Integrating remotely acquired and field data to assess effects of setback levees on riparian and aquatic habitats in glacial‐melt water rivers

Setback levees, in which levees are reconstructed at a greater distance from a river channel, are a promising restoration technique particularly for alluvial rivers with broad floodplains where river‐floodplain connectivity is essential to ecological processes. Documenting the ecological outcomes of restoration activities is essential for assessing the comparative benefits of different restoration approaches and for justifying new restoration projects. Remote sensing of aquatic habitats offers one approach for comprehensive, objective documentation of river and floodplain habitats, but is difficult in glacial rivers because of high suspended‐sediment concentrations, braiding and a lack of large, well‐differentiated channel forms such as riffles and pools. Remote imagery and field surveys were used to assess the effects of recent and planned setback levees along the Puyallup River and, more generally, the application of multispectral imagery for classifying aquatic and riparian habitats in glacial‐melt water rivers. Airborne images were acquired with a horizontal ground resolution of 0.5 m in three spectral bands (0.545–0.555, 0.665–0.675 and 0.790–0.810 µm) spanning from green to near infrared (NIR) wavelengths. Field surveys identified river and floodplain habitat features and provided the basis for a comparative hydraulic analysis. Broad categories of aquatic habitat (smooth and rough water surface), exposed sediment (sand and boulder) and vegetated surfaces (herbaceous and deciduous shrub/forest) were classified accurately using the airborne images. Other categories [e.g. conifers, boulder, large woody debris (LWD)] and subdivisions of broad categories (e.g. riffles and runs) were not successfully classified either because these features did not form large patches that could be identified on the imagery or their spectral reflectances were not distinct from those of other habitat types. Airborne imagery was critical for assessing fine‐scale aquatic habitat heterogeneity including shallow, low‐velocity regions that were not feasible or practical to map in the field in many cases due to their widespread distribution, small size and poorly defined boundaries with other habitat types. At the reach‐scale, the setback levee affected the amount and distribution of riparian and aquatic habitats: (1) the area of all habitats was greater where levees had been set back and with relatively more vegetated floodplain habitat and relatively less exposed sediment and aquatic habitat, (2) where levees confine the river, less low‐velocity aquatic habitat is present over a range of flows with a higher degree of bed instability during high flows. As river restoration proceeds in the Pacific Northwest and elsewhere, remotely acquired imagery will be important for documenting its effects on the amount and distribution of aquatic and floodplain habitats, complimenting field data as a quantitative basis for evaluating project efficacy. Copyright © 2008 John Wiley & Sons, Ltd.     

Quantification of channel bed morphology in gravel-bed rivers using airborne multispectral imagery and aerial photography

The potential for mapping in-channel morphology within shallow gravel-bed rivers using airborne multispectral imagery and aerial photography is illustrated using a case study from the River Tummel, Scotland. The technique described relies on a good correlation between observed light reflectance levels from a water body and water depth. Measured water depths are regressed against reflectance levels derived from airborne multispectral imagery and black-and-white aerial photographs, to obtain equations that can be used for mapping channel bathymetry. The technique has a great deal of potential for wide-ranging applications, including detailed morphological surveys, assessing in-channel changes and mapping riverine habitats.© 1997 John Wiley & Sons, Ltd.     

ESTIMATION OF JUVENILE SALMON HABITAT IN PACIFIC RIM RIVERS USING MULTISCALAR REMOTE SENSING AND GEOSPATIAL ANALYSIS

We conducted a regional classification and analysis of riverine floodplain physical features that represent key attributes of salmon rearing habitats. Riverine habitat classifications, including floodplain area and river channel complexity, were derived at moderate (30 m) spatial resolution using multispectral Landsat imagery and global terrain data (90 m) encompassing over 3 400 000 km² and most North Pacific Rim (NPR) salmon rivers. Similar classifications were derived using finer scale (i.e. ≤ 2.4‐m resolution) remote sensing data over a smaller set of 31 regionally representative flood plains. A suite of physical habitat metrics (e.g. channel sinuosity, nodes, floodplain width) were derived from each dataset and used to assess the congruence between similar habitat features at the different spatial scales and to evaluate the utility of moderate scale geospatial data for determining abundance of selected juvenile salmon habitats relative to fine scale remote sensing measurements. The resulting habitat metrics corresponded favorably (p < 0.0001) between the moderate scale and the fine scale floodplain classifications; a subset of these metrics (channel nodes and maximum floodplain width) also were strong indicators (R² > 0.5, p < 0.0001) of floodplain habitats defined from the finer scale analysis. These relationships were used to estimate the abundance and distribution of three critical shallow water floodplain habitats for juvenile salmon (parafluvial and orthofluvial springs, and shallow shore) across the entire NPR domain. The resulting database provides a potential tool to evaluate and prioritize salmon conservation efforts both within individual river systems and across major catchments on the basis of physical habitat distribution and abundance. Copyright © 2011 John Wiley & Sons, Ltd.     

EFFECTS OF FLOW DYNAMICS ON THE AQUATIC‐TERRESTRIAL TRANSITION ZONE (ATTZ) OF LOWER MISSOURI RIVER SANDBARS WITH IMPLICATIONS FOR SELECTED BIOTA

Sandbars are an important aquatic terrestrial transition zone (ATTZ) in the active channel of rivers that provide a variety of habitat conditions for riverine biota. Channelization and flow regulation in many large rivers have diminished sandbar habitats and their rehabilitation is a priority. We developed sandbar‐specific models of discharge‐area relationships to determine how changes in flow regime affect the area of different habitat types within the submerged sandbar ATTZ (depth) and exposed sandbar ATTZ (elevation) for a representative sample of Lower Missouri River sandbars. We defined six different structural habitat types within the sandbar ATTZ based on depth or exposed elevation ranges that are important to different biota during at least part of their annual cycle for either survival or reproduction. Scenarios included the modelled natural flow regime, current managed flow regime and two environmental flow options, all modelled within the contemporary river active channel. Thirteen point and wing‐dike sandbars were evaluated under four different flow scenarios to explore the effects of flow regime on seasonal habitat availability for foraging of migratory shorebirds and wading birds, nesting of softshell turtles and nursery of riverine fishes. Managed flows provided more foraging habitat for shorebirds and wading birds and more nursery habitat for riverine fishes within the channelized reach sandbar ATTZ than the natural flow regime or modelled environmental flows. Reduced summer flows occurring under natural and environmental flow alternatives increased exposed sandbar nesting habitat for softshell turtle hatchling emergence. Results reveal how management of channelized and flow regulated large rivers could benefit from a modelling framework that couples hydrologic and geomorphic characteristics to predict habitat conditions for a variety of biota. Published in 2011 by John Wiley & Sons, Ltd.     

Applying instream flow incremental method for the spawning habitat protection of Chinese sturgeon (Acipenser sinensis)

The Chinese sturgeon, Acipenser sinensis, is an anadromous species that spawns in the Yangtze River and Pearl River of China. Its population has declined dramatically since the construction of the Gezhouba Dam (GD) in 1981 and then with the impoundment of the Three Gorges Dam (TGD) upstream of the GD in 2003. This paper presents a quantitative method based on the instream flow incremental method to explore the relationship between the fish spawning habitat and the operations of the GD and TGD, aiming to find a solution for conservation of the species. A two‐dimensional hydrodynamic model was built with the River2D to simulate the hydraulic behaviour of the stream below the GD. Habitat suitability index was determined by the biological data of the fish collected in the field. The two parts were then integrated through a geographical information system developed via ArcGIS to outline the fish habitat area variation with flows. The decision support system is applied to set up a habitat time series for validating the assumption that more habitats have the potential to support more fish. The fish habitat results for alternative instream flow schemes are then compared with one another for defining the optimal flow requirements and evaluating effects of reservoir operation alternatives in order to improve the operation management for the GD and TGD projects. The results show that the optimal flow for spawning of the fish is about 7000–13000 m³/s and the optimal inlets combination is where the inflow comes from two power plants. Copyright © 2009 John Wiley & Sons, Ltd.     

Regulated scouring in a sand-bed river for channel habitat maintenance: A Platte River waterfowl case study

The planned Narrows Reservoir on the South Platte River in Colorado is expected to jeopardize the maintenance of scarce instream waterfowl habitat 400 km downstream in Nebraska's Big Bend reach of the mainstem Platte River. Extreme shrinkage of wide, shallow channel habitat for sandhill and endangered whooping cranes in the Central Flyway has resulted from past reservoir development along the North and South Platte Rivers in Wyoming and Colorado. Proposals to counteract further shrinkage caused by streamflow regulation and depletion include controlled annual peak flows designed to scour the banks, sandbars, and emerging vegetation. The criteria for effective channel maintenance by streambed scouring are examined, as well as the principles involved. The feasibility of preserving the remaining habitat by releasing scouring flows from the Narrows Dam is found seriously lacking. Alternative mechanical measures, such as dredging the channel and bulldozing the tree cover, may prove more practical, although possibly unaffordable.     

Evaluating Uncertainty in Physical Habitat Modelling in a High‐Gradient Mountain Stream

Predictions of habitat‐based assessment methods that are used to determine instream flow requirements for aquatic biota are uncertain, but instream flow practitioners and managers often ignore those uncertainties. Two commonly recognized uncertainties arise from (i) estimating the way in which physical habitat within a river changes with discharge and (ii) the suitability of certain types of physical habitat for organisms. We explored how these sources of uncertainty affect confidence in the results of the British Columbia Instream Flow Methodology (BCIFM), which is a commonly used transect‐based habitat assessment tool for small‐scale water diversions. We calculated the chance of different magnitudes of habitat loss resulting from water diversion using a high‐gradient reach of the North Alouette River, BC, as a case study. We found that uncertainty in habitat suitability indices for juvenile rainbow trout generally dominated uncertainty in the results of the BCIFM when large (>15) numbers of transects were used. In contrast, with small numbers of transects, variation in physical habitat among sampled transects was the major source of uncertainty in the results of the BCIFM. Presentations of results of the BCIFM in terms of probabilities of different amounts of habitat loss for a given flow can help managers prescribe instream flow requirements based on their risk tolerance for fish habitat loss. Copyright © 2015 John Wiley & Sons, Ltd.     

Macroinvertebrate assemblages of littoral habitats in the Macquarie and Mersey rivers,Tasmania: Implications for the management of regulated rivers

Littoral habitats in large rivers are influenced to varying degrees by changes in discharge. Irrigation abstractions can increase the amount of habitat that would naturally be dewatered during low flow periods and therefore it is important to have some knowledge of the potential impact this may have on riverine macroinvertebrates. The macroinvertebrate assemblages of common littoral habitats in riffles, pools and runs in two reaches each of the Macquarie and Mersey Rivers, northern Tasmania, Australia were compared from samples collected during the low flow and irrigation season, between December 1991 and April 1992. The area under water of these habitats, riffle substrata, macrophyte beds and coarse woody debris, responded differently to changes in discharge. Within a reach, the same taxonomic groups often dominated the total number of macroinvertebrates for all habitats, but there were differences in the proportions contributed by these taxa to the different habitats. In general, taxa characteristic of slow-flowing or lentic habitats, such as ostracods and amphipods, were dominant in macrophyte beds in pools and runs, whereas taxa such as larval elmid beetles and hydropsychid caddisflies were dominant in riffles. A substantial component of the fauna from each habitat within a reach was unique to that habitat, but there was always a similar number of taxa common to all habitats. Classification and ordination grouped samples from both rivers firstly by habitat and secondly by month and reach. Total density and family richness of invertebrates differed by reach, habitat and month in both rivers, except for richness in the Mersey River where habitat was not significant. Differences in densities and numbers of invertebrate families among habitats were not consistent between reaches for each river. This study has highlighted the differences in macroinvertebrate assemblages of several littoral habitats in two lowland rivers in Tasmania. Differences in taxonomic composition, density and richness among habitats within reaches strongly imply the uniqueness of these habitats in terms of the invertebrate faunas that occupy them. We suggest that if maintenance of biotic diversity is an aim of instream flow management, water allocations that address low flows should place a high priority on the maintenance of a diversity of habitats.     

Evaluation of Steelhead Passage Flows Using Hydraulic Modeling on an Unregulated Coastal California River

Passage and habitat connectivity flows for steelhead Oncorhynchus mykiss through depth sensitive natural, low gradient, critical riffle sites were investigated in the unregulated Big Sur River, California. The River2D two‐dimensional hydraulic habitat model, along with quantitative passage metrics and species‐specific and lifestage‐specific depth criteria, were used to evaluate and compare predicted fish passage flows with flows derived by a traditional empirical critical riffle fish passage method. Passage flows were also compared with historical unimpaired natural hydrology patterns to assess the frequency and duration of suitable passage flows under the naturally variable flow regimes characteristic of Central California coastal rivers. A strong relationship (r² = 0.93) was observed between flows predicted by hydraulic modeling and flows identified by the empirical critical riffle method. River2D provided validation that the flows derived using the traditional critical riffle methodology provided for contiguous passable pathways of suitable hydraulic (depth and velocity) conditions through complex cobble‐dominated riffle sites. Furthermore, steelhead passage flows were spatially and temporally consistent between lagoon and upstream riffles for adults, and were generally indicative of a river system in equilibrium with a naturally variable flow regime and associated intact ecological processes. An analysis of 25 years of continuous flow data indicated sufficient flows for upstream passage by young‐of‐year and juvenile steelhead were produced between 37% and 100% and between 1% and 95% of the time, respectively. September and October are the most challenging months for natural flows to meet young‐of‐year and juvenile passage and habitat connectivity flows. Careful consideration of seasonal and interannual flow variability dynamics, therefore, are critical components of an effective flow management strategy for the maintenance and protection of passage and habitat connectivity flows between lagoon and upriver habitats. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Effects of Environmental Water Transfers on Stream Temperatures

Low streamflows and warm stream temperatures currently limit habitat and productivity of trout, including native Lahontan cutthroat trout in Nevada's Walker Basin. Environmental water transfers, which market water from willing sellers to instream uses, are evaluated to improve instream habitat. We use River Modelling System, an hourly, one‐dimensional hydrodynamic and water quality model, to estimate current and potential environmental water transfer effects on stream temperatures. Model runs simulate a range of environmental water transfers, from 0.14 to 1.41 cms, at diversions and reservoirs for wet year 2011 and dry year 2012. Results indicate that critically warm stream temperatures generally coincide with low flows, and thermal refugia exist in East Walker River, a tributary of the Walker River. Environmental water transfers reduce maximum stream temperatures by up to 3 °C in dry years and are more effective in dry years than wet years. This research suggests that environmental water transfers can enhance instream habitat by improving water quality as well as increasing instream flow. Copyright © 2015 John Wiley & Sons, Ltd.     

Recent habitat association and the historical decline of Notropis simus pecosensis

Small‐bodied, riverine minnows that historically characterized fish assemblages of Great Plains rivers in North America have declined because of river fragmentation, dewatering, river channel degradation, river salinization and nonnative species introductions. The Pecos bluntnose shiner Notropis simus pecosensis, a member of this guild, persists in one segment of the Pecos River, New Mexico, USA. We characterized habitat associations for the species at two spatial scales. In general, N. s. pecosensis associated with fluvial habitats, but velocity association depended on body size, with larger individuals using swifter habitats. All N. s. pecosensis associated with relatively low depths (3–51 cm), which were most abundant in sites with relatively wide river channels (>25 m), especially when discharge was between 0.5 and 4.0 m³ s^?1. The Pecos River sub‐segment that is occupied by the core population of N. s. pecosensis (V‐ii) had a unique combination of being buffered from direct dam effects by intervening segments and sub‐segments, high sub‐segment length, substantial sediment inputs from numerous uncontrolled tributaries, substantial base flow provided by irrigation return flows and groundwater inflows, high channel width in relation to discharge and low salinity. Although no unoccupied Pecos River segment appears to be suitable for N. s. pecosensis, habitat restoration opportunities exist within all occupied sub‐segments (V‐i, V‐ii and V‐iii) via base flow enhancement and river channel restoration. Restoration that offsets chronic effects of dams may be necessary to conserve the species. Restoration would also benefit other rare riverine minnows that coexist with N. s. pecosensis. Copyright © 2008 John Wiley & Sons, Ltd.     

Urbanization in a great plains river: Effects on fishes and food webs

Spatial variation of habitat and food web structure of the fish community was investigated at three reaches in the Kansas River, USA to determine if δ¹³C variability and δ¹⁵N values differ longitudinally and are related to urbanization and instream habitat. Fish and macroinvertebrates were collected at three river reaches in the Kansas River classified as the less urbanized reach (no urban in riparian zone; 40% grass islands and sand bars, braided channel), intermediate (14% riparian zone as urban; 22% grass islands and sand bars) and urbanized (59% of riparian zone as urban; 6% grass islands and sand bars, highly channelized) reaches in June 2006. The less urbanized reach had higher variability in δ¹³C than the intermediate and urbanized reaches, suggesting fish from these reaches utilized a variety of carbon sources. The δ¹⁵N also indicated that omnivorous and detritivorous fish species tended to consume prey at higher trophic levels in the less urbanized reach. Channelization and reduction of habitat related to urbanization may be linked to homogenization of instream habitat, which was related to river food webs. Published in 2009 by John Wiley & Sons, Ltd.     

Response of sandhill crane (Grus canadensis) riverine roosting habitat to changes in stage and sandbar morphology

Over the past century, flow regulation and vegetation encroachment have reduced active channel widths along the central Platte River, Nebraska. During the last two decades, an annual program of in‐channel vegetation management has been implemented to stabilize or expand active channel widths. Vegetation management practices are intended to enhance riverine habitats which include nocturnal roosting habitat for sandhill cranes. Evaluating the success of other management treatments such as streamflow modification requires an understanding of how flow shapes the sandbars in the river and how sandbar morphology interacts with flow to create crane habitat. These linkages were investigated along a 1‐km managed river reach by comparing the spatial pattern of riverine roosts and emergent sandbars identified with aerial infrared imagery to variables computed with a two‐dimensional hydraulic model. Nocturnal observations made multiple years showed that the area and patterns of riverine roosts and emergent sandbars and the densities of cranes within roosts changed with stage. Despite sandbar vegetation management, low flows were concentrated into incised channels rather than spread out over broad sandbars. The flow model was used to compute hydraulic variables for identical streamflows through two sandbar morphologies; one following a period of relatively high flow and the other following the low‐flow period. Compared with the simulation using the morphology from the antecedent high flow, the simulation using the morphology from the antecedent low flow produced a smaller quantity of available wetted area. These remote‐sensing observations and hydraulic simulations illustrate the importance of considering flow history when designing streamflows to manage in‐channel habitat for cranes. Published in 2008 by John Wiley & Sons, Ltd.     

Comparison of empirical and theoretical remote sensing based bathymetry models in river environments

Knowledge of underwater morphology is an essential component of many hydrological and environmental applications such as flood modelling and lotic habitat mapping. Remote sensing allows modelling of bathymetry at spatial scales that are impossible to achieve with traditional methods. However, the use of passive remote sensing for modelling water depth in fluvial environments remains a challenge. Different methods of computing bathymetry models based on remotely sensed imagery combined with ground measurements for calibration were investigated in order to produce a digital bathymetry model of a reach of the river Tana in Lapland. An empirical deep water correction model was evaluated together with theoretical hydraulically assisted bathymetry (HAB) models. The empirical model produced good results, correlating to known depths at 0.98 (R ² = 0.96) with a mean error of ±12.0 cm. It was demonstrated that usable levels of accuracy can be achieved with data that had previously been considered unsuitable for bathymetry modelling. Some issues related to channel substrate were addressed. The models based on hydraulic theory were tested for the first time outside the area they were developed in. Both models were found to be rather sensitive to certain assumptions, such as the channel friction parameter. The HAB models are able to produce relative depth estimates that can under certain conditions approach actual depths at accuracies similar to the empirical model. Extensive accuracy assessment was performed in order to evaluate the vertical as well as the spatial accuracy of the three models. Copyright © 2010 John Wiley & Sons, Ltd.     

MODELLING CHANGES IN SALMON HABITAT ASSOCIATED WITH RIVER CHANNEL RESTORATION AND FLOW‐INDUCED CHANNEL ALTERATIONS

The River2D two‐dimensional hydraulic and habitat model was used to simulate fall‐run Chinook salmon (Oncorhynchus tschawytscha) spawning and fry and juvenile rearing habitat of the first phase of a stream channel restoration project on Clear Creek, California. Habitat was simulated for a range of stream flows: (1) before restoration; (2) based on the restoration design; (3) immediately after restoration; and (4) after one and two large flow events. Hydraulic and structural data were collected for three sites before restoration, and prerestoration habitat was simulated. Habitat simulated for these sites was extrapolated to the prerestoration area based on habitat mapping. The topographical plan for the restoration was used to simulate the anticipated habitat after restoration. Although the restoration increased spawning habitat, it was less successful for rearing habitat. Channel changes associated with high‐flow events did not entirely negate the benefits of the restoration project. The results of this study point out the need for models that can simulate the changes in channel topography associated with high‐flow events, which could then be used to simulate habitat over time. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Flow and water temperature simulation for habitat restoration in the Shasta River,California

Low instream flows and high water temperatures are two factors limiting survival of native salmon in California's Shasta River. This study examines the potential to improve fish habitat conditions by better managing water quantity and quality using flow and water temperature simulation to evaluate potential restoration alternatives. This analysis provides a reasonable estimate of current and potential flows and temperatures for a representative dry year (2001) in the Shasta River, California. Results suggest restoring and protecting cool spring‐fed sources provides the most benefit for native salmon species from a broad range of restoration alternatives. Implementing a combination of restoration alternatives further improves instream habitat. Results also indicate that substituting higher quality water can sometimes benefit native species without increasing environmental water allocations. This study shows the importance of focusing on the limitations of specific river systems, rather than systematically increasing instream flow as a one size fits all restoration approach. Copyright © 2009 John Wiley & Sons, Ltd.     

Hyperspatial Remote Sensing of Channel Reach Morphology and Hydraulic Fish Habitat Using an Unmanned Aerial Vehicle (UAV): A First Assessment in the Context of River Research and Management

In this paper, we assess the capabilities of an unmanned/uninhabited aerial vehicle (UAV) to characterize the channel morphology and hydraulic habitat of a 1‐km reach of the Elbow River, Alberta, Canada, with the goal of identifying the advantages and challenges of this technology for river research and management. Using a small quadcopter UAV to acquire overlapping images and softcopy photogrammetry, we constructed a 5‐cm resolution orthomosaic image and digital elevation model (DEM). The orthomosaic was used to map the distribution of geomorphic and aquatic habitat features, including bathymetry, grain sizes, undercut banks, forested channel margins, and large wood. The DEM was used to initialize and run River2D, a two‐dimensional hydrodynamic model, and resulting depth and velocity distributions were combined with the mapped physical habitat features to produce refined estimates of available habitat in terms of weighted usable area. Based on 297 checkpoints, the vertical root‐mean‐squared error of the DEM was 8.8 cm in exposed areas and 11.9 cm in submerged areas following correction of the DEM for overprediction of elevations as a result of the refractive effects of water. Overall, we find several advantages of UAV‐based imagery including low cost, high efficiency, operational flexibility, high vertical accuracy, and centimetre‐scale resolution. We also identify some challenges, including vegetation obstructions of the ground surface, turbidity, which can limit bathymetry extraction, and an immature regulatory landscape, which may slow the adoption of this technology for operational measurements. However, by enabling dynamic linkages between geomorphic processes and aquatic habitat to be established, we believe that the advantages of UAVs make them ideally suited to river research and management. Copyright © 2014 John Wiley & Sons, Ltd.     

基于黄河鲤栖息地水文-生态响应关系的黄河下游生态流量研究

本研究采用野外生物监测、栖息地同步观测和实验室控制实验等技术手段,应用生物学、鱼类生态学、生态水力学、水文学等多学科理论,基于河流栖息地模拟法,研究了黄河下游指示物种黄河鲤生态学特性及其栖息生境与流速、水深、水温等水文水环境因子之间的关系,将径流条件与目标物种不同生长阶段生物学信息相结合,建立了代表物种繁殖期、越冬期栖息地适宜度指数,构建了黄河下游重点河段河流栖息地模型,建立了指示物种栖息地状况与河川径流条件定量响应关系,提出黄河下游花园口和利津断面繁殖期最小生态流量为300 m³/s和100 m³/s、适宜生态流量为600~700 m³/s和190~250 m³/s。该研究在水生生物习性及其与河川径流响应关系方面实现突破,解决了黄河生态需水研究中关键技术问题。     

Hydraulic geometry of New Zealand rivers and its use as a preliminary method of habitat assessment

‘Downstream’ hydraulic geometry relationships describe the variation of water depth, velocity, and water surface width between rivers of different size at a characteristic discharge, whereas ‘at-a-station’ geometry describes the variation of hydraulic geometry with discharge within a reach. The instream flow incremental methodology (IFIM) also predicts the variation in water depth and velocity with discharge at a reach scale, so that hydraulic geometry relationships can potentially be used as a preliminary method of habitat assessment. Hydraulic geometry relationships were calculated from instream habitat surveys of 73 New Zealand river reaches with mean flows varying from 0.6 to 204 m³ s⁻¹ and an average gradient of 0.0047. The exponents of both at-a-station and downstream hydraulic geometry relationships were within the range of values reported in other international studies, although the exponents indicated that New Zealand rivers tended to experience greater changes in velocity and less in depth than the international average, probably because of high average gradient. The frequency distributions of water depth and velocity were positively skewed in most rivers, and on average the modal velocity was 90% of the mean velocity and the modal depth was 80% of mean depth. The use of at-a-station hydraulic geometry relationships for instream habitat assessment was compared to depth and velocity predictions using habitat simulation techniques (IFIM) in two streams. Measurements of stream width and depth at five cross-sections at two calibration discharges were used to establish at-a-station hydraulic geometry relationships. These predicted mean depth and velocity within 8% of the reach average values of the IFIM surveys within the range of calibration discharges and within 10–15% of the IFIM reach average when extrapolated beyond the calibration discharges. Hydraulic geometry can be used to indicate whether hydraulic conditions approach a ‘threshold’ such as a minimum acceptable depth or velocity, thus predicating the need for more extensive habitat survey and analysis. © 1998 John Wiley & Sons, Ltd.     

Coping with hydrogeomorphic variations in a prairie river: resiliency in young‐of‐the‐year fishes

Young‐of‐the‐year (YOY) fish in sand‐bed rivers of grassland ecoregions frequently encounter dynamic habitat conditions produced by highly variable flow and resulting changes in geomorphic complexity of the river. How these vulnerable life history stages cope with changes in habitat condition and location is largely unknown. Therefore, we examined biodiversity and abundance of YOY fish (larvae and juveniles) at two spatiotemporal scales in response to dynamic flow conditions in the Kansas River. During the summer of 2004, we collected and identified 13 745 YOY fish from various moderate to zero‐flow habitats. Patterns of community structure through time were compared with the river's hydrogeomorphic characteristics, as determined by aerial photographs and measurements of instream flow rate and other water quality characteristics. Our initial hypothesis was that species richness and total YOY abundance would be maximized in larger and longer‐lived slackwater habitats. In contrast, our data suggest that dominant fishes in these highly dynamic ecosystems have adapted to hydrological variations with a more generalist lifestyle by either: (i) temporarily using higher flow habitats (except possibly the thalweg) to cope with intermittent loss of prime habitat; or (ii) extending spawning through favourable and demanding conditions. In contrast to current theories, our results suggest that conditions for YOY success are optimized in dynamic prairie rivers during the intermediate flows that maximize habitat heterogeneity and availability. Copyright © 2007 John Wiley & Sons, Ltd.