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.     

Mitigation of cold‐water thermal pollution downstream of a large dam with the use of a novel thermal curtain

Hypolimnial releases from dams during periods of thermal stratification modify the downstream riverine thermal regime by decreasing water temperature and reducing natural diel thermal variability. This cold‐water thermal pollution in rivers can persist for hundreds of kilometres downstream of dams and impact important ecological processes such as fish spawning. To mitigate this problem, a first‐of‐its‐kind thermal curtain was fitted to the large bottom release Burrendong Dam on the Macquarie River, Australia. The thermal curtain acts by directing warmer, near‐surface epilimnial water to the low‐level hypolimnial offtake. This study aimed to test the efficacy of the thermal curtain by measuring temperatures before and after the curtains installation, quantifying the magnitude and extent of cold‐water thermal pollution along the Macquarie River downstream of Burrendong Dam. Epilimnial releases with use of the curtain increased diel temperature ranges and the mean monthly water temperature below the dam. Epilimnial releases with use of the curtain increased diel temperature ranges from 0.9°C to 2.5°C and reduced the difference between the mean monthly water temperature of an upstream control and a downstream site by up to 3.5°C. A comparison of the monthly temperature means along the river, indicated that thermal recovery, whereby temperatures returned to within the natural range of upstream temperatures occurred 45 km downstream of the dam during summer when the thermal curtain was deployed, compared with approximately 200 km prior to deployment of the curtain. Our study suggests that the use of thermal curtains can reduce cold‐water thermal pollution and improve ecological outcomes for river ecosystems downstream of dams.     

Serial discontinuities in a Rocky mountain river. II. Distribution and abundance of trichoptera

River regulation in the headwaters and middle reaches of the Gunnison River, Colorado, significantly altered distributions and abundances of Trichoptera fauna. Twenty-five species were collected from mainstream samples, with the greatest species richness occurring at an unregulated, rhithron segment above the central reach dams. At sites immediately below the three hypolimnial-release dams and a reregulation dam, species richness was reduced 35–90 per cent and abundance > 95 per cent. Net-spinning caddisflies were the dominant trichopterans at unregulated sites; Arctopsyche grandis in the upper reaches (218 organisms, 586 mg dry mass m^?2) and Hydropsyche cockerelli, H. occidentalis and Cheumatopsyche pettiti in the lower river (9041 total organisms, 6621 mg m^?2), downstream from the last dam. The observed distributional pattern of low trichopteran densities in dam tailwaters and high hydropsychid densities at sites 60–80 km below the central reach dams is a classic expression of continuum resets and adjustments in response to stream regulation as predicted by the Serial Discontinuity Concept.     

Serial discontinuities in a Rocky mountain river. I. Distribution and abundance of plecoptera

Samples were taken year-round at eleven sites along the altitudinal profile (2900-1400 m a.s.l.) of the Gunnison River, a 329 km tributary of the Colorado River, to document the distribution of the Plecoptera and to evaluate responses to hypolimnial-release dams in the headwaters and middle reaches. Twenty-two species were present, with the greatest species richness occurring in an unregulated segment upstream of the middle reach dams; average nymphal biomass over the study period (175 organisms, 395 mg dry mass m^?2) was also greatest in this segment. Only four species (58 organisms, 48 mg m^?2) were present in the tailwaters of the headwater dam and values were greatly reduced (nine species; 35 organisms, 180 mg m^?2) below the middle reach dams. The stonefly community recovered ca. 80 km downstream from the last dam (15 species; 244 organisms, 250 mg m^?2), apparently in response to natural resetting of environmental conditions corresponding to those above the middle reach dams. At the most downstream site (11) only four species (four organisms, 16 mg m^?2) were present. The observed distributional pattern is a classic serial discontinuity in response to hypolimnial stream regulation in a temperate latitude river.     

Effects of elevated water temperature on fish and macroinvertebrate communities below small dams

Many studies have investigated the ecological changes that occur below dams that release cold, hypolimnetic water, but very few studies have looked at the effects of the release of warm, surface waters. The effect of small, surface release dams on downstream thermal regimes is a major habitat concern for many cold‐water systems, however. The objective of this study was to examine the effects of summer temperature increases due to impoundment on downstream fish and macroinvertebrate communities in cold‐water streams. We sampled fish, macroinvertebrates and habitat upstream and downstream of dams on ten rivers during the summers of 1998 and 1999. Changes in mean summer temperature downstream varied from a cooling of 1 °C to an increase of more than 5 °C. Increasing temperatures downstream coincided with lower densities of several cold‐water fish species, specifically brown trout (Salmo trutta), brook trout (Salvelinus fontinalis) and slimy sculpin (Cottus cognatus) while overall fish species richness increased downstream. Density of mottled sculpin (Cottus bairdi), another cold‐water species, was not related to temperature changes below the dams. Macroinvertebrates showed shifts in community composition below dams that increased temperature. This study provides information useful for determining the extent of impact of these small, surface release dams, which are abundant across the country. Copyright © 2003 John Wiley & Sons, Ltd.     

A simplified water temperature model for the Colorado River below Glen Canyon Dam

Glen Canyon Dam, located on the Colorado River in northern Arizona, has affected the physical, biological and cultural resources of the river downstream in Grand Canyon. One of the impacts to the downstream physical environment that has important implications for the aquatic ecosystem is the transformation of the thermal regime from highly variable seasonally to relatively constant year‐round, owing to hypolimnetic releases from the upstream reservoir, Lake Powell. Because of the perceived impacts on the downstream aquatic ecosystem and native fish communities, the Glen Canyon Dam Adaptive Management Program has considered modifications to flow releases and release temperatures designed to increase downstream temperatures. Here, we present a new model of monthly average water temperatures below Glen Canyon Dam designed for first‐order, relatively simple evaluation of various alternative dam operations. The model is based on a simplified heat‐exchange equation, and model parameters are estimated empirically. The model predicts monthly average temperatures at locations up to 421 km downstream from the dam with average absolute errors less than 0.5°C for the dataset considered. The modelling approach used here may also prove useful for other systems, particularly below large dams where release temperatures are substantially out of equilibrium with meteorological conditions. We also present some examples of how the model can be used to evaluate scenarios for the operation of Glen Canyon Dam. Published in 2008 by John Wiley & Sons, Ltd.     

Ecological consequences of hydropower development in Central America: impacts of small dams and water diversion on neotropical stream fish assemblages

Small dams for hydropower have caused widespread alteration of Central American rivers, yet much of recent development has gone undocumented by scientists and conservationists. We examined the ecological effects of a small hydropower plant (Doña Julia Hydroelectric Center) on two low‐order streams (the Puerto Viejo River and Quebradon stream) draining a mountainous area of Costa Rica. Operation of the Doña Julia plant has dewatered these streams, reducing discharge to ～10% of average annual flow. This study compared fish assemblage composition and aquatic habitat upstream and downstream of diversion dams on two streams and along a ～4 km dewatered reach of the Puerto Viejo River in an attempt to evaluate current instream flow recommendations for regulated Costa Rican streams. Our results indicated that fish assemblages directly upstream and downstream of the dam on the third order Puerto Viejo River were dissimilar, suggesting that the small dam (< 15 m high) hindered movement of fishes. Along the ～4 km dewatered reach of the Puerto Viejo River, species count increased with downstream distance from the dam. However, estimated species richness and overall fish abundance were not significantly correlated with downstream distance from the dam. Our results suggested that effects of stream dewatering may be most pronounced for a subset of species with more complex reproductive requirements, classified as equilibrium‐type species based on their life‐history. In the absence of changes to current operations, we expect that fish assemblages in the Puerto Viejo River will be increasingly dominated by opportunistic‐type, colonizing fish species. Operations of many other small hydropower plants in Costa Rica and other parts of Central America mirror those of Doña Julia; the methods and results of this study may be applicable to some of those projects. Copyright © 2006 John Wiley & Sons, Ltd.     

Impacts of dam construction on streamflows during drought periods in the Upper Colorado River Basin,Texas

The purpose of this study was to assess the impacts of dam construction on streamflow during a severe drought in the upper Colorado River basin (TX), upstream of Lake Buchanan reservoir. The region has experienced severe, prolonged droughts over the past century. To ameliorate the effects of drought, and increase water storage for use during dry periods, several dams were constructed on the mainstem and tributaries of the Colorado River upstream of Lake Buchanan since the late‐1940s. Analysis of flow at the gauge above Lake Buchanan indicated streamflow was significantly reduced during the recent drought (2009–2014), compared with streamflows during the ‘drought of record’ (1950–1957). The construction of these upstream dams reduced streamflow by intercepting and storing water. It is concluded that building reservoirs in the western portion of the basin, largely in response to past droughts, although increasing water supplies for local uses, exacerbated the downstream effects of the hydrological drought, essentially making it the worst in recorded history.     

Fish Assemblage Structure and Habitat Associations in a Large Western River System

Longitudinal gradients of fish assemblage and habitat structure were investigated in the Kootenai River of northern Idaho. A total of 43 500‐m river reaches was sampled repeatedly with several techniques (boat‐mounted electrofishing, hoop nets and benthic trawls) in the summers of 2012 and 2013. Differences in habitat and fish assemblage structure were apparent along the longitudinal gradient of the Kootenai River. Habitat characteristics (e.g. depth, substrate composition and water velocity) were related to fish assemblage structure in three different geomorphic river sections. Upper river sections were characterized by native salmonids (e.g. mountain whitefish Prosopium williamsoni), whereas native cyprinids (peamouth Mylocheilus caurinus, northern pikeminnow Ptychocheilus oregonensis) and non‐native fishes (pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens) were common in the downstream section. Overall, a general pattern of species addition from upstream to downstream sections was discovered and is likely related to increased habitat complexity and additions of non‐native species in downstream sections. Assemblage structure of the upper sections were similar, but were both dissimilar to the lower section of the Kootenai River. Species‐specific hurdle regressions indicated the relationships among habitat characteristics and the predicted probability of occurrence and relative abundance varied by species. Understanding fish assemblage structure in relation to habitat could improve conservation efforts of rare fishes and improve management of coldwater river systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Bidirectional connectivity via fish ladders in a large Neotropical river

The conservation of migratory fish species worldwide has been threatened by the loss of longitudinal connectivity caused by dams intercepting large rivers. One environmental management strategy for reestablishing connectivity is providing passage through fish ladders. However, ladders in Neotropical rivers have been described as ascending one‐way routes. We analysed the movements of Prochilodus lineatus through a fish ladder at a large dam—Porto Primavera—in the heavily impounded Upper Paraná River, Brazil, to determine whether the ladder connected habitats downstream and upstream of the dam, in both directions. A total of 1,419 specimens of P. lineatus were PIT‐tagged above and below the dam, and continuously monitored for 4 years. We documented bidirectional movements of P. lineatus through the fish ladder. Many individuals repeated these movements annually; one individual as many as six times. It was estimated that the cumulative probability that P. lineatus would return from downstream after descending through the ladder was 0.38, 0.50, and 0.56 in 1, 2, and 3 years, respectively. Correspondingly, return probabilities from upstream were 0.15, 0.22, and 0.26 in 1–3 years, respectively. Although return probabilities from upstream were roughly half, our results suggest the Porto Primavera fish ladder contributes to habitat connectivity, bidirectional passage, and preservation of P. lineatus. These results deviate from the perception that fishways are ineffective in Neotropical rivers. We suggest that fishways can restore the bidirectional connectivity denied to some Neotropical species, and until the services of dams are no longer needed, environmental management through fish ladders could continue to be considered as an integral part of broader conservation strategy designed to preserve native fauna.     

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.     

Spawning Migration of Wild and Supplementary Stocked Landlocked Atlantic Salmon (Salmo Salar)

Upstream migration by adult salmonids is impeded by dams in many regulated rivers, as is the case for landlocked Atlantic salmon, Salmo salar, in the River Klarälven, Sweden. There, the salmon cannot reach the spawning grounds due to the presence of eight dams. Hence, hatchery‐reared smolts are released downstream of the dams, and upstream migrating spawners are caught in a trap at the lowermost dam before transported by truck to the spawning grounds past the dams. To identify the spawning grounds and compare the behavior of wild and hatchery‐reared Atlantic salmon during upstream migration and spawning, 34 wild and 28 hatchery‐reared, radio‐tagged Atlantic salmon were followed during their spawning migration from August to October 2011. Half (50%) of the hatchery fish, but only 11,8% of the wild fish ended up as fallbacks, i.e. they migrated past the first downstream power station, and did not spawn. A significantly higher proportion (21.4%) of hatchery‐ reared salmon moved in an erratic way, with several up and down stream movements, when compared to the wild salmon (5.9%). When looking at the salmon that stayed in the river (exc. fallbacks), wild individuals exhibited a holding behavior (little or no movements before presumed spawning) more often (86.7%) than the reared ones (50%). The wild salmon also held position (and presumably spawned) for longer time (25.4 days) than the reared salmon (16.1 days). Reared salmon held position, on average, 10 km further upstream than wild salmon, passing the presumed best‐quality spawning habitat. The migration speed (average 17.4 km/day) between two logger stations did not differ between wild and reared fish or between sexes. Our results suggest that the reproductive success of hatchery‐reared Atlantic salmon is relatively low and their capacity as supplementary spawners to the wild population in the Klarälven, is probably small. Copyright © 2015 John Wiley & Sons, Ltd.     

The impact of cold water releases on the critical period of post‐spawning survival and its implications for Murray cod (Maccullochella peelii peelii): a case study of the Mitta Mitta River,southeastern Australia

The effects of cold water releases, as a by‐product of storing irrigation water in large dams, has been a source of great concern for its impact on native freshwater fish for some time. The Mitta Mitta River, northeast Victoria, is impacted by altered thermal regimes downstream of the fourth largest dam in Australia, Dartmouth dam, with some daily temperatures 10–12°C below normal. Murray cod (Maccullochella peelii peelii) were endemic to the Mitta Mitta River; however, resident Murray cod have not been found in this river since 1992. The response of eggs and hatched larvae from Murray cod to different temperature gradients of water were measured and the post‐spawning survival recorded. As a case study, post‐spawning survival was then inferred from flow data for each year of operation of Dartmouth Dam, recorded since first operation in 1978, and included in a stochastic population model to explore the impact of the altered (historical) thermal regime on population viability. Experimental results revealed no egg and larval survival below 13°C and predicted historical temperature regimes point to more than 15 years of low temperatures in the Mitta Mitta River. Population modelling indicates that the impact of cold water releases on post‐spawning survival is a significant threatening process to the viability of a Murray cod population. Additionally, we consider changes to the thermal regime to explore how the thermal impact of large dams may be minimized on downstream fish populations through incrementally increasing the temperature of the releases. The modelled Murray cod population responds to minor increases in the thermal regime; however, threats are not completely removed until an increase of at least 5–6°C. Copyright © 2005 John Wiley & Sons, Ltd.     

Changes to the trophic structure of a desert fish community following river regulation and species turnover: Implications for an endangered top predator

A poor understanding of factors leading to species decline can result in inefficient or ineffective species restoration. Endangered Colorado pikeminnow (Ptychocheilus lucius) was nearly extirpated from the San Juan River, NM, USA and recent efforts to reestablish the species via hatchery augmentation of juveniles has yet to reach the target number of adults in the system. To assess how changes to the river's food web could be limiting reestablishment of this top predator, we used stable isotope (δ¹³C and δ¹⁵N) signatures of the fish community pre- (museum specimens) and post river regulation with coincident extirpations and invasions. Following river regulation, four of five community-wide trophic structure metrics that quantify overall resource use were reduced and species turnover likely had little effect per se. For species sampled in both time periods, shared trophic resources generally increased (mean niche overlap = +35.7%). Additionally, Colorado pikeminnow experienced the largest decrease in niche breadth (−72%) and diet mixing models suggested a shift from piscivory towards insectivory in contemporary collections. Our results suggest an overall reduction in basal resource availability after river regulation. We propose increased reliance on similar resources may be limiting fish prey for Colorado pikeminnow, ultimately contributing to the slow and limited reestablishment in the San Juan River. This study adds to the growing body of knowledge provided by museum specimens and stable isotope analyses to identify food-web dynamics that are a departure from historical conditions, which can provide novel information critical to imperiled species management in modified systems.     

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.     

A physical habitat model for predicting the effects of flow fluctuations in nursery habitats of the endangered Colorado pikeminnow (Ptychocheilus lucius)

Larval and juvenile Colorado pikeminnow (Ptychocheilus lucius) use shallow, low‐velocity, channel‐margin areas (backwaters) as nursery habitats. It is hypothesized that within‐day flow fluctuations caused by hydropower operations can directly affect the suitability of such habitats by altering water temperature and habitat geometry. Despite the importance of backwaters to juvenile fishes, there is a lack of established approaches for modelling how river management affects these habitats. Here, we describe a physical habitat model that predicts the effects of mainstem flow variation on backwater temperature, geometry and invertebrate availability. We specifically modelled these effects on habitat in a portion of the Green River in Utah below Flaming Gorge Dam. The overall model combines a cell‐based model of backwater geometry, a pond‐based temperature model and a model of invertebrate production. Results from a series of simulations suggest that the most important biological effects of within‐day flow fluctuations are likely to be those associated with the availability of invertebrate prey including (1) minimum wetted area, (2) the proportion of the backwater's volume exchanged with the mainstem, and, to a lesser degree, (3) backwater temperature. Taken together, such effects could have important implications for the growth and survival of juvenile fish when flow fluctuations are sufficiently large. Copyright © 2006 John Wiley & Sons, Ltd.     

Simulating effects of hydro‐dam alteration on thermal regime and wild steelhead recruitment in a stable‐flow Lake Michigan tributary

Hydroelectric dams may affect anadromous fish survival and recruitment by limiting access to upstream habitats and adversely affecting quality of downstream habitats. In the Manistee River, a tributary to Lake Michigan, two hydroelectric dams potentially limit recruitment of anadromous rainbow trout (steelhead) by increasing tailrace water temperatures to levels that significantly reduce survival of young‐of‐year (YOY) fish. The objectives of this study were to determine whether proposed restoration scenarios (dam removals or a bottom withdrawal retrofit) would alter the Manistee River thermal regime and, consequently, improve wild steelhead survival and recruitment. Physical process models were used to predict Manistee River thermal regimes following each dam alteration scenario. Empirical relationships were derived from historical field surveys to quantify the effect of temperature on YOY production and potential recruitment of Manistee River steelhead. Both dam alteration scenarios lowered summer temperatures and increased steelhead recruitment by between 59% and 129%, but total recruitments were still low compared to other Great Lakes tributaries. Considering only temperature effects, bottom withdrawal provides the greatest promise for increasing natural steelhead recruitment by decreasing the likelihood of year‐class failures in the warmest summers. Results of this study may allow managers to evaluate mitigation alternatives for Manistee River dams during future relicensing negotiations, and illustrate the utility of physical process temperature models in groundwater‐fed rivers. Copyright © 2004 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.     

Aquatic habitat change in the Arkansas river after the development of a lock‐and‐dam commercial navigation system

The McClellan–Kerr Arkansas River Navigation System (MKARNS), completed in 1971, required the construction of 17 locks and dams and associated navigation works to make the Arkansas and Verdigris Rivers navigable for barge traffic from the Mississippi River to Catoosa, Oklahoma. We used a Geographic Information System to assess habitat changes in the 477‐km portion of this system within Arkansas from 1973 to 1999. Total aquatic area declined by 9% from 42 404 to 38 655 ha. Aquatic habitat losses were 1–17% among pools. Greatest habitat losses occurred in diked secondary channels (former secondary channels with flow reduced by rock dikes) and backwaters adjacent to the main channel. Most of the area of dike pools (aquatic habitat downstream of rock dikes), diked secondary channels and adjacent backwaters were <0.9 m deep. Copyright © 2008 John Wiley & Sons, Ltd.     

Spatial patterns of fish communities in the Upper Mississippi River System: assessing fragmentation by low‐head dams

We assessed the similarity of fish communities among river reaches to assess community‐level fragmentation by low‐head dams in the Upper Mississippi River System (UMRS). The spatial coverage of standardized electrofishing sampling used in the Long Term Resource Monitoring Program (LTRMP) was extended for three of the six regional trend areas (RTA; pools 4, 13, and the Open River Reach) to include river reaches (outpools) immediately upstream and downstream from the standard RTA from 15 June to 31 October 2000. Additionally, pools 19 and 20 were sampled in September 2000. Cluster analysis and non‐metric multidimensional scaling of community composition and structure data revealed two major groups, upper and lower reaches, and four (for composition) or five (for structure) sub‐groupings of river reaches. In general, all outpools grouped with the nearest RTA for both community composition (no exception) and community structure (one exception). This suggests that fragmentation of fish communities from low‐head dams is minimal. Mantel correlations demonstrated strong inverse association between the similarity of fish communities with the distance between reaches. Habitat variables measured during electrofishing collections were significantly correlated with spatial variation of fish composition and community structure, but provided only marginal improvements to correlations with distance between reaches alone. Furthermore, habitat variability among river reaches also was related to distance between reaches. Determining the extent to which variation of fish communities is related to habitat or demographic processes (e.g. migration, larval drift, source‐sink dynamics) will be challenging for this system. Although low‐head dams on the UMRS may restrict movements for individuals and populations of certain fish species, we found little evidence that these effects have led to substantial, community‐level fragmentation. Copyright © 2006 John Wiley & Sons, Ltd.