Assessing morphologic complexity and diversity in river systems using three‐dimensional asymmetry indices for bed elements,bedforms and bar units

Geomorphologists are becoming increasingly interested in assessing morphologic structure and the diversity and/or complexity in morphologic structure across multiple scales within river systems. Unfortunately, many of our existing tools/variables are unsuitable for this task because they do not work across multiple scales or with changing discharges. Asymmetry is one variable that can be used to either include or exclude variations in flow stage and that can be assessed across multiple scales. Existing asymmetry indices, however, are limited in scope and largely focus on only cross‐sectional form. This study examines three existing asymmetry indices in the cross‐stream and downstream planes (for cross‐sections and riffle or pools, respectively) and develops nine new asymmetry indices that incorporate vertical, cross‐stream and downstream asymmetry for bed elements (e.g. riffle crests, pool troughs, riffle entrance slope), bedforms (pools or riffles) and bar units (pool‐riffle sequences) to investigate the utility of asymmetry as a measure of morphologic structure and diversity in fluvial systems. These 12 indices are field tested on the Embarras River in East Central Illinois, USA. The results of this study indicate that there is considerable morphologic diversity in bed elements, bedforms and bar units both at bankfull and also with varying flow stage. This multi‐scale, multidimensional, multistage variability in morphologic structure highlights the complexity of natural river systems. The highly variable nature of fluvial form within a reach has important implications for river restoration and/or assessments of physical habitat or river health especially in instances where pools, riffles or pool‐riffle sequences are the focus of study. In general the most robust and useful combination of asymmetry indices for most applications includes A* and A_L1 for bed elements and bedforms and A_L3, A_W and A_H for bar units. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of a 2D hydrodynamic model to design of reach‐scale spawning gravel replenishment on the Mokelumne River,California

In‐stream chinook salmon (Oncorhynchus tschawytscha) spawning habitat in California's Central Valley has been degraded by minimal gravel recruitment due to river impoundment and historic gravel extraction. In a recent project marking a new direction for spawning habitat rehabilitation, 2450 m³ of gravel and several boulders were used to craft bars and chutes. To improve the design of future projects, a test was carried out in which a commercial modelling package was used to design and evaluate alternative gravel configurations in relation to the actual pre‐ and post‐project configurations. Tested scenarios included alternate bars, central braid, a combination of alternate bars and a braid, and a flat riffle with uniformly spaced boulders. All runs were compared for their spawning habitat value and for susceptibility to erosion. The flat riffle scenario produced the most total, high, and medium quality habitat, but would yield little habitat under flows deviating from the design discharge. Bar and braid scenarios were highly gravel efficient, with nearly 1 m² of habitat per 1 m³ of gravel added, and yielded large contiguous high quality habitat patches that were superior to the actual design. At near bankfull flow, negligible sediment entrainment was predicted for any scenario. Copyright © 2004 John Wiley & Sons, Ltd.     

Drift and retention of pelagic spawning minnow eggs in a regulated river

Long‐distance drift of eggs and larvae has been identified as a possible cause of downstream displacement and poor recruitment of the endangered Rio Grande silvery minnow (Hybognathus amarus; silvery minnow). Seven experiments were conducted using artificial eggs to estimate silvery minnow egg drift and retention in the Albuquerque and Isleta reaches of the regulated Middle Rio Grande, New Mexico, USA over a range of flows during expected spawning times. Bead retention varied by reach, discharge, and shape of the hydrograph. Highest retention (6.9 and 9.7% per km in the Albuquerque and Isleta reaches, respectively) occurred on the ascending limb of a high flow in areas where there was substantial floodplain inundation. Retention was maximized at different flows in each reach (97 and 140 m³/s, respectively), possibly associated with reach‐specific floodplain inundation thresholds. Lowest retention in each reach (2.1 and 1.7%, respectively) occurred on the descending limb of low and high flows, respectively. Of the silvery minnow eggs produced in the combined Albuquerque and Isleta reaches in 2005, 8–14% are predicted to have been retained in the Albuquerque Reach (67 km) and 49–83% in the Isleta Reach (86 km) based on the distribution of adult fish and measured bead retention rates. Although silvery minnow propagules are capable of drifting long distances, our study suggests that considerable retention occurs in the Middle Rio Grande. Habitat restoration to increase channel habitat complexity, and flow management to promote floodplain inundation should help to retain a greater proportion of propagules in upstream reaches. Copyright © 2010 John Wiley & Sons, Ltd.     

Monitoring the channel process of a stream restoration project in an urbanizing watershed: a case study of Kelley Creek,Oregon, USA

Pacific Northwest (PNW) streams in the United States were impacted by the 20th century development, when removal of instream structure and channelization degraded an aquatic habitat. The lower Kelley Creek in southeast Portland, USA was channelized during the 1930's Works Progress Administration (WPA) projects. Stream restoration reintroduced pool‐riffle sequences and heterogeneous substrates to protect salmonids while mitigating impacts from flooding. We investigated whether the restored pool‐riffle morphology changed substantially following effective discharge events. We examined channel forms for four reaches representing three time periods—pre‐development (two reference reaches), development and restoration. We conducted thalweg profiles, cross‐sections and pebble counts along the reaches to examine how channel geometry, residual pool dimensions and particle size distribution changed following effective discharge events. The effective discharge flows altered the restoration reach more substantially than the reference reaches. The restoration reach decreased in median particle size, and its cross‐sectional geometry aggraded near its margins. However, the residual pool morphology remained in equilibrium. Richardson Creek's reference reach degraded at the substrate level, while Kelley Creek's reference reach remained in equilibrium. The restoration reach's aggradation may have resulted from sedimentation along the nearby Johnson Creek. In contrast, Richardson Creek's degradation occurred as upstream land use may have augmented flows. Stream channels with low gradient pool‐riffle morphologies are ideal for salmonid spawning and rearing and should be protected and restored within urban corridors. The findings of our study suggest that the connectivity of streams and the dynamic fluvial geomorphology of stream channels should be considered for stream restoration projects in humid temperate climates. Copyright © 2008 John Wiley & Sons, Ltd.     

Evaluation of pool‐riffle naturalization structures on habitat complexity and the fish community in an urban Illinois stream

Urbanization and its associated stressors such as flow alteration, channel modification and poor water quality is a leading cause of ecological degradation to rivers and streams. Driven by public concern to address this issue, there has been a dramatic increase in urban restoration projects since 1990 using in‐stream structures. Attempts at restoring the ecological condition of urban streams using structures have produced varied results, but projects do not often meet planned ecological goals. A major challenge to improving the ecological health of urban streams is to better understand how to incorporate ecological assessments into a ‘restoration’ design framework with reasonable expectations for ecological recovery. A naturalization design framework was used in a project on a 0.62‐km reach of the North Branch of the Chicago River in Northbrook, Illinois. Initial surveys of channel morphology, habitat and biota identified poor pool‐riffle bed structure and fish biodiversity, which became the basis for research and development of a pool‐riffle structure specifically designed for constrained, low‐gradient channels. Habitat and fish surveys were conducted pre‐ and post‐construction. The project improved mesohabitat structure, and fish abundance, and biomass and diversity were greater for 2 years following construction (2002–2003) compared to 3 years prior to construction (1999–2001). However, the improved fish metrics were in the low range when compared to rural streams in the same ecoregion, and the fish community consisted primarily of tolerant, slow‐water species. Absent were intolerant and riffle dwelling species, such as insectivorous cyprinids and darters. Assessment of pre‐ and post‐project ecological condition and the use of species information provided a basis for ecologically informed design and expanded our understanding of the limitations to restoring urban streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Large wood addition for aquatic habitat rehabilitation in an incised,sand‐bed stream,Little Topashaw Creek,Mississippi

Large wood (LW) is a key component of stream habitats, and degraded streams often contain little wood relative to less‐impacted ones. Habitat rehabilitation and erosion control techniques that emphasize addition of natural wood in the form of individual elements or structures are increasingly popular. However, the efficacy of wood addition, especially in physically unstable, warmwater systems is not well established. The effects of habitat rehabilitation of Little Topashaw Creek, a sinuous, sand‐bed stream draining 37 km² in northwest Mississippi are described herein. The rehabilitation project consisted of placing 72 LW structures along eroding concave banks of a 2‐km reach and planting 4000 willow cuttings in sandbars opposite or adjacent to the LW structures. Response was measured by monitoring flow, channel geometry, physical aquatic habitat and fish populations in treated and untreated reaches for 2 years before and 4 years after rehabilitation. Initially, LW structures reduced high flow velocities at concave bank toes. Progressive failure of the LW structures and renewed erosion began during the second year after rehabilitation, with only 64% of the structures and about 10% of the willow plantings surviving for 3 years. Accordingly, long‐term changes in physical habitat attributable to rehabilitation were limited to an increase in LW density. Fish biomass increased in the treated reach, and species richness approximately doubled in all reaches after rehabilitation, suggesting the occurrence of some sort of stressful event prior to our study. Fish community composition shifted toward one typical of a lightly degraded reference site, but similar shifts occurred in the untreated reaches downstream, which had relatively high levels of naturally occurring LW. Large wood is a key component of sand‐bed stream ecosystems, but LW addition for rehabilitation should be limited to sites with more stable beds and conditions that foster rapid woody plant colonization of sediment deposits. Published in 2006 by John Wiley & Sons, Ltd.     

Flow and spawning habitat relationships for Dolly Varden: Understanding habitat–population dynamics in the Canadian Western Arctic

Northern form Dolly Varden (Salvelinus malma malma) have been designated as a species of Special Concern in Canada due to declines in population abundance and potential threats. Concern over detrimental effects of low flows on population abundance prompted research on how variability in discharge regimes influence habitat availability. Habitat suitability indices for prespawning and spawning adult anadromous Dolly Varden from two streams were integrated into a two‐dimensional hydrodynamic habitat model to assess the effect of flow variability on usable habitat. Regional hydrographs were used to identify an ecologically relevant range of flows that provided optimal spawning habitat for these populations and examine the relationship between abundance and discharge. Adults spawned in the tail end of pools at moderate water depths and water velocities, and used pebble‐ to cobble‐sized substrate for building redds; whereas, prespawning adults occupied deeper pools with moderate velocities and used cobble for cover. Model outputs showed that spawning habitat availability was optimized at flow rates between 1.6 and 3.0 m³/s and between 1.0 and 6.0 m³/s in Fish Hole Creek (FHC) and Little Fish Creek, respectively. A positive relationship between flows during the fall spawning period and abundance of the FHC population suggests that higher flows coinciding with optimal habitat availability may have contributed to positive recruitment. To strengthen and refine this habitat–population relationship for Dolly Varden in this area requires investigation of a broader suite of variables associated with environmental regimes and physical habitat in reaches used for spawning.     

Geomorphic comparison of two Atlantic coastal rivers: Toward an understanding of physical controls on Atlantic salmon habitat

River channel substrate size and mobility are important to Atlantic salmon spawning and rearing success. We compare morphology and bed sediment between two North American Atlantic coastal streams (Narraguagus River, Maine, USA and Jacquet River, New Brunswick, Canada). The watersheds have similar drainage areas and mean annual precipitation, but differing relief structure, channel longitudinal profiles and numbers of returning salmon. The lower‐relief Narraguagus River is segmented into steeper (gradient >0.002) and flatter reaches (gradient <0.0005). Flat reaches, including mainstem lakes, act as sediment sinks, preventing the continuity of downstream sediment transport. Based on field measurements, the Narraguagus River has a larger high‐flow width to depth ratio than the Jacquet River, but this difference is principally the result of outliers from low‐gradient channel reaches. Measurements of substrate grain size reveal finer river‐bed sediments on the Narraguagus River, however, Shields parameter calculations indicate that bed sediment should be mobile during high flows in both streams. We use the Shields equation to predict grain size based on channel slope, width and drainage area measured from digital elevation models (DEM) and aerial photographs. Predictions of median grain size agree with field measurements within a factor of 2 for 62–70% of the survey stations. We suggest ways that model misfits may provide opportunities to prioritize reach‐based restoration efforts. Based on expected grain size, we estimate 62% spawning and 68% rearing habitat along the length of the Narraguagus River, and 28 and 95% respectively on the Jacquet. Overall, glacial history and relief structure appear to be the first‐order controls on substrate grain size and habitat quality in these two rivers. Copyright © 2010 John Wiley & Sons, Ltd.     

Low‐Head Dam Impacts on Habitat and the Functional Composition of Fish Communities

The natural flow regime of many rivers in the USA has been impacted by anthropogenic structures. This loss of connectivity plays a role in shaping river ecosystems by altering physical habitat characteristics and shaping fish assemblages. Although the impacts of large dams on river systems are well documented, studies on the effects of low‐head dams using a functional guild approach have been fewer. We assessed river habitat quality and fish community structure at 12 sites on two rivers; the study sites included two sites below each dam, two sites in the pool above each dam and two sites upstream of the pool extent. Fish communities were sampled from 2012 to 2015 using a multi‐gear approach in spring and fall seasons. We aggregated fishes into habitat and reproductive guilds in order to ascertain dams' effects on groups of fishes that respond similarly to environmental variation. We found that habitat quality was significantly poorer in the artificial pools created above the dams than all other sampling sites. Fast riffle specialist taxa were most abundant in high‐quality riffle habitats farthest from the dams, while fast generalists and pelagophils were largely restricted to areas below the downstream‐most impoundment. Overall, these dams play a substantial role in shaping habitat, which impacts fish community composition on a functional level. Utilizing this functional approach enables us to mechanistically link the effects of impoundments to the structure of fish communities and form generalizations that can be applied to other systems. Copyright © 2017 John Wiley & Sons, Ltd.     

SPATIAL VARIABILITY IN SPAWNING HABITAT SELECTION BY CHINOOK SALMON (ONCORHYNCHUS TSHAWYTSCHA) IN A WILDERNESS RIVER

Chinook salmon (Oncorhynchus tshawytscha) survival during early life stages depends largely on spawning habitat selection by adults, which has been linked to biophysical stream variables (e.g. stream flow, velocity and substrate composition) as well as hyporheic exchange associated with riffle/pool and run/pool transitions. To examine how physical habitat variables influenced spawning habitat choice in one central Idaho (USA) wilderness stream, we used remote sensing techniques to classify and quantify the total amount of each aquatic habitat type present to assess how habitat quantity changed as stream order increased. Additionally, we measured physical habitat variables at each redd throughout the entire stream length for one spawning season to assess whether Chinook salmon selected for the same habitat parameters at varying spatial scales. Run, riffle and pool habitat types contributed similar proportions to the total area in both the upper and lower basins. However, ‘transitional zones’ (i.e. pool‐riffle and pool‐run transitions) accounted for 16% of the total area in the upper basin and only 4% in the lower. Redds were built in multiple habitat types in each of the three primary spawning locations, but transitional zones were chosen most frequently only in the upper basin. Significant differences in habitat variables were seen between spawning groups, with stream wetted width and velocity accounting for the majority of the variation. The techniques described here could be used to locate features that serve as indicators of potential spawning habitat, although caution should be exercised when extrapolating spawning habitat needs over large spatial extents. Copyright © 2013 John Wiley & Sons, Ltd.     

THRESHOLD FLOWS FOR THE BREAKDOWN OF SEASONALLY PERSISTENT THERMAL STRATIFICATION: SHOALHAVEN RIVER BELOW TALLOWA DAM,NEW SOUTH WALES,AUSTRALIA

Reduced mixing of deep pools attributable to river regulation and downstream flow suppression can lead to an increase in the magnitude, frequency and duration of thermal stratification in riverine pools over summer. This study monitored hourly temperature profiles with five thermistor loggers in a 15 m deep natural pool over 12 months from May 2005. Detailed bathymetric and topographic survey data and HEC‐RAS hydraulic modelling of layer Richardson numbers were used to extend thermistor observations of flow‐related stratification breakdown in this single deep pool to a 20 km long pool‐riffle dominated river reach below the dam. Reach‐wide breakdown of persistent thermal stratification in deep pools over spring and summer was likely to be achieved by a flow rate of 3000 ML day^?1. This flow rate approximates the long‐term mean annual natural flow (2860 ML day^?1) and the 16th flow duration percentile (mean daily flows equalled or exceeded for 16% of time), indicating that thermal stratification of the deepest pools in the Shoalhaven River is a common, natural phenomenon not solely attributable to river regulation. Should reasonably consistent hydraulic geometry relationships exist between low salinity rivers in similar climatic, hydrologic and geomorphic settings, then we suggest that the mean annual natural flow is likely to achieve widespread breakdown of thermal stratification across lengthy reaches of similar pool‐riffle sequence rivers elsewhere. Hourly mean wind speeds of up to 65 km h^?1 recorded at an automated weather station 25 km from the study site were found to suppress of the degree of thermal stratification in the study pool but did not achieve deep mixing of persistent seasonal thermoclines. Large, rapid and sustained air temperature decreases associated with the passage of cold fronts across southeastern Australia in summer were found to be more effective than wind and achieved mixing to depths of at least 4.2 m. Copyright © 2011 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.     

Periphyton and invertebrate response to wood placement in large pacific coastal rivers

Wood abundance in aquatic systems has been dramatically reduced compared to historical levels due to anthropogenic activities that led to wood removal and stream simplification. As a result, reintroduction of wood to aquatic systems is now a widely used and relatively well‐studied restoration technique for increasing habitat complexity. Although stream periphyton (biofilm) and invertebrates serve as food sources for a variety of predators including fish, birds and bats, data on how lower trophic levels respond to wood placement are relatively scarce. The purpose of this study was to test the hypothesis that periphyton biomass and aquatic invertebrate density were higher on Engineered Log Jams (ELJs) than on inorganic substrates in two large Pacific Northwest river systems. Among years and rivers, periphyton biomass and invertebrate densities were significantly higher on ELJs than on cobbles within the same reach. Invertebrate communities on ELJs were dominated by meiofauna (<500 µm), whereas cobbles were dominated by larger chironomids. We attribute these trophic level differences to substrate type, as we did not detect taxonomic differences between cobbles in reaches with and without ELJs. We show that adding wood to reaches with little or no naturally occurring wood increased overall habitat surface area and thereby the potential for increased productivity relative to reaches with low levels of wood. Finally, wood supports a unique community of invertebrates that are often overlooked in lotic system studies but may be contributing substantially to overall biological diversity. Published in 2009 by John Wiley & Sons, Ltd.     

Salmonid habitat modelling studies and their contribution to the development of an ecologically acceptable release policy for Kielder Reservoir,North‐east England

1. Kielder Reservoir regulates the Rivers North Tyne and Tyne. It provides a regular supply of water for downstream users, supports abstractions for a major water transfer scheme and provides hydroelectric power (HEP). Kielder's release regime typically alternates between a 1.3 m³ s⁻¹ compensation flow and 10–15 m³ s⁻¹ HEP releases of between 3 and 7 days in duration. Occasionally releases of up to 30 m³ s⁻¹ are made for the purpose of encouraging fish runs, for recreational events or to help in water quality management. The impacts of this release regime on Atlantic salmon (Salmo salar) and brown trout (S. trutta) habitat at four sites on the North Tyne are assessed and alternative regimes, designed to minimize impacts, are presented. 2. There is no evidence that the compensation flow results in extreme loss of instream habitat. A discharge of 1.3 m³ s⁻¹ ensures that water is maintained over most of the channel area at sites representative of upper, middle and lower sections of the North Tyne. This discharge lies above breaks in slope of respective site discharge versus wetted area curves; thus, disproportionate increases in discharge would be needed to increase wetted area. Simulations using the Physical Habitat Simulation System (PHABSIM) suggest that the compensation flow provides between 50% and 90% of the maximum possible weighted usable area (WUA) for juvenile (0+) salmonids. 3. During HEP releases, juvenile salmonid habitat (WUA) apparently falls to between 20 and 40% of site maxima. Newly emerged juvenile fish (March and April) are most affected by HEP releases because they are relatively small (25 mm in length) and water temperatures are relatively low at this time of year. During March and April, critical near‐bed displacement velocities for newly emerged fish may be exceeded across large parts (80%) of sites up to 8 km downstream from Kielder Reservoir; fish would either be displaced downstream or forced to relocate to flow refuge areas. 4. The availability of Atlantic salmon spawning habitat (WUA) at a key site is limited by the compensation flow; 1.3 m³ s⁻¹ provides approximately one third of the habitat available at the optimum discharge (4 m³ s⁻¹). At this site, a discharge of approximately 2 m³ s⁻¹ is needed to ensure most of the bed is inundated by water. Regulation has reduced the duration of flows exceeding 2 m³ s⁻¹ from 90 to 60% of the spawning season. 5. Simulations suggest that when discharge drops from 30 m³ s⁻¹ to the compensation flow, up to 60% of the optimum spawning habitat available at the former discharge may be left stranded (dry). This could potentially lead to egg or alevin mortality. 6. PHABSIM simulations suggest that increasing the compensation flow to 4 m³ s⁻¹ during the spawning period (November and December) is likely to increase the availability of suitable spawning habitat. Also, increasing the compensation flow to 2 m³ s⁻¹ during the incubation period (January through March) would minimize redd stranding. Reductions in the number of HEP releases in March and April would limit the extent to which newly emerged fish are exposed to velocities that potentially displace them. Such changes to the Kielder release regime may have implications for water resource management. While it is important that the biological instream flow requirements of the North Tyne are incorporated into the Kielder operating policy, these should be integrated along with the need for channel maintenance flows, downstream water supply abstractions and HEP generation, as well as for transfers of water to other catchments. Copyright © 2000 John Wiley & Sons, Ltd.     

Refinement of the wetted perimeter breakpoint method for setting cease‐to‐pump limits or minimum environmental flows

Analysis of inflections or breakpoints apparent in relationships between measures of wetted perimeter and discharge can be used to assist in the determination of minimum environmental flows for perennial rivers. This paper suggests refinements and provides an example application of the wetted perimeter method for the determination of cease‐to‐pump limits in a perennial, unregulated gravel‐bed river subject to increasing levels of surface water extraction. HEC‐GeoRAS modelling outputs of riffle wetted area are used to illustrate that the magnitude of the discharge selected to represent 100% habitat availability is of crucial importance to the breakpoint method. Because of the dependence of the technique on this assumption, we suggest that it is prudent to use an upper and lower limiting discharge based on an assessment of the degree of flow variability to develop a flow range around the zone of diminishing return in the wetted perimeter to discharge relationship. For rivers exhibiting a low degree of flow variability, the mean and median daily flows are likely to provide appropriate discharges for representation of 100% habitat availability. For perennial rivers with a higher degree of flow variability and considerable differences between the mean and median daily flows we suggest use of the 50th and 80th flow duration percentiles. Wetted perimeter breakpoint results are also influenced by the degree to which areas of non‐riffle habitat are included in the analysis. Inclusion of excessive pool areas can lead to significant reductions in resultant recommendations for cease‐to‐pump limits or minimum environmental flows. Integration of hydraulic model outputs with GIS for wetted perimeter analysis of riffles provides a useful, rapid, field‐based approach that can assist with determination of cease‐to‐pump limits or minimum environmental flows in gravel‐bed rivers. However, care is needed in its application and interpretation as the technique is prone to numerous subjective choices that have a substantial influence on results. Copyright © 2004 John Wiley & Sons, Ltd.     

LARGE WOODY DEBRIS IN URBAN STREAM CHANNELS: REDEFINING THE PROBLEM

Large woody debris (LWD) is an important ecological element in rivers and streams. Despite its importance, LWD is often removed from urban stream channels for flood control or road maintenance purposes, an approach with high economic and ecological costs and one that is largely unsuccessful. We propose an approach to conserve LWD in channels by modifying infrastructure (culverts and bridges) to allow LWD passage, maintaining aquatic habitat and reducing flooding and road maintenance costs. In Soquel Creek (California, USA), which has a history of LWD‐related flooding, we compared long‐term LWD management costs of historical, current and a LWD‐passing approach whereby infrastructure is enlarged to accommodate LWD passage downstream. We estimated costs of infrastructure replacement, programmatic flood control (LWD removal), LWD‐related flood damage and lost aquatic habitat. The amount of lost aquatic habitat was determined by comparing LWD loading (pieces m^?1) in Soquel Creek (0.007 pieces m^?1) to nearby unmanaged streams (0.054 to 0.106 pieces m^?1). Estimated costs of infrastructure able to pass LWD were nearly double that of historical costs but comparable to current costs. The LWD‐passing approach was comparable to removal approaches in the short term (1 to 50 years) but much less in the long term (51 to 100 years), as expenditures in infrastructure replacement to accommodate LWD yielded reductions in flooding costs and habitat loss. Given the urgency to maintain and restore aquatic habitat, the proposed approach may be broadly applicable. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of methods for analysing salmon habitat rehabilitation designs for regulated rivers

River restoration practices aiming to sustain wild salmonid populations have received considerable attention in the Unites States and abroad, as cumulative anthropogenic impacts have caused fish population declines. An accurate representation of local depth and velocity in designs of spatially complex riffle‐pool units is paramount for evaluating such practices, because these two variables constitute key instream habitat requirements and they can be used to predict channel stability. In this study, three models for predicting channel hydraulics—1D analytical, 1D numerical and 2D numerical—were compared for two theoretical spawning habitat rehabilitation (SHR) designs at two discharges to constrain the utility of these models for use in river restoration design evaluation. Hydraulic predictions from each method were used in the same physical habitat quality and sediment transport regime equations to determine how deviations propagated through those highly nonlinear functions to influence site assessments. The results showed that riffle‐pool hydraulics, sediment transport regime and physical habitat quality were very poorly estimated using the 1D analytical method. The 1D and 2D numerical models did capture characteristic longitudinal profiles in cross‐sectionally averaged variables. The deviation of both 1D approaches from the spatially distributed 2D model was found to be greatest at the low discharge for an oblique riffle crest with converging cross‐stream flow vectors. As decision making for river rehabilitation is dependent on methods used to evaluate designs, this analysis provides managers with an awareness of the limitations used in developing designs and recommendations using the tested methods. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydraulic habitat composition and diversity in rural and urban stream reaches of the North Carolina Piedmont (USA)

The differences between urban and rural streams regarding hydrological process, channel morphodynamics and ecosystem functioning have been highlighted by a number of studies in recent decades. The need to understand lotic ecosystem functioning in these environments at scales relevant to individual organisms has led to research focusing on hydraulic composition and structure over small areas of channel bed. In this study we map and analyze the hydraulic biotope composition of two urban and two rural stream reaches in the North Carolina (USA) Piedmont to determine if urban flow regimes and attendant channel modification processes might translate into important differences in hydraulic environment, and if so, what those differences are. Hydraulic biotope assemblages were found to vary only moderately in diversity per unit stream length between sites, but were distinctly different in composition. One important control on the differences between rural and urban streams was found to be the localized incision of urban channels into bedrock and saprolite. Resistant rock outcrop in the beds of urban streams creates rapid and riffle biotopes and long stretches of upstream pool habitat by impoundment. Urban reaches were found to be more homogeneous than rural reaches in hydraulic composition and were dominated by pools. Rural reaches, characterized by copious sandy alluvium in the bed, were dominated by runs or glides. Quantitative differentiation of biotopes based on four hydraulic indices generally yields coherent associations, although these may vary in content. Comparisons between hydraulic and biotic diversities suggest relationships favoring biotic functional group aggregation over species richness‐based indices of diversity. Because the majority of published hydraulic diversity analyses are based on coarse‐bed streams, further study of hydraulic diversity in streams with finer substrate is likely to be beneficial. Copyright © 2008 John Wiley & Sons, Ltd.     

Juvenile masu salmon in a regulated river

We examined the relationship between the physical environment and habitat use of juvenile masu salmon, Oncorhynchus masou, in the Nobori River in Hokkaido, Japan to provide a perspective for the conservation of fish habitat in regulated streams. The study was undertaken during the autumn and winter, with an emphasis on the hierarchy of three spatial scales: microhabitat, channel‐unit and reach scales. The microhabitat‐scale analysis indicated juvenile masu salmon preferred a midstream habitat type, with a greater depth (Avg. ± SD: 35.4 ± 14.2 cm) and high (43.4 ± 23.1 cm s^?1) and uniform current velocities during the autumn, and a channel margin habitat type with a moderate current (about 20 cm s^?1) and submerged cover during winter. In addition, different cover types have different roles in determining juvenile salmon distributions during winter. Grass cover had extremely high carrying capacities, whereas coarse substrate cover provided winter habitat for larger juvenile salmon. Channel‐unit scale analyses showed that abundance of juvenile salmon tended to be higher in pools than runs in the autumn through winter. Reach‐scale analysis showed that abundance and mean body length of juvenile salmon significantly differed between differently regulated reaches during winter, associated with the dominant cover type in each reach. This study demonstrated that the habitat conditions determining juvenile masu salmon distribution differ according to the season and scale of analysis. Therefore, for conservation of fish communities, it is important to evaluate and conserve or create fish habitats in regulated reaches, with a focus on the hierarchy of spatial scales and seasonal differences. Copyright © 2007 John Wiley & Sons, Ltd.     

SPATIAL VARIATION OF STRUCTURAL AND FUNCTIONAL INDICATORS IN A LARGE NEW ZEALAND RIVER

The ecological responses of large rivers to human pressure can be assessed at multiple scales using a variety of indicators, but little is known about how the responses of ecological indicators vary over small spatial scales. We sampled phytoplankton, zooplankton and macroinvertebrates and measured river metabolism and cotton strip breakdown rates (loss in tensile strength) in contrasting habitats along a 21‐km urban‐industrial reach on a constrained section of the Waikato River, New Zealand's longest river. Rates of gross primary production (2.8–7.8 g O₂/m²/d) and ecosystem respiration (3.5–12.7 g O₂/m²/d) did not differ consistently between near‐shore (2–3 m from river side) and far‐shore (ca. 10 m from side) locations, urban and industrial reaches or between autumn and spring sampling occasions. Rates of cotton decay (?k) ranged from 0.014 to 0.112 per day and were typically faster at far‐shore locations and in the section of river receiving industrial inputs, but slower in spring compared with autumn. Nonmetric multidimensional scaling analysis of phytoplankton and zooplankton data did not reveal spatial patterns relating to pressure or location (embayment, edge, mid‐river). However, the macroinvertebrate ordination suggested distinct communities for the mid‐river benthos compared with near‐shore communities and a distinction between sites in the urban reach and the industrial reach. Our results suggest that large‐river macroinvertebrate communities and cotton decay rates can be influenced to varying degrees by reach‐scale pressures and local habitat conditions. Monitoring designs in spatially complex rivers should account for habitat heterogeneity that can lead to differences in structural and functional indicator responses. Copyright © 2012 John Wiley & Sons, Ltd.