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Stream bank erosion has been anecdotally identified as an important source of sediment in New Zealand catchments, however, there have been few attempts to quantify its contribution. Here we use a radionuclide-based sediment tracing approach to determine the relative contribution of stream bank- and hillslope-derived sediment within three catchments in the upper North Island of New Zealand. Both lithogenic (radium-226 and radium-228) and fallout radionuclides (caesium-137 and excess lead-210) were used to differentiate sediment derived from stream bank and hillslope sources. The relative contribution of stream banks and hillslopes to fluvially transported suspended sediment were predicted using a mixing model approach. Our results indicate that both fallout and lithogenic radionuclides provide good source differentiation. We demonstrate that stream bank erosion can contribute very high proportions of sediment within New Zealand catchments. We used independent assessments of bank erosion from each of the study catchments to support the sediment source fingerprinting results. Further work is required to determine the spatial and temporal variability of the contribution of sediment from stream banks. Information on the importance of different sediment sources is needed to target limited catchment rehabilitation resources where they will have the most impact. [ABSTRACT FROM AUTHOR]

Deep‐water sedimentation on active margins often entails complex sediment transport pathways through slope accommodation. Sedimentation in such settings is commonly differentiated into 'fill and spill' vs. 'tortuous corridor' models. To investigate the utility of these models in convergent settings 15,344 km2 of 3D seismic data is used to investigate sedimentation and erosion patterns across the Hikurangi subduction margin. A series of thrust‐bound trench‐slope basins, each tens of kilometres long by kilometres wide, have been diachronously forming, filling and deforming through the Neogene until today. Five primary input points delivered sediment to the basins along the studied part of the margin. Channels display both axial and transverse orientations, the run‐out lengths of which vary temporally. At various times, relatively coarse‐grained sediment was trapped in the interior basins, occasionally then to be cannibalised during landsliding or erosion of growing structures. At other times, coarse‐grained sediment was bypassed to distal basins or the trench. Multiple sediment input points and occasionally tortuous sediment dispersal corridors result in the evolution of convoluted depositional systems, often with similar styles of sedimentation occurring contemporaneously in proximal and distal basins, contrary to simple models of basin fill. A hierarchy of controls on sediment distribution can be distinguished. At the highest level, sediment distribution is controlled by external factors, for example, glacio‐eustacy and tectonics. At basin scale, the interaction of sedimentary systems with local relief (e.g. evolving seafloor structures and landslides) dictates the location and style of deposition. At the lowest level, autocyclic factors (e.g. flow response to earlier deposits) influence the spatiotemporal variation in erosion and sedimentation. The complex interplay of these factors dictates whether basins were filling, spilling or some combination at any point in time, whilst basins that were filled and spilled may subsequently resume filling due to changes in the bounding conditions. Hence simple use of 'fill and spill' or 'tortuous corridor' models to tectonically active margins is not advised. Furthermore, as sedimentation may influence structure growth, constraining the controls on sediment distribution may improve understanding of the broader evolution of convergent margins and their resource distribution. [ABSTRACT FROM AUTHOR]

Intertidal zones by definition are exposed to air at low tide, and the exposure duration can be weeks (e.g. during neap tides) depending on water level and bed elevation. Here we investigated the effect of varying exposure duration (6 h to 10 days) on intertidal mudflat erosion (measured using the EROMES device), where the effects of water content and biofilm biomass (using chlorophyll‐a content as a proxy, Chl‐a μg g−1) were taken into account. Sediments were collected between spring and summer (in October 2018, January 2019 and February 2019) from an intertidal site in the Firth of Thames, New Zealand. Longer exposure duration resulted in more stable sediments [higher erosion threshold (Ƭcr, N m−2) and lower erosion rate (ER, g m−2 s−1)]. After 10 days, exposure increased Ƭcr by 1.7 to 4.4 times and decreased ER by 11.6 to 21.5 times compared with 6 h of exposure. Chl‐a and water content changed with exposure duration and were significantly correlated with changes in Ƭcr and ER. The stability of sediments after two re‐submersion periods following exposure was also examined and showed that the stabilizing effect of exposure persisted even though water content had increased to non‐exposure levels. Re‐submersion was associated with an increase in Chl‐a content, which likely counteracted the destabilizing influence of increased water content. A site‐specific model, which included the interplay between evaporation and biofilm biomass, was developed to predict water content as a function of exposure duration. The modelled water content (WMod.) explained 98% of the observed variation in water content (WObs.). These results highlight how the exposure period can cause subtle changes to erosion regimes of sediments. An understanding of these effects (e.g. in sediment transport modelling) is critical to predicting the resilience of intertidal zones into the future, when sea‐level rise is believed to exacerbate erosion in low‐lying areas. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Overbank fine sediment deposition in the lower Oroua River leads to decreased flood capacity between stopbanks and increased flood risk. Identifying the dominant sources of erosion contributing to this deposited sediment can inform targeting of erosion management strategies most likely to reduce deposition. Sediment fingerprinting quantifies proportional contributions of catchment sources to suspended and deposited sediment transported downstream. Here, it is used to determine source contributions to recent overbank sediment deposits in the lower Oroua River, North Island, New Zealand. The analysis comprised six sources (channel bank; greywacke mountain range; hill subsurface; hill surface; mudstone; and, unconsolidated sand/silt) and two particle size fractions (< 63 µm and 125-300 µm) using bulk geochemical and radionuclide tracers. The dominant contributions to overbank deposition for the < 63 µm size fraction, which represents 42% of the deposits, are estimated to be from unconsolidated sand/silt (36 ± 8%) and hill subsurface (27 ± 8%), followed by channel bank (12 ± 12%), mudstone (11 ± 8%), hill surface (11 ± 6%), and greywacke mountain range (3 ± 4%). The overbank deposits for the 125-300 µm sediment fraction, which represents 28% of the deposits, show a different order of dominance with the largest contributions from hill subsurface (37 ± 7%) and channel bank (27 ± 10%), followed by mountain range (17 ± 10%), unconsolidated sand/silt (10 ± 7%), mudstone (7 ± 6%), and hill surface (2 ± 4%). Combining these two size fractions by their proportions in the overbank sediment provides an overall sediment source contribution of hill subsurface (31 or 38%), unconsolidated sand/silt (26 or 29%), channel bank (18%), mudstone (9 or 10%), mountain range (9 or 15 %), and hill surface (7 or 8%) (including/excluding channel bank, respectively). Hill subsurface, which represents shallow landslides in steep hill country, is a significant sediment source for both particle size ranges. Therefore, targeting erosion from this source is likely to have the most significant impact on sediment supply to overbank deposition. Differences in source contribution estimates for the two particle size fractions may reflect the composition of source material, variation in erosion and deposition processes, and discrimination challenges caused by channel bank sediment. These differences highlight the importance of accounting for particle size distributions when using sediment fingerprinting. Particle size also needs to be considered when implementing erosion management strategies because sources may have disproportionate downstream impacts depending on their particle size distribution and the aim of the mitigation. Specific sediment yield was estimated using sediment fingerprinting results distributed across a slope-weighted model of catchment outlet sediment yield. The highest specific sediment yield is from unconsolidated sand/silt (1,900 t km-2 yr-1), followed by mudstone (1,100 t km-2 yr-1) and hill subsurface (1,000 t km-2 yr-1). The presentation of sediment fingerprinting outputs can be improved by incorporating results into spatial modelling to produce maps of sediment yield. [ABSTRACT FROM AUTHOR]

The Soil and Water Assessment Tool (SWAT) model is widely used worldwide to model erosion, sediment transport and sediment yields from catchments. SWAT requires non-trivial compilation and pre-processing of Digital Elevation Models, land use, soils, topography and climate input data. This paper discusses GIS challenges applying SWAT to predict national-scale sediment loads and concentrations in New Zealand rivers and streams. It concerns compulsory input data in SWAT, with an emphasis on freely available datasets. Limitations with New Zealand input datasets for SWAT modelling are identified, in light of the international SWAT literature. Key challenges include sourcing and processing GIS input data on stream networks, slope, soils and land use representing relevant erosion processes. A particular challenge is geo-synching with digital stream networks developed for other classification and modelling systems. [ABSTRACT FROM AUTHOR]

Recent studies have shown that luminescence signals have potential beyond dating to trace earth surface processes and quantify sediment transport [1,2]. Here, we tested potential and limitation of using single-grain feldspar luminescence to study river landscape dynamics based on two approaches. First, we have tested the link between erosion dynamics and proxies derived from single-grain luminescence signal. For this, we measured feldspar post-infrared infrared luminescence signals (pIRIR) of modern fluvial sediments in 8 catchments in the Southern Alps of New Zealand (SANZ). We compared proxies derived from single-grain pIRIR with catchment-wide erosion rates obtained from 10Be cosmogenic nuclide concentrations in modern fluvial quartz grains. We found a high correlation between pIRIRbased proxies and 10Be erosion rates. It shows that pIRIR luminescence signal is reflecting the erosion and sediment fluxes of a catchment, and probably the landslide dynamics. Second, we also study how the pIRIR based proxies vary alongstream wide braided river system. We measured pIRIR in modern sediment from the Rakaia and Waimakariri rivers, draining the SANZ. We observe that the pIRIR based proxies vary in function of the downstream distance of sediments in the plain, with a better bleaching of the grains downstream. Based on a simple numerical model that considers the evolution of bleaching during transport and storage, we were able to estimate a mean transport length of the particles between 2 and 10 km and a mean resting time between 20 and 150 years. Our work illustrates the great potential of single-grain feldspar luminescence signal to study erosion dynamics and sediment transport processes in rivers. In the future we aim to carry out more tests with the perspective to use single-grain pIRIR luminescence signal as a new tool to quantify erosion and sediment transport parameters in a wide variety of rivers. [ABSTRACT FROM AUTHOR]

Improvements in riparian management, such as shrub/tree planting and livestock exclusion, are often assumed to result in reduced stream bank erosion and associated catchment sediment yield. Studies that quantify the effectiveness of riparian interventions aimed at reducing bank erosion and river sediment yields are, however, rare. This paper discusses how bank erosion processes can vary throughout catchments (with particular reference to their scale dependence) and hence how the effectiveness of different riparian interventions can be variable. The findings of known published accounts of the effectiveness of riparian management interventions for reducing stream bank erosion in New Zealand are also summarised. Only nine relevant studies were identified and most used qualitative or semi-quantitative analysis methods. Most studies compared stream banks in pasture catchments (with unlimited livestock access) with stream banks where livestock were excluded and riparian shrubs/trees were present. Many studies reported that managed stream banks were in better condition than unmanaged banks. The exclusion of livestock from riparian areas was generally reported as the principal factor in the measured improvements or differences. Only two studies specifically attributed reduced stream bank erosion to the presence of riparian vegetation. The dearth of research identified here highlights the need for further quantitative studies to determine the effectiveness of riparian management measures for reducing bank erosion. [ABSTRACT FROM AUTHOR]

Information on the effect of human-induced disturbances on sediment yields and sediment sources from New Zealand catchments is limited. This study analyses the sediment yields and event sediment dynamics for a 12-year period from two paired headwater catchments within the Waikato region. One catchment (Whakakai) was entirely vegetated in native forest, while the other (Mangaotama) was of mixed land use. Near the beginning of the study period, an integrated catchment management plan was implemented with the Mangaotama catchment. This plan included riparian planting, planting of degraded land in Pinus radiata and changes in stock management. Long-term in-stream turbidity data was calibrated with suspended sediment concentration (SSC) measurements and used to determine event and annual sediment yields. Hysteresis of the discharge (Q)–SSC relationship during events was used to infer differences in sediment sources. Event sediment yields within the Mangaotama catchment were similar to Whakakai catchment during small events (c. 1.01-year return period). For larger and less frequent events (>5-year return period), the event yields from Mangaotama exceeded Whakakai by about a factor of three, probably due to mass movement initiation. The specific sediment yield (±1σ) from Mangaotama (97±39 t/km2/year) was around 60% higher than Whakakai (60±22 t/km2/year). Clockwise hysteresis in the Q–SSC relationship dominated at Mangaotama, indicating the importance of near-channel sources. The persistence of near-channel source dominance throughout the study period (despite major land use changes) was attributed to switching of erosive mechanisms (i.e. from cattle-induced mechanical disturbance to subaerial processes). The prevalence of anti-clockwise hysteresis in the Q–SSC relationship of events at Whakakai suggests that hillslope sources are likely to be a principal source of eroded sediment. Such information provides valuable information that may help guide future decisions on the application of catchment rehabilitation measures. [ABSTRACT FROM AUTHOR]

This study relates connectivity of sediment sources to the channel to the high variability in short-term channel dynamics and stream bed stability in mountain stream reaches of the eastern Ruahine Range in New Zealand's North Island. Despite similar geographical and geological characteristics of the catchments upstream of these reaches, the morphological response to flood events ranges from almost none to frequent migration of the channel in a wide active channel zone. These dynamics are mainly driven by sediment supply from slopes and gullies over the last decade and from in-channel storage. The latter is particularly important where recent erosion on hill slopes has decreased. In the upper Waipawa catchment 22% of the area, dominantly gullies, could potentially supply sediment to the studied reach whereas in the Tamaki and Mangapuaka catchments, 3.7% and 1.9% of the area was affected by erosion and coupled to the respective study reaches. In contrast, in the Manawatu and Coppermine catchments this figure was less than 0.3%. In these potentially connected catchments where sediment supply is scarce, coupling between slopes and channel exerts an important control upon channel dynamics. In the connected catchments such as Tamaki, Mangapuaka and Waipawa, reach-to-reach connectivity is effective, with transport barriers such as large woody debris having a relatively short-term modulating effect on coarse sediment conveyance. [ABSTRACT FROM AUTHOR]

The Wairau River is one of the most engineered river systems in the Marlborough region. Historical river control works, particularly the blocking of the Opawa Breach and the subsequent construction of the Wairau Diversion, have had a significant impact on flow and sediment transport in the Lower Wairau. Currently, the Lower Wairau is aggrading as a result of an increase in sediment concentration and a decrease in the duration for which flows exceed the critical threshold to keep sediment entrained. These changes are attributable to the substantial reduction in flow in the Lower Wairau that is a result of over half the total Wairau flow now passing down the Wairau Diversion. During the previous period, following the blocking of the Opawa Breach up until the construction of the Wairau Diversion, the opposite occurred, with significant increases in flow down the Lower Wairau and widespread erosion. This paper reviews the geomorphic setting of the Wairau deposition zone and the impacts of the Opawa Breach and the Wairau Diversion on the Lower Wairau by examining historical cross-sectional data (1927 to present) and assessing the distinct flow and sediment transport regimes driving the aggradation and erosion. The effects on channel slope, width and bed level through the different regimes are also examined, along with their implications for sustainable management decisions. [ABSTRACT FROM AUTHOR]

The evolution of the coast of Hawke's Bay, New Zealand, and its erosion problems have been governed by multiple factors, including its tectonic setting with an earthquake in 1931 that altered land elevations along its shore, ranging from a 2-m uplift at its north end to 1-m subsidence at its south. Human environmental impacts have also been important, including the deforestation of the watersheds of rivers and the mining of gravel and sand from their channels, having decreased the sediment supplies to the beaches. Significant erosion has occurred at the south end of the Hawke's Bay shore, attributed in part to its subsidence in 1931, together with the net northward longshore transport of the beach sediment that is greater than the volumes being supplied by the rivers, the beach-sediment budget being significantly ''in the red.'' Midway along the Bay's shore, the construction of the Port of Napier's breakwater in 1887-90, extending seaward from the Buff Hill headland within that city, is interpreted by some investigators to have been the cause of the erosion experienced at Westshore, a development located immediately north of the breakwater. The assumption has been that this erosion was the result of the breakwater having blocked the northward longshore transport of sediment that formerly had bypassed Bluff Hill, this being an example of down-drift beach erosion. However, a reexamination of the history of the Port's development, the resulting shoreline changes, differences in beach gravels on the shores north and south of Bluff Hill, as well as other evidence, support the conclusion that the breakwater was not the cause of the erosion; instead, the impacts to Westshore at the time of the breakwater construction can be attributed primarily to a series of major storms, which produced erosion at a number of sites along the Hawke's Bay shore. There has been minimal erosion at Westshore since the completion of the breakwater a century ago, its northward extending arm acting to shelter the development from the waves of major storms. Investigations by coastal scientists and engineers of the Hawke's Bay coast, together with a comprehensive beachsurvey monitoring program, support its sound management based on an understanding of the multiple factors that are important to its responses during storms, and to its long-term changes. [ABSTRACT FROM AUTHOR]

Abstract: The last decade has witnessed the development of a series of cellular models that simulate the processes operating within river channels and drive their geomorphic evolution. Their proliferation can be partly attributed to the relative simplicity of cellular models and their ability to address some of the shortcomings of other numerical models. By using relaxed interpretations of the equations determining fluid flow, cellular models allow rapid solutions of water depths and velocities. These can then be used to drive (usually) conventional sediment transport relations to determine erosion and deposition and alter the channel form. The key advance of using these physically based yet simplified approaches is that they allow us to apply models to a range of spatial scales (1–100 km2) and time periods (1–100 years) that are especially relevant to contemporary management and fluvial studies. However, these approaches are not without their limitations and technical problems. This paper reviews the findings of nearly 10 years of research into modelling fluvial systems with cellular techniques, principally focusing on improvements in routing water and how fluvial erosion and deposition (including lateral erosion) are represented. These ideas are illustrated using sample simulations of the River Teifi, Wales. A detailed case study is then presented, demonstrating how cellular models can explore the interactions between vegetation and the morphological dynamics of the braided Waitaki River, New Zealand. Finally, difficulties associated with model validation and the problems, prospects and future issues important to the further development and application of these cellular fluvial models are outlined. [Copyright &y& Elsevier]

Field measurements of tidal current velocities are used to infer sediment transport characteristics in the lower section of a large, tidally dominated estuarine system at Whangarei Harbour, New Zealand. Recent (2002) developments at the harbour entrance included a 32.6 ha intertidal reclamation and a 31.8 ha dredged turning basin. Residual distance vectors indicate that the postdevelopment, large-scale pattern of sediment transport dynamics remains consistent. Minor, localised modification of transport potentials has been observed immediately adjacent to the developments, however. These modifications include a slight realignment of current flows near the reclamation wall and some leakage from a previously identified transport loop near the dredged basin. The potential for scour is identified along the eastern margin of the dredged basin, which could act to remove material moving downslope into the basin from its western edge. These data are consistent with numerical model results that predicted minimal consequences resulting from the developments. Lower harbour sediment dynamics are consistent with established patterns for tide-dominated inlets, with separation of the channel into areas of ebb and flood dominance, and typical transport patterns over the flood tidal delta. Broad-scale inlet geomorphology has been maintained, which is consistent with other dredged tide-dominated inlets. [ABSTRACT FROM AUTHOR]

Relationships between the surface area and age of alluvial deposits were used to estimate the residence time of alluvium in the 2205 km² Waipaoa River basin, New Zealand. The contemporary Waipaoa River is an efficient transporter of sediment to the continental shelf, but the basin has been characterized by rapid channel and valley aggradation in the historic period, and by extensive mid- to late Holocene alluvial storage in the lower reaches. The area-weighted mean age of alluvial deposits in the lower part of the river basin is ∼4400 yr. These deposits comprise terrace remnants isolated by downcutting, and Holocene to Recent sediments that are potentially remobilizable by the modern river. Even though the amount of storage is small relative to downstream transport, the majority of the potentially remobilizable alluvium is likely to remain in storage for >100 yr, and its half-life (time for 50 per cent removal) is >2000 yr. Within the confines of the floodplain, the apparent 'loss' of older deposits is due primarily to burial, but losses of the most recent deposits are due almost entirely to remobilization (30-40 per cent), with the remainder preserved in the alluvial record for at least 104 yr. Most of this sediment is likely to remain in storage until there is a shift to a degradational state. [ABSTRACT FROM AUTHOR]

The intractable obstacle to understanding cirque evolution has been the lack of an independent timescale against which to measure morphometric change. By using known rates of transport and uplift of the Southern Alps, combined with the oxygen isotope record contained in marine sediments, it is possible to estimate the duration of glacial occupancy of cirques in the Ben Ohau Range. The pattern is one of very limited glaciation of the southern range, with glacial occupancy increasing northwards. Map-based measurement of 18 morphometric properties in each of 92 cirques reveals systematic northward trends in cirque size, form and altitude, which are explained as reflections of increasing cirque 'age' (i.e. length of glacial occupancy). Thus, the validity of space-time substitution is demonstrated rather than assumed, and morphometric trends define evolutionary stages of cirque development. For cirques with up to c. 750 ka occupancy, rates of downcutting average 0.29 mm a-1 and headwall retreat 0.44 mm a-1. There is no indication of attainment of an equilibrium form within this time: both cirque size and form continue to evolve throughout, with few parameters showing a convergence of value with cirque 'age'. Some morphometric parameters perform much better than others: the k coefficient of the long-profile descriptor is the most powerful predictor of cirque 'age', and simple measures correlate better than derived measures (e.g. ratios of dimensions). The trends identified concur with some previously published ideas, but for the first time provide empirically quantified rates of change at an appropriate timescale. [ABSTRACT FROM AUTHOR]

A series of large rainfall simulator experiments was conducted in 2002 and 2003 on a small plot located in an experimental catchment in the North Island of New Zealand. These experiments measured both runoff and sediment transport under carefully controlled conditions. A physically based hydrological modelling system (SHETRAN) was then applied to reproduce the observed hydrographs and sedigraphs. SHETRAN uses physically based equations to represent flow and sediment transport, and two erodibility coefficients to model detachment of soil particles by raindrop erosion and overland flow erosion. The rate of raindrop erosion also depended on the amount of bare ground under the simulator; this was estimated before each experiment. These erodibility coefficients were calibrated systematically for summer and winter experiments separately, and lower values were obtained for the summer experiments. Earlier studies using small rainfall simulators in the vicinity of the plot also found the soil to be less erodible in summer and autumn. Limited validation of model parameters was carried out using results from a series of autumn experiments. The modelled suspended sediment load was also sensitive to parameters controlling the generation of runoff from the rainfall simulator plot; therefore, we found that accurate runoff predictions were important for the sediment predictions, especially from the experiments where the pasture cover was good and overland flow erosion was the dominant mechanism. The rainfall simulator experiments showed that the mass of suspended sediment increased post-grazing, and according to the model this was due to raindrop detachment. The results indicated that grazing cattle or sheep on steeply sloping hill-country paddocks should be carefully managed, especially in winter, to limit the transport of suspended sediment into watercourses. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper examines the behaviour of wind flow within a trough blowout on the Manawatu coastline, lower west coast, North Island, New Zealand. The trough blowout is 40 metres long, 20 metres wide and 7-8 metres deep. Virtually any wind flow approaching the blowout from a seaward, alongshore or slightly offshore direction in an arc of at least 200 degrees to the entrance orientation will be directed into the blowout. This happens for a number of reasons including (i) the relatively deep trough topography is an area of intense localised low pressure and winds are "sucked" into the blowout; (ii) topographic steering by the blowout erosional walls; (iii) local flow separation over the blowout wall crests and within the blowout during alongshore and offshore winds, and (iv) re-direction/deflection of low to moderate obliquity approach flows to seawards and over the foredune. This research indicates that winds from a wide range of approach directions will influence blowout flow, sand transport, erosion and deposition patterns, and evolution. It cannot be assumed that only "onshore" winds will transport sediment up a blowout. The implications for routine determination of sand transport by remote sensing in blowouts are significant. [ABSTRACT FROM AUTHOR]

Steeghs, L. and Healy, T., 2007. Tidal inlet configuration and bathymetric changes resulting from active sedimentation, Buffalo Bay, New Zealand. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 716 – 720. Gold Coast, Australia, ISSN 0749.0208 The Whitianga tidal inlet lies within Mercury Bay, on the northeast coast of New Zealand. The Whitianga Estuary harbours many recreational and commercial vessels. Terrestrial sediment inputs into the Whitianga Estuary catchment are high, due to the erodible tertiary sediments and the steep nature of the catchment, which suffers intense rainfall that results in high sediment influxes, much of which becomes deposited within the estuary. In recent years active shoals have appeared in Buffalo Bay and the inlet channel has noticeably shallowed, both of which inhibit the use of the inlet by boat traffic. The past and present bathymetry of Buffalo Bay is compared to establish changes in morphology and zones of accretion and erosion within Buffalo Bay. The outcomes of this research contribute to a larger research programme which aims to use numerical model simulations to identify sediment transport pathways and patterns within Buffalo Bay. This research will particularly provide information which can be used to calibrate a sediment transport model. Comparison of the Buffalo Bay bathymetries over the 38 year period from 1938 to 1979 and Whitianga inlet bathymetries between 1979 and 1995, show extensive accretion in the ebb discharge channel which is predominantly associated with the migration of the tidal inlet channel and growth of the ebb delta, and within the inner inlet area which are attributed to the growth of the flood delta system. [ABSTRACT FROM AUTHOR]