the advance in scientific knowledge, much too has its roots in our collective past. [ABSTRACT FROM AUTHOR]

Many multi-regional studies investigating how available habitat area, energy availability, and historical refugia drive freshwater fish diversity have emphasized Northern Hemisphere and tropical areas. Furthermore, while many such studies have examined diversity drivers on basin-scale species richness (i.e., gamma diversity), they typically have not evaluated beta diversity or phylogenetic diversity nor included representative numbers of Southern Hemisphere basins unassociated with tropical rainforests. Here, we examine 784 basins and present the first comprehensive evaluation of the driving effects of gamma diversity and the patterns of beta and phylogenetic diversity across extra-tropical Southern Hemisphere (ETSH) freshwater fish communities. We find that ETSH gamma diversity does not show a strong historical legacy associated with glaciations, and is influenced more by factors related with habitat area and available energy. Additionally, ETSH regions show high beta and phylogenetic diversity and low levels of diadromy, except New Zealand, which shows narrow species diversity and cosmopolitan species dominance. Finally, phylogenetic diversity indicates how the endemism of the three ETSH Mediterranean-climate regions is characteristic of long-term isolation and persistence. These results demonstrate ETSH freshwater fish diversity to be distinct from both the tropics and their Northern Hemisphere latitudinal counterparts. [ABSTRACT FROM AUTHOR]

The article discusses the history of the railway system and incidents of flooding in New Zealand in the 19th century. Topics discussed include the task of Colonial Under Secretary George Sisson Cooper to enumerate the deaths by drowning which occurred since the inception of Pakeha colonization in 1840, the rapid expansion of rail transport and railways which served as bridges to many rivers, and the impact of railways on the flow of river water and vice versa.

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnäsite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems. [ABSTRACT FROM AUTHOR]

Thirsty Creek Norite is a small, undeformed, mafic intrusion intruding gneisses of the Granite Hill Complex in the Buller Terrane. An age of 107.1 ± 4.1 Ma has been obtained using Rb-Sr geochronology. A relatively high MSWD of 4.5 is attributed to sericitization and alteration of plagioclase. This date is consistent with other ages for the timing of high-grade metamorphism in the Granite Hill Complex and may represent a metamorphic age, thus providing a minimum age for metamorphism and emplacement of the norite. However, the relatively undeformed nature of the Thirsty Creek Norite suggests it may postdate metamorphism and we speculate that the date could represent a crystallisation age which is contemporaneous with Rahu Suite granitic magmatism in the adjacent Hohonu Batholith. Several geochemical characteristics, including overlapping initial Sr and Nd isotope compositions, suggest the Thirsty Creek Norite could be a relatively mafic yet contaminated melt that is petrogenetically related to Rahu Suite magmatism. Overlaps in the isotopic compositions of the Thirsty Creek Norite and Rahu Suite granitoids therefore imply that any mafic components involved during granitic magmatism must have had relatively enriched isotopic compositions, and that evolution to more felsic compositions occurred without significant interactions with upper crustal components. [ABSTRACT FROM AUTHOR]

We assessed the reproducibility of river state-of-environment (SoE) water quality measurements in the Wellington Region, New Zealand (NZ). Field staff from GWRC and NIWA conducted 29 side-by-side water sampling and in-situ measurements at six river sites of diverse water quality for 12 variables measured routinely in river SoE monitoring across NZ. Field measurements of water temperature, dissolved oxygen, electrical conductivity and visual clarity agreed closely with strong numerical similarity (within 10%). Numerical similarity ranged widely for laboratory measurements, from strong for nitrate-nitrite-nitrogen to weak for turbidity, dissolved reactive phosphorus, and ammoniacal-nitrogen. Numerical agreement was very weak for laboratory pH (which is problematic) and E. coli–which is 'tolerable' for many applications given good correlation (R = 0.94) over a 2000-fold concentration range. The findings of our inter-agency comparison have contributed to quality assurance recommendations in the NZ National Environmental Monitoring Standard (NEMS) for water quality. [ABSTRACT FROM AUTHOR]

From 2011, lamprey (Geotria australis) populations in Southland, New Zealand have been affected by reddening along the length of the body and increased mortalities, termed lamprey reddening syndrome (LRS). Molecular testing, bacteriology, histopathology and epidemiology were used to determine if an infectious agent was present and to increase the understanding of this syndrome. An atypical Aeromonas salmonicida was detected from affected lamprey in 2011. This organism has not previously been reported from New Zealand. Investigative molecular testing indicated the organism was an uncharacterised, unculturable, atypical A. salmonicida and a likely incidental finding. Histopathology did not indicate an infectious process was involved, but suggested the reddening may be due to blunt trauma. Epidemiological investigation found the Mokoreta River had a significantly higher prevalence of LRS than others in the Southland region, but there was no clear reason why. To date, no infectious aetiology for this syndrome has been identified. [ABSTRACT FROM AUTHOR]

Legislation in New Zealand dictates that the Whanganui River is a living entity and a legal person. Guardians uphold the river's environmental, social, cultural and economic well-being. We provide a conceptual discussion of the river's economic well-being, understood as the mutual enhancement of natural and human elements through community entrepreneurship that is based on human and non-human capabilities. We discuss human economic activity that preserves the right of the river to be free from pollution and form an integral part of the Māori culture and tradition, the improvement of Māori living conditions, and their rights to self-determination and prior consent. [ABSTRACT FROM AUTHOR]

The enrichment of phosphorus (P) in streams and rivers can impair water quality, but concentrations have decreased. We found little evidence that this improvement was caused by a decrease in soil Olsen P concentrations or imported P (e.g. fertiliser), a change to low water-soluble P fertilisers, or that greater nitrate loads were assimilating P from groundwater or sediments. Possible causes of improvement were that land use change had decreased erosion, more nitrogen fertiliser use was assimilating soil P, and a greater awareness of P as an environmental issue. However, the most probable causes were that strategies were mitigating P loss from land, guidelines were directing where to best use strategies, and policy instruments were including P management. These findings support the development and implementation of mitigations, supported by voluntary guidelines and regulation. However, our findings can be strengthened if data are referenced to equivalent, and finer spatial and temporal scales. [ABSTRACT FROM AUTHOR]

Issues regarding water quality and supply have been raised within the Pourewa catchment, following ongoing landuse change and associated demand for freshwater. Nitrogen loss from farms due to leaching of nitrate into the subsurface has increased in conjunction with landuse intensification. The stream dissects Whanganui Basin fill, including alternating coarse and fine-grained sediments deposited throughout Quaternary climatic cycles, resulting in interbedded aquifers and aquitards in a geologically young landscape. The Rangitikei River dominates the landscape, forming a series of aggradational terraces. Fault lines and basement highs provide a control on both the underlying Plio-Pleistocene strata and modern-day drainage network. A 1:25 000 hydrogeological map and corresponding structural interpretation are presented. Analysis of catchment lithologies demonstrates the importance of grain size, porosity and age in dictating hydraulic conductivity. Organic carbon, illite and glauconite present within the succession indicate attenuation of nitrate within the local groundwater system is taking place. [ABSTRACT FROM AUTHOR]

That biodiversity declines with latitude is well known, but whether a metacommunity process is behind this gradient has received limited attention. We tested the hypothesis that dispersal limitation is progressively replaced by mass effects with increasing latitude, along with a series of related hypotheses. We explored these hypotheses by examining metacommunity structure in stream invertebrate metacommunities spanning the length of New Zealand’s two largest islands (∼1,300 km), further disentangling the role of dispersal by deconstructing assemblages into strong and weak dispersers. Given the highly dynamic nature of New Zealand streams, our alternative hypothesis was that these systems are so unpredictable (at different stages of post-flood succession) that metacommunity structure is highly context dependent from region to region. We rejected our primary hypotheses, pinning this lack of fit on the strong unpredictability of New Zealand’s dynamic stream ecosystems and fauna that has evolved to cope with these conditions. While local community structure turned over along this latitudinal gradient, metacommunity structure was highly context dependent and dispersal traits did not elucidate patterns. Moreover, the emergent metacommunity types exhibited no trends, nor did the important environmental variables. These results provide a cautionary tale for examining singular metacommunities. The considerable level of unexplained contingency suggests that any inferences drawn from one-off snapshot sampling may be misleading and further points to the need for more studies on temporal dynamics of metacommunity processes. [ABSTRACT FROM AUTHOR]

Abstract The Waikari River tsunami was caused by a landslide triggered by the 1931 Hawke's Bay earthquake, New Zealand's deadliest natural disaster to date. Although it was reported in newspapers and personal diaries at the time, this is the first study to examine the tsunami using a multi-proxy approach aimed at investigating the physical evidence of this event. Sedimentological, geochemical, chronological and microfossil analyses were carried out on an anomalous gravel layer within the sedimentary sequence of Waikari Station located in a meander bend close to the river mouth. The chronology was established by extrapolation from 137Cs data coupled with historical artefacts found within the gravel layer. Diatoms within the deposit were brackish, and most probably sourced from the adjacent tidally-influenced Waikari River. Principal component analysis performed on geochemical data showed that the anomalous gravel layer at all sites is associated with a cluster of elements not related to detrital input nor organic matter, but indicative of an estuarine source. A second, older, gravel layer was also identified. It is considered to have been deposited by an earlier tsunami, quite likely caused by a landslide generated by the 1863 Hawke's Bay earthquake. The deposit has similar sedimentological, geochemical and microfossil characteristics to the gravel layer deposited by the 1931 event. This study adds valuable insights into the poorly understood topic of landslide generated tsunamis by investigating the largest historical event of its kind to occur in New Zealand. [ABSTRACT FROM AUTHOR]

In 2017, rivers in New Zealand, India and Colombia received legal rights and were granted the status of legal persons. The increased legal powers, often a result of groundbreaking agreements or settlements with Indigenous peoples, may improve environmental protection and river management, but they can also challenge the legitimacy of laws and regulations that protect the rivers. In this paper, we compare the new legal rights with two long-standing uses of legal personality in river management, to explore the effects of legal personality in terms of environmental resource management. We argue that governments must ensure that they get the right balance between giving rivers a voice (and the power to be heard), and creating collaborative governance arrangements that strengthen and maintain community support overtime. [ABSTRACT FROM AUTHOR]

Understanding the determinates of community structure and function is a central theme in community ecology. The form in which energy is supplied to food webs can strongly influence community structure and function. Trophic stability is also thought to be affected by the nature of food web energy pathways and whether they are external (allochthonous) or internal (autochthonous) to the ecosystem. In this study, we assessed whether stream invertebrate assemblages differ in taxonomic composition, energetic network structure, trophic network stability, and assemblage temporal variability based on whether the streams they occupy occur under forested canopies or in open grasslands. We assumed that forested sites would receive more allochthonous inputs, whereas grassland sites would rely more on autochthonous resources. We also tested whether food web stability (robustness, the conservation of energy flow) changed after simulated species extinctions. We found that the forest stream assemblages were dominated by filter feeders and shredders, whereas the grassland assemblages were dominated by grazers. In spite of these differences in assemblage composition, we found no significant differences in trophic network structure, stability, or temporal variability among different site types. Many stream systems, particularly in mountainous areas, such as New Zealand, are exposed to regular and large physical disturbance from flooding, which may result in assemblages with similarly generic diets irrespective of the energy source. [ABSTRACT FROM AUTHOR]

The importance of chromophoric dissolved organic matter (CDOM) and water parameters for the structure and function of rivers and lake ecosystems has led to the development of a way to estimate the amount of CDOM, TP (total phosphorus) and TN (total nitrogen) in rivers and lakes over large geographic areas. Operational Land Imager (OLI) images have been obtained of the North and South Islands of New Zealand for the March 2014 and 2015, with in-situ measurements of bio-optical properties and the water parameters of 18 stations. Satellite images of the date closest to the water sampling were used to statistically correlate the in-situ measurements with various combinations of Landsat 8 bands in order to develop algorithms that best describe those relationships and calculate accurately the aforementioned water quality components. Optimal models were selected based on statistical criteria and indices; finally, the predictive models of CDOM, TP and TN concentrations involving the combination of ultra-blue (B1), blue (B2) and green (B3) OLI bands of Landsat 8 satellite sensor were achieved. As a result of the validation process, CDOM absorption at 420 nm (R = 0.35), TP (R = 0.57) and TN (R = 0.71) concentrations were shown to be the most accurately estimated components, respectively. [ABSTRACT FROM AUTHOR]

Abstract We analyse recent morphological evolution of braiding rivers of disparate regions of the Earth to develop and address the hypothesis that braiding of rivers tends to be reduced by human presence and related activities. Firstly, through a large-scale literature survey we observe generalised paths of bed degradation, channel narrowing and shift towards single-thread configuration in braided reaches due to multiple anthropogenic stressors. Secondly, we select three rivers from different geographic contexts characterised by complementary anthropic stressors for a detailed analysis (the lower Waitaki River in New Zealand, the middle Piave River in Italy and the lower Dunajec River in Poland) which shows that these rivers have undergone very similar trajectories of morphological change. In previous works, these morphodynamic changes have been related to the alteration of the fundamental physical processes of braided rivers, due to anthropogenic changes in constraints and controls. Here, a closer analysis of these alterations shows that analogous morphological evolutionary trajectories can result from very different paths of causation, i.e., from different management causes and different alteration of physical processes. Through the use of pattern predictors we analyse observed morphological trajectories and potential for recovery. We highlight the role of different geographic contexts as sources of constraints and drivers to the river evolution, with reference both to the physical and human environment, showing that the observed similar trajectories are the product of different local conditions and characteristics. These observations have implications for river management and restorations. [ABSTRACT FROM AUTHOR]

Spatial regression models were used to predict yields (kg ha−1 yr−1) of nitrogen (N) and phosphorus (P) discharged from catchments throughout New Zealand under natural and current conditions. The models were derived using loads (kg yr−1) of TN, NO3-N, TP and DRP calculated for 592 river water quality monitoring sites. Anthropogenic increases in yields above natural levels were associated with the proportions of catchments occupied by the intensive agricultural land cover and were unevenly distributed across regions. Anthropogenic increases in national loads of TN, NO3-N, TP and DRP exported to the ocean were 74%, 159%, 48% and 18%, respectively. Increases in loads exported to the ocean varied considerably at smaller scales, with catchments having significant load increases between 4- and 26-fold for N and 6- to 9-fold for P. Predictions of yields and loads reported here have utility in the development of strategies to manage nutrients. [ABSTRACT FROM AUTHOR]

Escherichia coli concentrations in rivers are known to vary considerably. Much research has focused on storm events and the relationships between fecal microbe concentrations and flows. However, there is still considerable variability in microbial concentrations during base-flow conditions, and little research has been conducted to understand this short-term variability in rivers. We investigated the variability of E. coli concentrations in base flows at the time scales of minutes, hours, and days and compared this to variability from laboratory replication of the measurement methods. This was conducted in three different-sized rivers in both summer and winter seasons. Estimates of variability were analyzed using the coefficient of variation (CV). The variability at the minute time scale was 17%, compared with the laboratory replication variability of 15%. The CV then increased to approximately 32 and 60% at the hourly and daily time scales, respectively. There is strong evidence that both time scale (p < 0.001) and river (p < 0.001) significantly affect the variation in E. coli concentrations. The E. coli concentrations were higher in summer than winter, with a marked effect in the smallest stream, where at one site the concentrations were >2000 most probable number (mpn) 100 mL-1 in all summer samples. This variability of concentrations should be considered when interpreting the results from a one-off grab sample used to compare against water quality standards or for calibrating models. [ABSTRACT FROM AUTHOR]

The morphology and abundance of streams control the rates of hydraulic and biogeochemical exchange between streams, groundwater, and the atmosphere. In large river systems, the relationship between river width and abundance is fractal, such that narrow rivers are proportionally more common than wider rivers. However, in headwater systems, where many biogeochemical reactions are most rapid, the relationship between stream width and abundance is unknown. To constrain this uncertainty, we surveyed stream hydromorphology (wetted width and length) in several headwater stream networks across North America and New Zealand. Here, we find a strikingly consistent lognormal statistical distribution of stream width, including a characteristic most abundant stream width of 32 ± 7 cm independent of discharge or physiographic conditions. We propose a hydromorphic model that can be used to more accurately estimate the hydromorphology of streams, with significant impact on the understanding of the hydraulic, ecological, and biogeochemical functions of stream networks. [ABSTRACT FROM AUTHOR]

Background and aims: Soil development follows a predictable pattern of nutrient availability over hundreds to thousands of years, which drives changes in the composition and productivity of associated plant communities. However, our understanding of the influence of soil forming factors such as parent material on ecosystem development is constrained by the scarcity of appropriate chronosequences.Methods: We studied a chronosequence of coastal beach ridges at the mouth of the Arawhata River in Westland, New Zealand. The Arawhata chronosequence is formed of coarse gravel, so comparison with the nearby Haast chronosequence, formed of sand of the same lithological origin, presents a rare opportunity to isolate the influence of the physical nature of the parent material on ecosystem development.Results: Soils at Arawhata are Entisols with a shallow organic horizon over unconsolidated rounded coarse gravel, although old soils demonstrate features of podsolization and are morphologically similar to Spodosols. The fine earth fraction is a small proportion of the total volume, but increased in surface mineral soils from 5% to 20% over approximately 2000 years of pedogenesis. Corresponding soil phosphorus concentrations in the fine-earth fraction declined markedly, from >900 mg P kg−1 in young soils to 135 mg P kg−1 in older soils. However, on a profile basis the fine-earth fraction at Arawhata contained little phosphorus (10 g P m−2 on a 300-year-old soil) compared to the same-aged soil at Haast (253 g P m−2). Total nitrogen concentrations declined in surface mineral soil but not in the organic horizon, so nitrogen to phosphorus ratios increased with pedogenesis only in the organic horizon. Tree basal area at Arawhata was lower than on similar-aged soils at Haast, and the tree community on the youngest undisturbed ridge at Arawhata included Nothofagus menziesii, an ectomycorrhizal species that appears only on old infertile soils at Haast. Similarly, tree ferns were rare on the youngest soil at Arawhata and became more abundant along the chronosequence, yet at Haast they were most abundant on the phosphorus-rich young soils and decreased with soil age.Conclusions: Collectively, these results suggest that ecosystem development along the Arawhata chronosequence represents ‘backward succession’, because phosphorus stocks in the fine earth increase during the early stages of pedogenesis, even as mass-based concentrations decline. We conclude that the nature of the parent material plays a key role in determining the basal area and composition of tree communities during succession in the region, although further studies are required to isolate the relative importance of chemical and physical characteristics of the substrate. [ABSTRACT FROM AUTHOR]

Toxic benthic cyanobacterial proliferations, particularly of the genus Phormidium , are a major concern in many countries due to their increasing extent and severity. The aim of this study was to improve the current understanding of the dominant physicochemical variables associated with high Phormidium cover and toxin concentrations. Phormidium cover and anatoxin concentrations were assessed weekly for 30 weeks in eight predominately cobble-bed rivers in the South Island of New Zealand. Phormidium cover was highly variable both spatially (among and within sites) and temporally. Generalized additive mixed models (GAMMs) identified site, month of the year, conductivity and nutrient concentrations over the accrual period as significant variables associated with Phormidium cover. Cover was greatest under low to intermediate accrual dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) concentrations. Accrual nutrients had a strong, negative effect on cover at concentrations > 0.2 mg L − 1 DIN and 0.014 mg L − 1 DRP. The effect of flow was generally consistent across rivers, with cover accruing with time since the last flushing flow. Total anatoxins were detected at all eight study sites, at concentrations ranging from 0.008 to 662.5 mg kg − 1 dried weight. GAMMs predicted higher total anatoxin concentrations between November and February and during periods of accrual DRP < 0.02 mg L − 1 . This study suggests that multiple physicochemical variables may influence Phormidium proliferations and also evidenced large site-to-site variability. This result highlights a challenge from a management perspective, as it suggests that mitigation options are likely to be site-specific. [ABSTRACT FROM AUTHOR]

The Stream Ecological Valuation (SEV) provides water resource managers with a standardised measure of the ecological functions of a stream reach. Reaches are assessed in terms of their hydrological, biogeochemical, habitat and biological functions. SEV has been widely used for resource consent applications and monitoring in the Auckland region since 2006. End-user feedback, expert review, an accumulated database of SEV data and advances in the underlying science of stream functions prompted a review of the method in 2010. The revised SEV is simpler than the original, and based on slightly fewer functions and variables. Definitions of terms and field methods have been clarified, improving interpretability and reducing subjectivity and field time. In a field test of 19 sites, the revised SEV showed a similar mean (0.68 compared to 0.64) but greater range (0.77 compared to 0.40) of scores than the original, indicating little or no information loss but improved ability to discriminate between pristine and degraded streams. [ABSTRACT FROM PUBLISHER]

We present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the Kaipara River (New Zealand) under both low and high freshwater inputs and a neap tidal cycle. During the first day, we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence. During the second day, longitudinal surveys were conducted over a distance of 6 km centred on the upstream station. The data reveal a turbidity maximum in the form of a high-concentration ‘plug’ of suspended mud that was advected downstream on the ebbing tide past the upper (HB) measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide. The data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low-flow conditions. On the third day of measurements we repeated the experiments of the first day but later in the year, for a much higher freshwater flow. This interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended-sediment signals, including resuspension of local bed sediment, advection by the tidal current, settling of suspended sediment over a long timescale compared to the advection timescale, advection of longitudinal gradients in suspended sediment, and suppression of vertical mixing by density stratification of the water column. The level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed-point sensors. [ABSTRACT FROM AUTHOR]

A sound understanding of the effects of hydrogeological factors on loss, transport and transformation of farm nutrients is essential for predicting their impacts on ecosystem health of receiving waters. We assessed the potential of groundwater to attenuate nitrate through denitrification, and the distribution of this potential across the Tararua Groundwater Management Zone (GWMZ) in the Manawatu River catchment, New Zealand. We combined a number of methods in an unprecedented manner to confirm findings and obtain supporting evidence for the features that determine the subsurface denitrification characteristics. Our results showed that the denitrification characteristics of groundwater varied considerably in the Tararua GWMZ. The southern part of the Tararua GWMZ contained mainly oxic groundwater with low potential to denitrify, whereas the middle and northern parts of the Tararua GWMZ contained reduced groundwater with high denitrification potential. The hydrogeological features that influence denitrification potential in groundwater were identified as soil texture and drainage class, and the aquifer material or rock type. Low dissolved oxygen levels and nitrate concentrations were found in groundwater where the combinations of soil and rock types had poor drainage characteristics as opposed to higher concentrations in groundwater under well-drained soils and rocks (e.g. gravels). Intensive pastoral farming over well-drained soils and rocks showed high nitrate concentration in groundwater. This spatial variability in denitrification potential of groundwater offers a targeted management of nutrients runoff and leaching from pastoral lands to reduce their impacts on receiving surface waters. [ABSTRACT FROM AUTHOR]

Projected changes in 21st century climate are likely to impact water resources substantially, although much uncertainty remains as to the nature of such impacts. A relatively under-explored source of uncertainty is the method by which current and scenario evapotranspiration (ET) are estimated. Using the Waikaia River (New Zealand) as a case study, the influence of a potential ET (PET) method is investigated for a scenario of a 2°C increase in global mean temperature (the presumed threshold of 'dangerous' climate change). Six PET methods are investigated, with five general circulation models (GCMs) used to provide an indication of GCM uncertainty. The HBV-Light hydrological model is used to simulate river runoff. Uncertainty in scenario PET between methods is generally greater than between GCMs, but the reverse is found for runoff. The cause of the reduction in uncertainty from PET to runoff is unclear: the catchment is not water-limited during the summer half-year, indicating that it is not because of actual ET failing to reach the potential rate. Irrespective of the cause, these results stand in contrast to previous estimations of relatively high sensitivity of runoff projections to PET methods, indicating that further work is required to understand the controls on this source of uncertainty. [ABSTRACT FROM AUTHOR]

We sampled natural and reconstructed side-arms during different stages of hydrological connectivity with a large floodplain river in northern New Zealand, to determine whether re-establishment of connectivity would be an effective strategy for restoring plankton communities in former side-arms. Connectivity between side-arms and the river was moderated by water level and influenced flow rates and closure of inlets and outlets. Physicochemical conditions were more strongly related to the connectivity phase than to habitat type (river, natural or reconstructed side-arm), except during low connectivity when natural side-arms in particular were characterised by higher ammonium (NH4-N) and total phosphorus (P) concentrations, as well as specific conductivity. Dissolved reactive phosphorus (PO4-P), water temperature, conductivity and dissolved oxygen were identified as explanatory variables of phytoplankton and zooplankton community composition, which along with total nitrogen (phytoplankton) or total suspended solids (zooplankton) explained 44-52% of variation. Phytoplankton community composition and the abundance of several dominant or discriminatory taxa were affected by connectivity but not habitat type, whereas habitat and connectivity both had significant effects on zooplankton communities and abundances of the cladoceran Bosmina meridionalis. Significant interactions between connectivity and paired habitat types occurred for abundances of the diatom Asterionella, the cryptophyte Cryptomonas, the rotifer Synchaeta oblonga and cyclopoid copepods, reflecting differential responses to connectivity among habitats by these taxa. Overall, these results underscore the importance of hydrological connectivity between side-arms and rivers in moderating plankton community composition, and highlight unpredictable trajectories of community development and alternative transient states that can occur soon after side-arm reconnection. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]