USING OTOLITH CHEMISTRY TAGS AND GROWTH PATTERNS TO DISTINGUISH MOVEMENTS AND PROVENANCE OF NATIVE FISH IN THE GRAND CANYON

Fish otolith and water chemistry were assessed in the Grand Canyon reach of the Colorado River and its tributaries. Aqueous strontium and selenium (in ratio to calcium) and carbon stable isotopic ratios were identified as markers with excellent potential to track the provenance and movements of the endangered humpback chub Gila cypha. Although otolith δ¹³C and Sr/Ca varied proportionately to water chemistry and provided a framework for detailed study of humpback chub movements, otolith Se/Ca showed ambiguous tracking of known water chemistries. As an application, we document the natal source and movement dynamics of n = 10 humpback chub and compare these findings from otolith microchemistry with the current paradigm of humpback chub spawning ecology. We found that seven of ten fish follow the current early life history paradigm and were spawned in the Little Colorado River and subsequently emigrated to the main stem Colorado River as juveniles. However, the otolith markers of three fish suggest an alternative early life trajectory with unknown provenance. Age and growth analyses demonstrate seasonally higher growth rates in the warmer Little Colorado River compared with the Colorado River. Combining natural markers with age and growth reconstructions provides a powerful tool for assessing the habitat use and success of humpback chub in the Grand Canyon. Copyright © 2012 John Wiley & Sons, Ltd.     

Assessing Juvenile Native Fish Demographic Responses to a Steady Flow Experiment in a Large Regulated River

The Colorado River below Glen Canyon Dam, Arizona, is part of an adaptive management programme which optimizes dam operations to improve various resources in the downstream ecosystem within Grand Canyon. Understanding how populations of federally endangered humpback chub Gila cypha respond to these dam operations is a high priority. Here, we test hypotheses concerning temporal variation in juvenile humpback chub apparent survival rates and abundance by comparing estimates between hydropeaking and steady discharge regimes over a 3‐year period (July 2009–July 2012). The most supported model ignored flow type (steady vs hydropeaking) and estimated a declining trend in daily apparent survival rate across years (99.90%, 99.79% and 99.67% for 2009, 2010 and 2011, respectively). Corresponding abundance of juvenile humpback chub increased temporally; open population model estimates ranged from 615 to 2802 individuals/km, and closed model estimates ranged from 94 to 1515 individuals/km. These changes in apparent survival and abundance may reflect broader trends, or simply represent inter‐annual variation. Important findings include (i) juvenile humpback chub are currently surviving and recruiting in the mainstem Colorado River with increasing abundance; (ii) apparent survival does not benefit from steady fall discharges from Glen Canyon Dam; and (iii) direct assessment of demographic parameters for juvenile endangered fish are possible and can rapidly inform management actions in regulated rivers. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling effects of discharge on habitat quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River,Grand Canyon

A two‐dimensional hydrodynamic model was applied to seven study reaches in the Colorado River within Grand Canyon to examine how operation of Glen Canyon Dam has affected availability of suitable shoreline habitat and dispersal of juvenile humpback chub (Gila cypha). Suitable shoreline habitat typically declined with increasing discharges above 226–425 m³/ s, although the response varied among modelled reaches and was strongly dependent on local morphology. The area of suitable shoreline habitat over cover types that are preferred by juvenile humpback chub, however, stayed constant, and in some reaches, actually increased with discharge. In general, changes in discharge caused by impoundment tended to decrease availability of suitable shoreline habitat from September to February, but increased habitat availability in spring (May–June). Hourly variation in discharge from Glen Canyon Dam substantially reduced the amount of persistent shoreline habitat at all reaches. Changes in suitable shoreline habitat with discharge were shown to potentially bias historical catch per unit effort indices of native fish abundance up to fourfold. Physical retention of randomly placed particles simulating the movement of juvenile humpback chub in the study reaches tended to decline with increasing discharge, but the pattern varied considerably due to differences in the local morphology among reaches and the type of swimming behaviour modelled. Implications of these results to current hypotheses about the effects of Glen Canyon Dam on juvenile humpback chub survival in the mainstem Colorado River are discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal regime suitability: Assessment of upstream range restoration potential for Colorado pikeminnow,a warmwater endangered fish

Dams have reduced distribution of the endangered Colorado pikeminnow Ptychocheilus lucius in the upper Colorado River basin: low‐head diversion dams blocked upstream passage and large dams inundated free‐flowing segments and cooled downstream reaches with deep‐water releases. To date, range restoration efforts in the Colorado and Gunnison Rivers have focused on building fish ladders around diversion dams to allow recolonization of upstream reaches. Upstream thermal suitability for this warmwater cyprinid was assessed using temperature data and existing distributional information from river reaches where Colorado pikeminnow movements were unrestricted. Among‐site thermal regime comparisons were made using mean annual thermal units (ATU), derived from mean daily temperatures during 1986–2005 and the relation between temperature and Colorado pikeminnow growth. Upstream distributional limits in the Yampa and Gunnison Rivers occurred where in‐channel thermal regimes fell below a long‐term mean of 47–50 ATU, suggesting that two Colorado River fish ladders will make available an estimated 17 km of thermally suitable habitat. A Gunnison River fish ladder successfully re‐established access to 54 km of suitable habitat, but 32 km of critical habitat upstream remains unsuitable. Suitability there could be achieved by raising temperatures only 1–2°C from late May to mid‐October with installation of a temperature control device on an upstream dam. Maximum, main‐channel, summer temperatures did not limit Colorado pikeminnow distribution in downstream reaches of the upper Colorado River. Published in 2010 by John Wiley & Sons, Ltd.     

Habitat relationships of subadult humpback chub in the Colorado River through Grand Canyon: spatial variability and implications of flow regulation

We examined subadult humpback chub densities along 24 kms of the Colorado River in the Grand Canyon to: (1) identify geomorphic conditions in the study area; (2) determine associations between subadult humpback chub (<200 mm TL) habitat use and geomorphic differences; and (3) determine how discharge, during base flow conditions, was related to subadult humpback chub habitat conditions. Habitat was categorized at two nested spatial scales: geomorphic reach and shoreline type. Within reaches, shoreline types were categorized according to geomorphology. We measured water depth, velocity and cover attributes along all shoreline types over a range of discharges to determine if habitat quality of reaches and shoreline types varied with discharge. Reaches 1 and 3 had narrow, deep corridors, whereas Reach 2 was a wide, shallow reach. Among shoreline types, depth, velocity and cover varied; however, differences were not consistent between reaches. Fish densities also varied among shoreline types and reaches. Vegetation, talus and debris fan shorelines had the highest densities of subadult humpback chub in a pattern similar to that of cover. In addition, subadult humpback chub presence was associated with a high frequency of cover regardless of shoreline designation. However, these relationships explained little of the overall variation in subadult densities. Lack of a strong association between fish density and geomorphology may be partially due to effects of discharge on habitat quality. The overall trend among shorelines (without regard to type) showed that cover decreased with increasing discharge, whereas depth and velocity increased. However, no consistent pattern between discharge and depth, velocity and cover among individual shoreline types was evident. Vegetated shorelines, consisting mainly of non-native tamarisk (Tamarix chinensis), had nearly twice the fish densities of talus and debris fan. Reasons are discussed as to why subadult humpback chub occupy naturalized habitat like vegetated shorelines in greater densities than natural habitats. The relationships observed in this study have important implications for humpback chub recovery and management of the Colorado River through Grand Canyon. © 1998 John Wiley & Sons, Ltd.     

Do Management Actions to Restore Rare Habitat Benefit Native Fish Conservation? Distribution of Juvenile Native Fish Among Shoreline Habitats of the Colorado River

Many management actions in aquatic ecosystems are directed at restoring or improving specific habitats to benefit fish populations. In the Grand Canyon reach of the Colorado River, experimental flow operations as part of the Glen Canyon Dam Adaptive Management Program have been designed to restore sandbars and associated backwater habitats. Backwaters can have warmer water temperatures than other habitats, and native fish, including the federally endangered humpback chub Gila cypha, are frequently observed in backwaters, leading to a common perception that this habitat is critical for juvenile native fish conservation. However, it is unknown how fish densities in backwaters compare with that in other habitats or what proportion of juvenile fish populations reside in backwaters. Here, we develop and fit multi‐species hierarchical models to estimate habitat‐specific abundances and densities of juvenile humpback chub, bluehead sucker Catostomus discobolus, flannelmouth sucker Catostomus latipinnis and speckled dace Rhinichthys osculus in a portion of the Colorado River. Densities of all four native fish were greatest in backwater habitats in 2009 and 2010. However, backwaters are rare and ephemeral habitats, so they contain only a small portion of the overall population. For example, the total abundance of juvenile humpback chub in this study was much higher in talus than in backwater habitats. Moreover, when we extrapolated relative densities based on estimates of backwater prevalence directly after a controlled flood, the majority of juvenile humpback chub were still found outside of backwaters. This suggests that the role of controlled floods in influencing native fish population trends may be limited in this section of the Colorado River. Copyright © 2014 John Wiley & Sons, Ltd.     

Do Hydropeaking Flows Alter Juvenile Fish Growth Rates? A Test with Juvenile Humpback Chub in the Colorado River

Riverine ecosystems have been altered in many large catchments by dam development to provide water, power, flood control and navigational benefits to humans. Conservation actions in these river ecosystems are commonly focused on minimum releases of water to downstream ecosystems. Increasingly minimum release approaches are being replaced with ‘experimental’ flows that mimic natural conditions in order to benefit riverine ecosystems. While these new policies are intuitive in their design, there is limited data of how riverine ecosystems actually respond to more natural flows. A test of more natural steady‐flow water release was compared with typical fluctuating hydropower flows in the adaptive management programme at Glen Canyon Dam, Arizona, during 2008–2011 to assess growth improvements of endangered juvenile humpback chub Gila cypha. Our results are counterintuitive and show that more natural steady flows reduced growth rates of juvenile humpback chub compared with fluctuating flows when both treatments occurred within the same year. Daily growth rates during steady flows of 2009 and 2010 were 0.05 and 0.07 mm day^?1 slower, respectively, than fluctuating flows those same years, despite similar water temperatures. Juvenile humpback chub also grew more slowly during steady flows that occurred in the same season. During the summer, juvenile humpback chub grew 0.12 and 0.16 mm day^?1 in fluctuating flow regimes in 2009 and 2010, respectively, and only 0.07 mm day^?1 in the experimental steady flow regime in 2011, despite higher water temperatures. Our results suggest that optimal conservation management policies for endangered species in regulated rivers may not always be achieved with more natural flows. Copyright © 2014 John Wiley & Sons, Ltd.     

Fish populations of a small lowland channelized river in England subject to long-term river maintenance and management works

Fish populations in the River Perry, a small lowland channelized river in England, were affected by long-term river maintenance and improvement works. Habitat diversity at two ‘old channelized’ sites was low compared with a downstream ‘partially channelized’ site and an unmodified site, where natural river features such as the pool-riffle pattern were more apparent. The fish community was low in diversity at all sites, with two running-water cyprinids, dace Leuciscus leuciscus (L.) and chub Leuciscus cephalus (L.), being the dominant fish species. Relative species composition varied between sites, with dace predominating at the ‘old channelized’ sites and chub increasingly dominant at the ‘partially channelized’ and unmodified sites. Population density and biomass estimates of dace were similar at all sites, but chub were up three times more abundant at the ‘partially channelized’ and unmodified sites than at the ‘old channelized’ sites. Populations of dace and chub at the ‘old channelized’ sites contained a higher proportion of small fish than populations at the downstream sites, which exhibited a more balanced population structure. The growth rates of both dace and chub varied between sites. Low recaptures of marked dace suggested a highly mobile movement pattern while higher recapture rates of chub, particularly at the ‘partially channelized’ and unmodified sites, seemed to indicate a more sedentary activity pattern. The conclusion of the study is that long-term river maintenance and management works may delay the morphological and biological recovery of lowland channelized rivers.     

Dam and geomorphological influences on colorado river waterbird distribution,Grand Canyon,Arizona, USA

Impoundment effects override natural, reach-based channel geomorphology influences on seasonal waterbird distribution in Grand Canyon along the Colorado River downstream from Glen Canyon Dam. Large winter waterbird populations were rare or non-existent prior to completion of Glen Canyon Dam in 1963, and pre-dam summer breeding was rare. Post-dam river corridor surveys of 13 geomorphological reaches from 1973 to 1994 detected 58 species of waterfowl, waders, shorebirds and piscivorous raptors, with a grand mean of 138·2 waterbirds/reach (SE=31·0, n=727 reach surveys), and a mean area-adjusted rate of encounter (AARE) of 372·8 waterbirds km⁻¹ h⁻¹ of observation per reach (SE=69·1). The post-dam assemblage has been dominated by Anseriformes (13 diving and 12 dabbling species) and includes regionally significant populations of wintering waterfowl and bald eagle, and breeding mallard. Most wading birds and shorebirds occur primarily as migrants or summer vagrants. Total waterbird AARE was greatest in the productive clear water (CW) and variably turbid (VT) segments upstream from the Little Colorado River (LCR) (km 98), decreasing downstream on the usually turbid (UT) lower Grand Canyon segment. Mean total winter waterfowl AARE was 1076·8, and decreased by three orders of magnitude from the CW to the UT segments (p=0·0001). Mean total summer AARE was 2·7, and also decreased across the turbidity segments (p=0·066). In contrast, AARE varied little between wide and narrow geomorphological reaches. Total AARE was only 1·4 and 1·3-fold greater in wide versus narrow reaches within the VT and UT turbidity segments, respectively (p<0·0002). Winter AARE was threefold greater (p=0·0002), while summer AARE was equivalent between wide and narrow reaches. These tributary-related turbidity and geomorphological reach width factors contributed to a non-linear, circuitous shift in the waterbird assemblage over distance downstream from the dam, differentially affecting the seasonal distribution of waterbird feeding guilds. We discuss flow regulation and habitat management implications. © 1997 John Wiley & Sons, Ltd.     

Waterfall formation at a desert river–reservoir delta isolates endangered fishes

Unforeseen interactions of dams and declining water availability have formed new obstacles to recovering endemic and endangered big‐river fishes. During a recent trend of drying climate and declining reservoir water levels in the Southwestern United States, a large waterfall has formed on two separate occasions (1989–1995 and 2001–present) in the transition zone between the San Juan River and Lake Powell reservoir because of deposited sediments. Since recovery plans for two large‐bodied endangered fish species, razorback sucker (Xyrauchen texanus) and Colorado pikeminnow (Ptychocheilus lucius), include annual stockings in the San Juan River, this waterfall potentially blocks upstream movement of individuals that moved downstream from the river into the reservoir. To quantify the temporal variation in abundance of endangered fishes aggregating downstream of the waterfall and determine population demographics, we remotely monitored and sampled in spring 2015, 2016, and 2017 when these fish were thought to move upstream to spawn. Additionally, we used an open population model applied to tagged fish detected in 2017 to estimate population sizes. Colorado pikeminnow were so infrequently encountered (<30 individuals) that population estimates were not performed. Razorback sucker captures from sampling (335), and detections from remote monitoring (943) showed high abundance across all 3 years. The razorback sucker population estimate for 2017 alone was 755 individuals and, relative to recent population estimates ranging from ~2,000 to ~4,000 individuals, suggests that a substantial population exists seasonally downstream of this barrier. Barriers to fish movement in rivers above reservoirs are not unique; thus, the formation of this waterfall exemplifies how water development and hydrology can interact to cause unforeseen changes to a riverscape.     

Recolonization Process and Fish Assemblage Dynamics in the Guadiamar River (SW Spain) After the Aznalcóllar Mine Toxic Spill

The Guadiamar River (SW Iberian Peninsula) received a major toxic spill (6 hm³) from a tailing pond in 1998 that defaunated 67 km of the main stem. Following early mud removal works, the fish assemblage was annually monitored at four affected sampling sites and one located in the upstream non‐affected reach of the Guadiamar River as reference. Fish abundance and assemblage structure were analysed. Principal response curve was applied to assess the recovery trends and to identify the most influential species. A non‐metric multidimensional scaling ordination and permutational multivariate analysis of variance were applied to evaluate changes in fish assemblage structure between sites and years. Overall, the affected reaches harboured fish within 2 years of the spill. Colonists arrived mainly from the upstream and downstream non‐affected Guadiamar River reaches and, to a lesser extent, from three lateral tributaries. It is likely that the proximity, connectivity and environmental conditions of non‐affected fish sources greatly influenced the recolonization process in each site. The structure of the fish community in the affected sites was initially similar to that in the unaffected reference stretch but changed dramatically with time, and each site followed its own trajectory. Currently, long‐term threats such as mining leachates, urban sewage, agricultural pollution and exotic fish species expansion have probably exceeded the initial spill effect. This highlights the large effect of anthropogenic factors on freshwater ecosystem resilience, and the need to significantly reduce both pollution and exotic species if the affected reach of the Guadiamar River is to recover fully. Copyright © 2015 John Wiley & Sons, Ltd.     

Distribution,movements and habitat use of channel catfish in a river with multiple low‐head dams

Channel catfish Ictalurus punctatus is a highly mobile species and is known to make extensive seasonal movements in lotic systems. Dams have been suggested to detrimentally affect this species, although abundant channel catfish populations are known to occur in many fragmented rivers. To examine factors that allow channel catfish to persist in impounded rivers, we assessed relative abundance of channel catfish in three impounded and three flowing sites of the Fox River, Illinois, USA. Radiotelemetry was used to determine movement and habitat use patterns of channel catfish among flowing and impounded areas. Relative abundance of channel catfish was consistently higher at flowing sites than at impounded sites during summer. Several radio‐tagged channel catfish moved downstream into impounded areas in fall, and all tagged individuals were found in impounded areas during winter. The majority of tagged channel catfish moved upstream into flowing areas during spring. Channel catfish used a wide range of depths (0.28–2.60 m), and were always found in current velocities less than 0.50 m s^?1. They selected most strongly for coarse substrates, but were infrequently found near cover. Although low‐head dams restrict the movements of channel catfish, impounded areas appear to provide overwintering habitats that may eliminate the need for seasonal long‐distance movements. Small run‐of‐river impoundments, however, may contain unsuitable conditions for channel catfish during other seasons. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of pool formation and flash flooding on relative abundance of young‐of‐year flannelmouth suckers in the Paria River,Arizona

Flannelmouth sucker, Catostomus latipinnis, a fish endemic to the Colorado River basin in the western United States, appears to experience poor recruitment to adult size in the Colorado River, downstream of Glen Canyon Dam. Lack or impermanence of rearing areas for young‐of‐year (YOY) fish is hypothesized to be the problem. Knowing the importance of tributary mouths as rearing areas in other river systems, we studied use of the mouth of the Paria River, a tributary of the Colorado River, by YOY flannelmouth suckers, and the availability of rearing area in the mouth at different flow levels in the Colorado River in 1996 and 1997. We also examined the relationship between flash floods in the Paria River and catch‐per‐unit‐effort (CPUE) of YOY in the Paria River between 1991 and 1996. Maximum mean daily discharge in the Paria River was inversely correlated with CPUE of YOY flannelmouth suckers (Spearman Rho=?0.9856, p=0.0003) during their critical rearing period (15 March–30 June). Thus, it appears that YOY flannelmouth suckers rear longer in the Paria River in years when flash flooding is minimal. Recruitment of YOY flannelmouth suckers at the Paria River may also be improved by enhancing pool formation during spring and summer rearing seasons. YOY flannelmouth sucker was captured in a pool created by high Colorado River flows (≥336 m³/s) that inundated the mouth of the Paria River during spring and summer, 1996. In 1997, high flows (about 550–750 m³/s) in the Colorado River during winter and spring initially inundated the Paria River and formed a pool in the mouth. However, these high flows eventually caused 0.5–1.0 m of suspended sediment from the incoming Paria River to deposit in the mouth. Thus, despite higher flows than 1996, the slackwater area formed only occasionally in 1997. Differences in pool formation between 1996 and 1997 demonstrate that pool formation cannot be inferred solely from Colorado River flows. Copyright © 2001 John Wiley & Sons, Ltd.     

Natal environments of age‐0 paddlefish in the middle Mississippi River inferred from dentary microchemistry

Effective management and conservation of riverine fish species relies on identification of habitats that contribute recruits to fish populations. Paddlefish are an important commercial and recreational species inhabiting North American large rivers. However, despite the knowledge of adult paddlefish movement patterns in large rivers, their principal natal environments and early life dispersal patterns remain unknown. Paddlefish dentary microchemistry can be used to identify natal environment of fish in large river networks such as the middle Mississippi River (MMR) and tributaries. The goals of this study were to (a) use dentary microchemistry (strontium:calcium ratios; Sr:Ca) to determine natal environment and potential drift for age‐0 paddlefish collected from the MMR and (b) assess whether MMR reach or year of collection influenced the percentage of recruits originating from different rivers. Age‐0 paddlefish were collected during 2010–2011 from two reaches of the MMR (upstream and downstream of the Kaskaskia River confluence). Water samples from the MMR and tributaries (upper Mississippi, Missouri, Illinois, Osage, and Kaskaskia Rivers) were collected during 2006–2016. Water Sr:Ca differed among rivers, enabling identification of natal environment for individual fish using dentary core Sr:Ca. The MMR (44–69% of fish sampled) and Missouri River (25–45% of fish sampled) were the primary natal environments for age‐0 paddlefish across both river reaches and collection years. The upper Mississippi River and smaller tributaries contributed few recruits (<13% of fish sampled). Conservation of paddlefish populations should include maintenance or improvement of connectivity between river reaches used for spawning and juvenile rearing and stock assessments of riverine paddlefish may need to be conducted at a riverscape scale because multiple rivers can contribute to paddlefish recruitment in a particular river reach.     

Plant dispersal along rivers fragmented by dams

To understand the influence of dams on connectivity of riparian plant communities along rivers, we examined plant dispersal by water (hydrochory) and riparian plant community attributes upstream and downstream from dams on two rivers in the southern Rocky Mountains, Colorado, USA. Drifting plant propagules were collected from the water column along reaches upstream and downstream from dams to examine the longitudinal and temporal variation in seed‐pool species composition and concentration of water‐transported seeds. Similarities between species composition of the hydrochoric seed pool and local standing riparian vegetation were used to evaluate the degree of longitudinal connectivity along river corridors and to isolate the relative contributions of local versus regional species pools to hydrochoric species composition. Furthermore, several synthetic attributes (longevity, origin, life‐form and dispersal mode) and species composition of riparian plant communities were examined to explore the effects of interrupted propagule dispersal on standing vegetation. We estimated that as many as 120 million seeds were transported via hydrochory along free‐flowing reaches of the Rocky Mountain streams in a single growing season. Seed concentration (seeds/m³) in the water column was reduced by 70–94% along reaches downstream from dams compared to free‐flowing reaches. The similarity in species composition of hydrochoric seeds and local standing vegetation was nearly two times greater downstream from reservoirs compared to upstream. This suggests that hydrochory complements local species pools by importing seeds from throughout the upstream catchment area along free‐flowing river reaches, but that hydrochoric seeds are derived primarily from local sources along regulated river reaches. Species richness recovers as a function of downstream distance from contributions of standing vegetation and seeds from tributary streams. Hydrochory may extend the period over which viable seeds of a parent population are dispersed. Even after dispersal of parent populations has terminated, seeds may continue to be available due to residence time in water transport. This extension of the ‘effective dispersal window’ of some species may exceed two weeks or more and may influence the likelihood of successful establishment. In this study, synthetic attributes of riparian vegetation did not differ significantly between free‐flowing and regulated reaches, whereas formal statistical comparisons of community composition upstream and downstream from reservoirs indicate that there are differences in community composition upstream and downstream from dams. These findings suggest that the consequences of 50 to 100 years of fragmentation result in community‐wide effects along Rocky Mountain streams and that these effects may be partially explained by dam‐caused disruption in connectivity of plant populations. Copyright © 2005 John Wiley & Sons, Ltd.     

Downstream effects of diversion dams on sediment and hydraulic conditions of Rocky Mountain streams

Reduced streamflow via flow diversion has the potential to limit the sediment‐transport capacity of downstream channels and lead to accumulation of fine sediments and habitat degradation. To investigate, we examined the effects of variable levels of flow diversion on fine‐sediment deposition, hydraulic conditions and geomorphic alteration. Our study consisted of a detailed field analysis pairing reaches above and below diversion dams on 13 mountain streams in north‐central Colorado and southern Wyoming USA. Diversions are ubiquitous across the American West, yet previous comparative studies on the effects of flow diversion have yielded mixed results. Through application of strict site‐selection criteria, multiple fine‐sediment measures, and an intensive sampling scheme, this study found that channels downstream of diversions contained significantly more fine sediment and slow‐flowing habitat as compared to upstream control reaches. Susceptibility to fine‐sediment accumulation was associated with decreasing basin size, decreasing bankfull depth and smaller d₈₄, and it appears to be magnified in streams of less than 3% slope. Copyright © 2010 John Wiley & Sons, Ltd.     

Influences of land use/cover on water quality in the upper and middle reaches of River Njoro, Kenya

Data from 10 sampling sites along the River Njoro are used to examine the contribution of nutrients from upstream land uses draining each of the sampling sites. The data also are used to assess whether both the proportion of land uses and the size of the subwatersheds account for the variability in water quality in the River Njoro watershed. Geographical Information System analysis was used to determine the spatial distribution of land‐cover types and subwatersheds contributing run‐off to the sampling sites in the River Njoro. Standard Digital Elevation Model‐based routines were used to establish the watershed area contributing run‐off to each sampling site. Water and sediment samples were collected for chemical analysis, and the nutrient levels were related to the upstream land‐use types and the size of the subwatersheds. The mid‐stream portion of the River Njoro (near Egerton University) accounts for the highest nutrient contributions. The percentage contribution is magnified by additions from industrial, human settlements and agricultural land uses around the University. There is a significant decrease in nutrient levels downstream, however, indicating natural purification as the river flows through an area of large‐scale farming with intense, well‐preserved riparian and in‐stream vegetation. Steep slopes of the land upstream of Egerton University enhance erosion and nutrient losses from those subwatersheds. Mixed small‐scale agricultural and bare lands contribute over 55% of the phosphorus load to the upper and mid‐reaches of the River Njoro. The size of the subwatershed accounts for about 53% of the variability in the soluble phosphorus in the river. The land‐use subwatershed proportions are important for characterizing and modelling water quality in the River Njoro watershed. Upland land uses are as important as near‐stream land uses. We suggest that conservation of intact riparian corridor along the river and its tributaries contributes significantly to natural purification processes and recovery of the ecological integrity of the River Njoro ecosystem.     

Colorado river benthic ecology in Grand Canyon,Arizona, USA: dam,tributary and geomorphological influences

The serial discontinuity concept (SDC; Ward and Stanford, in Ecology of River Systems, 1983) predicts that recovery of large regulated rivers over distance downstream from a dam is limited by relative tributary size; however, channel geomorphology may also influence the recovery process. We examined the spatial variation in water quality, benthic composition and ash-free dry standing biomass (AFDM) among the bedrock-defined geomorphological reaches in three turbidity segments of the Colorado River between Glen Canyon Dam and Diamond Creek, Arizona, including most of the Grand Canyon. This 387-km long study area supported virtually no Ephemeroptera, Plecoptera or Trichoptera, probably because cold, stenothermic, hypolimnetic releases limited maximum aestival warming to 17·1°C. The benthos displayed abrupt, physically related decreases in AFDM over distance from the dam and in the varial zone. The 26-km long clear water segment between the dam and the Paria River supported a depauperate Cladophora glomerata/epiphyte/chironomid/Gammarus lacustris/lumbricine/Physella sp. assemblage, and ooze-dwelling oligochaetes. This segment contained 6·9% of the aquatic habitat below the 140 m³/s (normal minimum) discharge stage of the Colorado River study area, but supported 63·5% of the benthic primary producer AFDM and 87% of the benthic consumer AFDM in the entire study area. Turbidity increased and light penetration decreased immediately downstream from the confluence of the small, turbid Paria River, and further downstream from the Little Colorado River confluence. The benthos downstream from the Paria River was abruptly replaced by an Oscillatoria/Simuliium assemblage with a mean AFDM of <0·12 g C/m². Dam-related effects on water clarity, varial flow and water temperature overrode geomorphological influences on habitat availability. These results generally support the SDC, in that recovery of the benthos did not take place over distance in this large river ecosystem; however, geomorphological differences in substratum availability between reaches mediated dam and tributary effects on water clarity and benthic AFDM. Interactions between flow regulation and geomorphology produce a pattern of circuitous recovery of some physical river ecosystem characteristics over distance from the dam, but not of the benthos. Improving discharge management for endangered native fish populations requires detailed understanding of existing and potential benthic development, and trophic interactions, throughout the geomorphological reaches and turbidity segments in this river. © 1997 John Wiley & Sons, Ltd.     

EFFECTS OF FLOW RELEASES ON MACROINVERTEBRATE ASSEMBLAGES IN THE INDIAN AND HUDSON RIVERS IN THE ADIRONDACK MOUNTAINS OF NORTHERN NEW YORK

The effects of flow releases (daily during spring and four times weekly during summer) from a small impoundment on macroinvertebrate assemblages in the lower Indian River and upper Hudson River of northern New York were assessed during the summers of 2005 and 2006. Community indices, feeding guilds, dominant species and Bray–Curtis similarities at three sites on the Indian River, below a regulated impoundment, were compared with those at four control sites on the Cedar River, below a run‐of‐the‐river impoundment of comparable size. The same indices at four less‐likely affected sites on the Hudson River, below the mouth of the Indian River, were compared with those at an upstream control site on the Hudson River. Results show that the function and apparent health of macroinvertebrate communities were generally unaffected by atypical flow regimes and/or altered water quality at study reaches downstream from both dams in the Indian, Cedar and Hudson Rivers. The lentic nature of releases from both impoundments, however, produced significant changes in the structure of assemblages at Indian and Cedar River sites immediately downstream from both dams, moderate effects at two Indian River sites 2.4 and 4.0 km downstream from its dam, little or no effect at three Cedar River sites 7.2–34.2 km downstream from its dam, and no effect at any Hudson River site. Bray–Curtis similarities indicate that assemblages did not differ significantly among sites within similar impact categories. The paucity of scrapers at all Indian River sites, and the predominance of filter‐feeding Simulium gouldingi and Pisidium compressum immediately below Abanakee dam, show that only minor differences in dominant species and trophic structure of macroinvertebrate communities occurred at affected sites in the Indian River compared to the Cedar River. Thus, flow releases had only a small, localized effect on macroinvertebrate communities in the Indian River. Copyright © 2011 John Wiley & Sons, Ltd.     

Particulate organic matter transport in the lower Colorado river,South-Western USA

The lower Colorado River from Laughlin, Nevada, to the border with Mexico is one of the most regulated rivers in North America. An analysis of particulate organic matter (POM) transport through the system was made to determine POM concentrations, composition and point of origin, as well as to determine the effect dams had on this transport. Particulate organic matter concentrations generally increased from upstream to downstream, and were dominated by very fine organic detritus (< 25μm size fraction). Reservoirs released more POM than they received during spring and summer, but less during autumn and winter. Reservoirs recycled POM, taking in organic detritus and discharging small limnoplankton. Backwater complexes functioned as filters during storm events, trapping coarse material and releasing fine material. Agricultural drains made only minor contributions of POM, but provided important nutrients for in-channel production of POM. Water diversions forced POM off-channel, whereas sluicing operations reduced inorganic loads in the canals. Despite the presence of six mainstem dams, the lower reaches support a diverse assemblage of detritivorous fishes, similar to that found in the lowermost reaches of natural, unregulated rivers. The food base for this fishery stems from autochthonous production of POM.