Physical modelling of pool and weir fishways with rock weirs

The flow characteristics of pool and weir fishways with rock weirs were studied through physical modelling. Detailed flow measurements were obtained using an acoustic Doppler velocimeter to understand how weir geometry, discharge, and bed slope affect flow patterns, velocity, turbulence kinetic energy, turbulence intensity, and Reynolds shear stresses in the fishway. The weir geometries used in this study are similar to those typically used for river restoration projects. The use of a V‐shaped rock weir was found to reduce the mean streamwise velocity in the pools by about 20% but more than double the maximum velocity magnitude. Two stage–discharge relationships were developed using the standard weir equation and a modified discharge coefficient to account for both flow over the weir and orifice flow through the base of the weir. The use of V‐shaped rock weirs has the potential to offer significant advantages in assisting multispecies fish migration. The results of this study can be applied to the hydrotechnical design of pool and weir fishways with rock weirs and for river restoration projects.     

Performance of a prototype surface collector for juvenile salmonids at Bonneville Dam's first powerhouse on the Columbia River,Oregon

During April–July 2000, we radio‐tagged and released juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) to evaluate a prototype surface flow bypass at Bonneville Dam on the Columbia River. The mock bypass, called a prototype surface collector (PSC), had six vertical slot entrances that were each 6 m wide and 12 m deep. The PSC was retrofitted to the upstream face of Bonneville Dam's First Powerhouse. Our objectives were to: (1) assess species‐specific differences in movement patterns and behaviour of fish within 6 m of the face of the PSC, (2) estimate the efficiency and effectiveness of the PSC and (3) evaluate factors affecting the performance of the PSC. We found that 60–72% of the fish, depending on species, detected within 6 m of the PSC entered it. Of the fish that passed the First Powerhouse at turbines 1–6, 79–83% entered the PSC. Diel period was a significant contributor to PSC performance for all species, and day of year was a significant contributor to PSC performance for subyearling Chinook salmon. The PSC was twice as effective (%fish/%flow) as the spillway, passing 2.5:1 steelhead and subyearling Chinook salmon and 2.4:1 yearling Chinook salmon per unit of water. If fully implemented, the PSC would increase the percentage of fish that pass the First Powerhouse through non‐turbine routes from 65–77% (without the PSC) to 76–85% (with the PSC), depending on species. Published in 2008 by John Wiley & Sons, Ltd.     

Hydraulic and turbulence structure of triangular labyrinth weir‐pool fishways

Detailed laboratory experiments were conducted to analyze and evaluate a new weir‐pool fishway design. The new design consisted of a series of one‐cycle, triangular labyrinth weirs that formed a weir‐pool fish passage. Hydraulic characteristics of flow over the proposed fishway were examined for three pool lengths and a wide range of discharges. It was found that the new design had superior discharge capacity over classical weir‐pool fishways in plunging flow regime. Spatial and point analyses of measured velocity data were carried out to understand turbulence structure and distribution of turbulent flow. The power dissipation per unit volume was calculated for different tests, and it varied nonlinearly with discharge. Based on the limiting criteria for energy dissipation rates of different fish species, the maximum operating discharge for different slopes was extracted for the proposed design. The model‐to‐prototype scaling analysis was performed based on the recommended scaling factor of three for prototype. It was found that the approach velocity of prototype increased by 12%, whereas the total discharge increased by more than five times. The contour plots of time‐averaged velocity indicated that the proposed fishways offered diversified flow fields without exceeding the maximum velocity above the standards recommended for weir‐pool fishways. To study the size, direction, and periodicity of turbulent eddies, magnitude of the most energetic eddy and dominant frequencies were extracted using power spectrum analysis. Quadrant analysis consistently showed that Reynolds shear stresses in ejection and sweep events became weaker as the pool length increased. The small region of strong ejection and sweep events evidenced a good resting place for fish migration, whereas the generated turbulence was large enough to attract fish species to use the passage.     

Body size,experience, and sex do matter: Multiyear study shows improved passage rates for alewife (Alosa pseudoharengus) through small‐scale Denil and pool‐and‐weir fishways

Alewife (Alosa pseudoharengus) passage through 3 fishways was assessed during the 2013–2016 spawning runs in 3 rivers of the Isthmus of Chignecto, Canada. From April 24 to June 10, 5,423 alewife with a mean ± SD fork length of 227 ± 18 mm were tagged with passive integrated transponders. During their tagging year, approximately half of individuals (40% to 64%) went undetected whereas those detected used fishways from April 16 to July 8. Detected alewife were significantly longer than those undetected. Attraction rates to fishway entrances in 2015 and 2016 ranged from 85% to 98%. Annual fishway passage rates for pooled fish tagged that year and returnees, varied from 64% to 97% for 2 Denil style fishways. A pool‐and‐weir fishway that was dysfunctional (2013), repaired (2014), and replaced (2015–2016) yielded 0.5%, 25%, 60%, and 73% annual pooled passage rates, respectively. Larger individuals, previously tagged returnees, and males compared to females of a similar size had higher passage success suggesting some fishways may apply population‐level selective pressures. Alewife passage rates related to fishway style, design, and proper function, with greater passage for the 2 Denils than the pool‐and‐weir fishway in our study. Regular structural maintenance and fish passage reviews are essential management considerations to ensure fishway functioning and river connectivity. Replacement of a fishway with poor fish passage may be the best option to improve passage rates. Future research should address the effects of multiple anthropogenic instream obstructions, environmental variables, negative sublethal post‐tagging effects, and the importance of returnees on fish passage rates in fishways.     

Numerical study of the effect of a V-shaped weir on turbulence characteristics and velocity in V-weir fishways

Fishways are used to allow fish to migrate around water infrastructure, whether movements are in the upstream or downstream direction. Hydrodynamic conditions within various fishways, including turbulence levels, are important for successful fish passage. A numerical hydrodynamic study was conducted for V-weir fishways, which assist species migrating upstream. The variables investigated included, fishway slopes of 4%, 7%, and 10%, relative spacing between weirs of 1.3, 2.6, and 4 (D/L, ratio of distance between weirs to pool width), and weir angles of 22.5°, 40°, and 60°. Turbulence characteristics, including turbulent kinetic energy (TKE), Reynolds shear stress (RSS), turbulent intensity (TI), turbulent dissipation (TD), as well as maximum water velocity, were examined by CFD (computational fluid dynamics) simulation using a re-normalized group or RNG turbulence model. The CFD was calibrated with flow measurements made in a physical model of the V-weir fishway in laboratory control experiments. Based on inferences from fish passage literature, the results showed that, in the range of parameters studied, a weir angle of 22.5^°, a slope of 10% and relative spacing between weirs of 1.3 and 2.6 were assessed as offering the best potential performance for several species and sizes of fish, while the latter has lower cost as it requires fewer weirs. The V-weir fishway may be adaptable for smaller fish.     

An assessment of the effectiveness of a vertical‐slot fishway for non‐salmonid fish at a tidal barrier on a large tropical/subtropical river

Fishways for salmon in temperate rivers have often been successful, but salmonid‐type fishways for non‐salmonid species in tropical and subtropical rivers have frequently failed. This study assessed the effectiveness of modifying a salmonid‐type pool‐and‐weir fishway into a vertical‐slot design on a tidal barrage on the subtropical Fitzroy River, in Queensland, north‐eastern Australia. In 38 paired samples of the top and bottom of the fishway, over 16 months, 29 fish species and over 23 000 fish were collected at a maximum rate of 3400 per day. This study shows much greater potential for success with a vertical‐slot fishway as relatively few fish negotiated the original pool‐and‐weir design. Common species using the vertical‐slot fishway included blue‐catfish (Arius graeffei [Ariidae]), bony herring (Nematalosa erebi [Clupeidae]), striped mullet (Mugil cephalus [Mugilidae]), barramundi (Lates calcarifer [Centropomidae]), and long‐finned eels (Anguilla reinhardtii [Anguillidae]). Freshwater shrimp (Macrobrachium australiense [Palaemonidae]), juvenile crabs (Varuna litterata [Grapsidae]) and long‐finned elvers did not ascend the full length of the fishway and specific fishways for these species are recommended. Fish between 25 and 640 mm in length ascended the fishway, although the passage of smaller size classes of immature fish was restricted and this may be important for the sustainability of these migratory populations. The barramundi (200–640 mm) which ascended the fishway were all immature fish. However, during a period of low river flows enlarging the width of the vertical‐slot from 0.15 to 0.45 m only encouraged a small number of larger fish (890 mm maximum length) to enter. The strong diel movement patterns of many species will need to be considered in future fishway design. Blue‐catfish could ascend the fishway in 2 h, but many fish remained in the fishway and this behaviour may cause crowding and a reduction in fishway capacity. Further work is needed to assess the proportion of fish finding the fishway entrance. However, the findings suggest that vertical‐slot fishways with lower water velocities and turbulence than salmonid fishways have great potential to pass the diverse migratory fish fauna of subtropical and tropical rivers. Copyright © 1999 John Wiley & Sons, Ltd.     

Dam Operations Affect Route‐specific Passage and Survival of Juvenile Chinook Salmon at a Main‐stem Diversion dam

Diversion dams can negatively affect emigrating juvenile salmon populations because fish must pass through the impounded river created by the dam, negotiate a passage route at the dam and then emigrate through a riverine reach that has been affected by reduced river discharge. To quantify the effects of a main‐stem diversion dam on juvenile Chinook salmon in the Yakima River, Washington, USA, we used radio telemetry to understand how dam operations and river discharge in the 18‐km reach downstream of the dam affected route‐specific passage and survival. We found evidence of direct mortality associated with dam passage and indirect mortality associated with migration through the reach below the dam. Survival of fish passing over a surface spill gate (the west gate) was positively related to river discharge, and survival was similar for fish released below the dam, suggesting that passage via this route caused little additional mortality. However, survival of fish that passed under a sub‐surface spill gate (the east gate) was considerably lower than survival of fish released downstream of the dam, with the difference in survival decreasing as river discharge increased. The probability of fish passing the dam via three available routes was strongly influenced by dam operations, with passage through the juvenile fish bypass and the east gate increasing with discharge through those routes. By simulating daily passage and route‐specific survival, we show that variation in total survival is driven by river discharge and moderated by the proportion of fish passing through low‐survival or high‐survival passage routes. Copyright © 2016 John Wiley & Sons, Ltd.     

HIGH‐HEAD DAMS AFFECT DOWNSTREAM FISH PASSAGE TIMING AND SURVIVAL IN THE MIDDLE FORK WILLAMETTE RIVER

Many high‐head dams in Oregon's Willamette River basin were constructed without fish passage facilities for downstream migrants. Instead, fish pass dams via hydroelectric turbines, surface spillways or deep‐water regulating outlets. The availability of these routes varies seasonally with dam operations and reservoir depth, which can fluctuate by tens of meters. To assess how dam and reservoir operations affect fish movement timing and survival, we used rotary screw traps below three Willamette basin dams and at two riverine sites above reservoirs. Traps were operated 2950 days over 8 years, and >195 000 fish were collected. Samples above reservoirs were primarily native salmonids (Oncorhynchus spp.), daces (Rhinichthys spp.) and sculpins (Cottus spp.), while those below dams were often dominated by non‐native Centrarchidae. Capture rates at riverine sites were highest from late winter to early summer, coincident with juvenile Chinook salmon emigration. Conversely, collection below dams was largely restricted to late fall and winter when reservoirs were drawn down to annual lows and discharge was high. We hypothesize that winter operations facilitated fish access to dam turbines and regulating outlets, whereas spring–summer operations entrapped fish in reservoirs and restricted volitional downstream passage. Total fish mortality was ≤2% at riverine sites and was 36–69% below dams. Estimates were highest for non‐native species and juvenile Chinook salmon. Fatal injuries were consistent with traumas related to pressure, shear and contact and there were size‐related and morphology‐related risk differences. Mitigation opportunities include fish bypass system development, retrofits for existing routes and seasonally appropriate reservoir draw down to allow fish passage. Copyright © 2012 John Wiley & Sons, Ltd.     

The behaviour of Pacific salmonid smolts during passage over two experimental weirs under light and dark conditions

Little is known of how fish respond to the hydraulic environment associated with diversion or bypass structures at hydroelectric power installations. To address this lack of knowledge, this paper presents results from a study to assess how three species of Pacific salmonid smolt (Oncorhynchus spp.) responded to distinct gradients of velocity and depth associated with two submerged weirs as they passed through an experimental flume at McNary Dam (Columbia River, USA) under illuminated and dark conditions. Migrating smolts entered one of two available treatment channels as coherent schools from which individuals would either disassociate from the group and pass over the weirs, or would reject them by swimming upstream. Alternatively, fish maintained position at the upstream end of the flume by swimming into the flow. The response of smolts to velocity and depth gradient and light condition varied between species, and route of passage was influenced by fork length. Initial channel selection and school size was not influenced by weir type, although schools resided longer within the short‐weir channel. The majority of smolts (70%) entered the treatment channels facing downstream (negative rheotaxis), but switched orientation at the crests of the weirs. This switch in orientation occurred farther downstream in the short‐weir treatment and for the largest smolts. The variation in response of different species of smolts to hydraulic gradients has important implications for the design of screening mechanisms used at hydroelectric power installations to divert migrant juvenile salmonids. Published in 2006 by John Wiley & Sons, Ltd.     

Evaluation of Central Valley Spring‐Run Chinook Salmon Passage Through Lower Butte Creek Using Hydraulic Modelling Techniques

River2D was used to develop a hydraulic model of an upstream passage impediment for adult spring‐run Chinook salmon (Oncorhynchus tshawytscha) on Butte Creek, Tehama County, California. Topographic data were collected by using a total station, survey‐grade real‐time kinematic global positioning system, and terrestrial light detection and ranging. Stage‐discharge relationships were developed at the upstream and downstream ends of the site to use as boundary conditions and to calibrate the 2D model. A pressure transducer was installed at the downstream boundary of the site to provide a time series of flow and water temperatures. Parameters of the hydraulic model were examined to assess upstream passage including minimum thalweg depth along the least width‐limiting pathway, velocity, and water surface elevation above and below a jump, and flow partitioning between a split in the main flow paths through the site. The results of the River2D model were used to identify flow levels that met the minimum depth and width thresholds needed for adult spring‐run Chinook salmon (SRCS) to migrate upstream through the study reach. A minimum passage depth criterion of 0.27 m was used for adult SRCS. Site‐specific passage width criteria were derived from the literature for the study site and ranged from 0.3 to 0.9 m. Model results indicated that a flow of 3.40 cms met the depth criterion and the lower bound of the width criterion. A flow level of 6.8 cms met the depth criterion and the upper bound of the width criterion. Data from the VAKI Riverwatcher fish passage counting device installed just upstream of the study site were related to the stage/passage limiting width and water temperature monitoring data. The monitoring data and results of the predictive modelling will be used by the California Department of Fish and Wildlife to recommend flow criteria that protect migrating adult SRCS. Copyright © 2016 John Wiley & Sons, Ltd.     

Food habits of migrating salmonid smolts passing bonneville dam in the Columbia river, 1984

The food habits of migrating juvenile steelhead (Salmo gairdneri), sockeye salmon (Oncorhynchus nerka), coho salmon (O. kisutch), and chinook salmon (O. tshawytscha) were identified from April through August 1984 at Bonneville Dam in the Columbia River. During the spring (April-June), the gammarid amphipods Corophium salmonis and C. spinicorne were the dominant prey for all species. Many insect taxa were also consumed, but in small quantities. Significant diet overlap occurred between all species during the spring due to the importance of Corophium. In summer (July-August), the importance of Corophium declined in the diet of subyearling chinook salmon and was replaced with Daphnia spp. and adult dipterans (primarily chironomids).     

To swim or to jump? Passage behaviour of a potamodromous cyprinid over an experimental broad‐crested weir

Physical stressors, such as man‐made obstacles, are considered one of the main causes that negatively affect freshwater fish. Even small weirs may impact fish populations, including potamodromous cyprinids, by partially or totally blocking upstream migratory movements. Some studies have addressed the effect of key hydraulic parameters on upstream movements past small weirs, but little is known on how these parameters interact to induce swimming or jumping behaviour in negotiating such obstacles. This study aims to evaluate the passage behaviour (swimming vs. jumping) of Iberian barbel (Luciobarbus bocagei), a potamodromous species, over an experimental small broad‐crested weir, considering the effect of different plunge pool depths (D), waterfall heights (H), and weir discharges (Q). Results revealed that passage behaviour was highly dependent on combinations of plunge pool depths and waterfall heights. Barbel navigated most configurations by swimming (overall outcomes: 81.5% swimming vs. 18.5% jumping), except the ones with the higher waterfall heights (25 cm) tested. Therefore, higher waterfalls proved to be preponderant in the switching of passage behaviour from swimming to jumping. Regarding the discharge over the 1‐m‐wide weir (overall outcomes: 85% swimming vs. 15% jumping), contrary to what was expected, there was no evidence that passage behaviour was discharge‐related, for the range of discharges tested (25–100 L/s). These results are useful to identify potential migration obstacles and should be taken into consideration in river restoration projects and when designing fishways for potamodromous cyprinids.     

Estimating adult Chinook salmon exposure to dissolved gas supersaturation downstream of hydroelectric dams using telemetry and hydrodynamic models

Gas bubble disease (GBD) has been recognized as a potential problem for fishes in the Columbia River basin. GBD results from exposure to gas supersaturated water created by discharge over dam spillways. Spill creates a downstream plume of water with high total dissolved gas supersaturation (TDGS) that may be positioned along either shore or mid‐channel, depending on dam operations. We obtained spatial data on fish migration paths and migration depths for adult spring and summer Chinook salmon, Oncorhynchus tshawytscha, during 2000. Migration paths were compared to output from a two‐dimensional (2‐dimensional) hydrodynamic and dissolved gas model to estimate the potential for GBD expression and to test for behavioural avoidance of the high TDGS plume. We observed salmon swam sufficiently deep in the water column to receive complete hydrostatic compensation 95.9% of the time spent in the Bonneville Dam tailrace and 88.1% of the time in the Ice Harbor Dam tailrace. The majority of depth uncompensated exposure occurred at TDGS levels >115%. Adult Chinook salmon tended to migrate near the shoreline and they tended to remain in relatively deep water. Adults moved into the high dissolved‐gas plume as often as they moved out of it downstream of Bonneville Dam, providing no evidence that adults moved laterally to avoid areas with elevated dissolved gas levels. When water depths decreased due to reduced river discharge, adults tended to migrate in the deeper navigation channel downstream from Ice Harbor Dam. The strong influence of dam operations on the position of the high‐TDGS plume and shoreline‐orientation behaviours of adults suggest that exposure of adult salmonids to high‐TDGS conditions may be minimized using operational conditions that direct the spilled water mid‐channel. Our approach illustrates the potential for combined field and modelling efforts to estimate the fine‐scale environmental conditions encountered by fishes in natural and regulated rivers. Published in 2007 by John Wiley & Sons, Ltd.     

Comparative Diets of Subyearling Chinook Salmon (Oncorhynchus tshawytscha) and Steelhead (O. mykiss) in the Salmon River,New York

Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) have established naturalized populations throughout the Great Lakes. Young-of-year of these species occur sympatrically for about one month in Lake Ontario tributaries. This study examined the diets of subyearling Chinook salmon and steelhead relative to available food in the Salmon River, New York. Terrestrial invertebrates and trichopterans were the major prey of Chinook salmon, whereas steelhead fed primarily on baetid nymphs and chironomid larvae. Diet overlap was low (0.45) between the species. The diet of Chinook was closely associated to the composition of the drift (0.88). Steelhead diet drew equally from the drift and benthos during the first year of the study, but more closely matched the benthos during the second year. Differences in prey selection, perhaps associated with differences in fish size, in addition to apparent differences in feeding mode (drift versus benthic), likely reduce competitive interactions between these species.     

Habitat use by subyearling Chinook and coho salmon in Lake Ontario tributaries

The habitat use of subyearling Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) was examined in three tributaries of Lake Ontario. A total of 1781 habitat observations were made on Chinook salmon (698) and coho salmon (1083). During both spring and fall, subyearling coho salmon used pool habitat with abundant cover. During spring, principal component analysis revealed that water depth was the most important variable governing subyearling Chinook salmon habitat use. Substrate materials used by Chinook salmon in the spring and coho salmon in the fall were significantly smaller than were present on average within the study reaches. When the two species occurred sympatrically during spring they exhibited similar habitat selection. Although the habitat used by coho salmon in Lake Ontario tributaries was consistent with observations of habitat use in their native range, higher water velocities were less important to Chinook salmon than has previously been reported.     

Effects of Dam Removal on Tule Fall Chinook salmon Spawning Habitat in the White Salmon River,Washington

Condit Dam is one of the largest hydroelectric dams ever removed in the USA. Breached in a single explosive event in October 2011, hundreds‐of‐thousands of cubic metres of sediment washed down the White Salmon River onto spawning grounds of a threatened species, Columbia River tule fall Chinook salmon Oncorhynchus tshawytscha. We investigated over a 3‐year period (2010–2012) how dam breaching affected channel morphology, river hydraulics, sediment composition and tule fall Chinook salmon (hereafter ‘tule salmon’) spawning habitat in the lower 1.7 km of the White Salmon River (project area). As expected, dam breaching dramatically affected channel morphology and spawning habitat due to a large load of sediment released from Northwestern Lake. Forty‐two per cent of the project area that was previously covered in water was converted into islands or new shoreline, while a large pool near the mouth filled with sediments and a delta formed at the mouth. A two‐dimensional hydrodynamic model revealed that pool area decreased 68.7% in the project area, while glides and riffles increased 659% and 530%, respectively. A spatially explicit habitat model found the mean probability of spawning habitat increased 46.2% after dam breaching due to an increase in glides and riffles. Shifting channels and bank instability continue to negatively affect some spawning habitat as sediments continue to wash downstream from former Northwestern Lake, but 300 m of new spawning habitat (river kilometre 0.6 to 0.9) that formed immediately post‐breach has persisted into 2015. Less than 10% of tule salmon have spawned upstream of the former dam site to date, but the run sizes appear healthy and stable. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Changes in fish communities of the Shoalhaven River 20 years after construction of Tallowa Dam,Australia

This study was undertaken as part of a long‐term investigation of the ability of high‐level fishways to rehabilitate fish communities upstream of high dams. Effects of Tallowa Dam on fish of the Shoalhaven River system were studied by comparing species abundances, population size‐structures and the structure of fish communities above and below the dam. Fish were sampled twice yearly for two years at 12 sites throughout the catchment. Species richness was greater downstream of the dam, with 21 species, compared to 16 species upstream of the dam. Ten diadromous species are believed to be extinct above the dam because of obstructed fish passage. Another four migratory species capable of climbing the wall have reduced abundances upstream. Accumulations of fish, particularly juveniles, directly below the dam were evident for nine species. Fish communities upstream and downstream of the dam differed significantly, identifying the dam as a significant discontinuity in the available fish habitats within the system. Historical evidence suggests that before the dam was built, fish communities from the tidal limit to at least 130 m elevation were largely continuous. This study has demonstrated that Tallowa Dam is a major barrier to fish migration and has had adverse effects on the biodiversity of the system. The creation of Lake Yarrunga by Tallowa Dam has resulted in distinctive fish communities in riverine and lacustrine habitats. Populations of five species that occur both upstream and downstream of the dam have developed differences in their size structures. The fish community downstream of the dam also differs from its historical condition because of the virtual disappearance of Australian grayling (Prototroctes maraena) and the establishment of non‐native species. A high‐level fishway is now being designed for the dam to restore fish passage. Data from this study will serve as a baseline against which to assess the effectiveness of the fishway in rehabilitating fish communities of the river system. Copyright © 2002 John Wiley & Sons, Ltd.     

The influence of discharge and temperature on the ability of upstream migrant adult river lamprey (Lampetra fluviatilis) to pass experimental overshot and undershot weirs

The efficiencies of fish passes specifically designed and constructed to facilitate the movement of a limited number of species and life‐stages past structural barriers are likely to decline as site‐specific conditions shift with a changing climate. There is a need to develop realistic fish passage criteria based on understanding swimming capability and behaviour of multiple species in relation to temperature and flow. The influence of temperature and discharge on behaviour and ability of groups of migrating adult river lamprey (Lampetra fluviatilis), a threatened species, to pass a small overshot or undershot weir at night was investigated using a large experimental flume. Lamprey approached the weirs less, and more often maintained station by using the oral disk to attach to structure, under high flows. Oral disk attachment was more commonly observed during tests employing the undershot weir. Upstream movement tended to be in close proximity to the channel walls and floor where, compared to the mid‐channel, velocities were generally lower and velocity vectors more likely to be in a direction other than the bulk flow. Upstream movement was positively related to temperature, and was higher for the overshot weir. Weir passage rate was higher for undershot than overshot weirs, and negatively related to the maximum velocity at the weir. Passage rate was low when maximum velocities at the weir exceeded 1.5 m s^?1, although some fish passed at c. 1.7 m s^?1. Passage efficiency, the number of weir passes as a percentage of the number of approaches, was also higher for the undershot weir, but was not affected by discharge because lamprey approached less frequently at high flows. This study provides fish passage criteria under realistic conditions for an infrequently studied anguilliform species of conservation concern and provides a methodological perspective by which to improve fishway suitability for a wider range of species subject to changing climate. Copyright © 2010 John Wiley & Sons, Ltd. This article was published online on March 3, 2010. An error was subsequently identified in the Article title. This notice is included in the online and print versions to indicate that both have been corrected [April 12, 2010].     

The impact of small and large impoundments on freshwater mussel distribution in the Hawkesbury‐Nepean River,southeastern Australia

The influence of weirs on the distribution of freshwater mussels was investigated in the Hawkesbury‐Nepean River, Australia. Distribution of species and densities of size classes were strongly correlated with catchment level factors (e.g. location around a major impoundment, stream order). At catchment scale, weir height, presence of a fish barrier, fish ladder type and position above or below small weirs did not influence the presence/absence of mussel populations. Lower mussel densities in the upper catchment may therefore reflect inhibition of host fish migration. Where present, weir height and geomorphic reach type were linked to differences in densities among species. Geomorphic reach‐based differences were reflected by the Hyridella species, but not Velesunio ambiguus. When population structure was described by size class distribution, there were significant differences between densities of small and medium mussels from weirs above, compared to weirs below, a major impoundment, but not for large mussels. Upstream populations may therefore be functionally extinct. Distribution of mussel size classes differed among geomorphic reach types with highest densities for each class found in the least human‐impacted reaches. Small mussels were almost invariably found below the major impoundment, most frequently below weirs. Distribution patterns were inconsistent across species, suggesting habitat preference. V. ambiguus and Hyridella australis were most abundant in shale reaches, where assemblages were influenced by fish ladder type. Hyridella depressa and H. australis dominated in sandstone gorges and straights with assemblage density related to weir height. In upper catchment sandstone reaches, mussel assemblages comprising predominantly V. ambiguus were influenced by fish ladder type and weir height. While multiple factors defined localized distribution, large impoundments were linked with reduced population densities. The probable mechanism is the restriction of host fish movement and resulting lack of recruitment. In the Hawkesbury‐Nepean River, smaller weirs also seriously impacted recruitment. Copyright © 2008 John Wiley & Sons, Ltd.