The importance of fluvial hydraulics to fish-habitat restoration in low-gradient alluvial streams

• 1 A major cause of degradation and loss of stream fish is alteration of physical habitat within and adjacent to the channel. We describe a potentially efficient approach to fish restoration based upon the relationship between fluvial hydraulics, geomorphology, and those habitats important to fish.
• 2 The aquatic habitat in a low-gradient, alluvial stream in the Ozark Plateaus physiographical province was classified according to location in the channel, patterns of water flow, and structures that control flow. The resulting habitat types were ranked in terms of their temporal stability and ability to be manipulated.
• 3 Delineation and quantification of discrete physical spaces in a stream, termed hydraulic habitat units, are shown to be useful in stream restoration programmes if the ecological importance of each habitat unit is known, and if habitats are defined by fluvial dynamics so that restoration is aided by natural forces.
• 4 Examples, using different taxa, are given to illustrate management options.

     

Effects of travertine and flow on leaf retention in Fossil Creek,Arizona

Leaf retention is important in transferring energy from riparian trees to stream food webs. Retention increases with geomorphic complexity such as substrate coarseness, sinuosity, and the presence of debris dams. High discharge can reduce retention, particularly when streams lack physical trapping features. Travertine formations, caused by calcium carbonate deposition, can alter stream morphology. To date, however, we know of no study testing the effect of travertine on leaf retention. This study capitalized on a river restoration project in Fossil Creek, Arizona, where water was returned to the channel after a century of diversion. We examined how the fixed factors Flow (before and after restoration) and Morphology (travertine and riffle-pool sites) affected leaf retention. Leaf retention was higher in sites where travertine forms barriers across the river, relative to sites with riffle-pool morphology. Most leaves retained in travertine reaches were concentrated at the bottom of pools formed between dams. Although flow restoration did not alter retention rates across all sites, it diminished them at travertine sites, indicating an interaction between stream flow and morphology. We conclude that stream complexity and leaf retention are enhanced by travertine deposition but that high discharge can reduce the retentive capacity of in-stream structures. Handling editor: Darren Ryder     

The importance of physical habitat assessment for evaluating river health

1. Physical habitat is the living space of instream biota; it is a spatially and temporally dynamic entity determined by the interaction of the structural features of the channel and the hydrological regime. 2. This paper reviews the need for physical habitat assessment and the range of physical habitat assessment methods that have been developed in recent years. These methods are needed for assessing improvements made by fishery enhancement and river restoration procedures, and as an intrinsic element of setting environmental flows using instream flow methods. Consequently, the assessment methods must be able to evaluate physical habitat over a range of scales varying from the broad river segment scale (up to hundreds of kilometres) down to the microhabitat level (a few centimetres). 3. Rapid assessment methods involve reconnaissance level surveys (such as the habitat mapping approach) identifying, mapping and measuring key habitat features over long stretches of river in a relatively short space of time. More complex appraisals, such as the Physical Habitat Simulation System (PHABSIM), require more detailed information on microhabitat variations with flow. 4. Key research issues relating to physical habitat evaluation lie in deciding which levels of detail are appropriate for worthwhile yet cost-effective assessment, and in determining those features that are biologically important and hence can be considered habitat features rather than simple geomorphic features. 5. The development of new technologies particularly relating to survey methods should help improve the speed and level of detail attainable by physical habitat assessments. These methods will provide the necessary information required for the development of the two-and three-dimensional physical and hydraulic habitat models. 6. A better understanding of the ways in which the spatial and temporal dynamics of physical habitat determine stream health, and how these elements can be incorporated into assessment methods, remains a key research goal.     

Simple measures of channel habitat complexity predict transient hydraulic storage in streams

Stream thalweg depth profiles (along path of greatest channel depth) and woody debris tallies have recently become components of routine field procedures for quantifying physical habitat in national stream monitoring efforts. Mean residual depth, standard deviation of thalweg depth, and large woody debris (LWD) volumes are potential metrics of habitat complexity calculated from these survey data. We used 42 intensive dye-transit studies to demonstrate the relevance of these easily measured channel habitat complexity metrics to transient hydraulic (“dead zone”) storage, a channel process important for biotic habitat as well as retention and “spiraling” of dissolved and particulate nutrients. We examined transient storage and channel morphology in small gravel and cobble-bedded upland streams (wetted width 2–5 m; slopes 2.6–8.3%) representing a wide range of flow stages, LWD loading, and channel complexity, including measurements before and after LWD was added to enhance fish habitat. While transient storage volume fraction decreased as flow stage increased in simple channels, those with complex morphology and well-developed riparian vegetation maintained high transient storage fractions even during storm flows. LWD additions increased transient storage and channel complexity over the 2 years of post-treatment measurements. We predict with considerable precision two different formulations of transient hydraulic storage fraction using single-variable linear regressions on residual depth (R ² = 0.61–0.89), thalweg depth variance (R ² = 0.64–0.91), or large woody debris volume (R ² = 0.48–0.74). Demonstration of these likely causal associations contributes to understanding the process of transient storage and redefines the use of thalweg profile metrics as a new approach to quantifying morphologic and hydraulic complexity in streams.     

Effects of woody debris on the habitat of juvenile masu salmon (Oncorhynchus masou) in northern Japanese streams

1. The effects of woody debris on stream habitat of juvenile masu salmon ( Oncorhynchus masou ) were examined at two spatial scales, stream reach and channel unit, for first to thirdorder tributaries of the Teshio River in northern Hokkaido, Japan. The fortyeight study reaches were classified into three distinct types: coarsesubstrate steppool (CSP), coarsesubstrate poolriffle (CPR) and finesubstrate poolriffle (FPR) reaches. Each reach type included reaches with different riparian settings, broadly classified as forest (relatively undisturbed forest and secondary forest after fires) or grassland (bamboo bushland and pasture).
2. The reachscale analyses showed that neither total pool volume nor pooltopool spacing was correlated with woody debris abundance in any of the three reach types. Masu salmon density was positively correlated with both woodydebris cover area and total cover area, but not with total pool volume in the reaches.
3. Channelunitscale analyses revealed that woody debris reduced nonpool velocity, increased pool depth and retained fine sediment in pools in FPR reaches, where the size of woody debris was very large relative to the substrate material size. However, woody debris did not influence any of the hydraulic variables (depth, velocity, substrate) in either nonpools or pools of CSP and CPR reaches. Habitat use by masu salmon in nonpools or pools was affected by woodydebris cover area or total cover area rather than by hydraulic variables in any of the reach types.
4. The effects of woody debris on habitat at the reach and channelunit scales in the study area were less than those indicated by previous work in the Pacific Northwest, North America, owing to the relatively small size of the riparian trees. However, the overall results suggested that woody debris in the study area contributed to masu salmon habitat by providing cover at the smaller, microhabitat scale.     

Response of fishes and aquatic habitats to sand-bed stream restoration using large woody debris

Effects of habitat rehabilitation of Little Topashaw Creek, a sinuous, sand-bed stream draining 37 km² in northwest Mississippi are described. The rehabilitation project consisted of placing 72 large woody debris structures along eroding concave banks and planting 4000 willow cuttings in sandbars. Response was measured by monitoring flow, channel geometry, physical aquatic habitat, and fish populations. Initially, debris structures reduced high flow velocities at concave bank toes, preventing further erosion and inducing deposition. Physical response during the first year following construction included creation of sand berms along eroding banks and slight increases in base flow water width and depth. Fish collections showed assemblages typical of incising streams within the region, but minor initial responses to debris addition were evident. Progressive failure of the structures and renewed erosion were observed during the second year after construction.     

Spatio-temporal effects of river regulation on habitat quality for Atlantic salmon fry

Many upland rivers in the Northern Hemisphere contain important habitat for Atlantic salmon (Salmo salar L.). Owing to their sensitivity to environmental change, salmon are often used as bio-indicators. In Scotland, rivers containing potentially suitable habitat for salmon fry are often also regulated for hydropower. Regulated flow regimes can differ substantially spatially and temporally. Thus, where river management may be needed to maintain, restore, and protect their ecological functioning, this needs to be based on evidence of such spatio-temporal effects. This study investigated the effects of different types of river regulation on the hydraulic characteristics of downstream river reaches and the inferred consequences for salmon fry using hydraulic habitat quality models. The study focussed on the River Lyon (390 km²), a tributary of the Tay (4587 km²), Scotland, UK. Hydraulic habitat variability was assessed for three reach-scale sites with contrasting flow regimes characterised by (a) releases from hydropower generation, (b) compensation flow and (c) partly re-naturalised flow conditions. For each site, high resolution Digital Terrain Models (DTMs) were developed from bathymetric surveys and 2D hydraulic models were used to assess hydraulic characteristics. Discharge time series were used to simulate hydraulic conditions for regulated and simulated natural flows. Depth and velocity data were extracted from the hydraulic models and used to infer habitat quality using a habitat model developed for Atlantic salmon fry in similar-sized Scottish rivers. Results showed the effects of regulation can vary substantially within reaches and between seasons. Comparison to natural flow regimes suggested that flow alteration has a variable influence on habitat quality depending on the type of regulation and time of year. This work has improved understanding of the effects of regulation on biophysical processes and may also be useful for managing trade-offs between management, restoration, and societal benefits.     

Exploration and rehabilitation of hydraulic habitats in streams using principles of fluvial behaviour

• 1 The combination of elements from geomorphology, open-channel hydraulics, and hydraulic habitat requirements of stream fish forms the basis for an ecologically sound ‘soft engineering’ of river channels.
• 2 Interpreting and mapping the hydraulic geometry of streams and locally varied flow conditions can be accomplished with plane table surveys and customized field-data sheets. This information can serve to manage hydraulic habitats preferred by fish.
• 3 The use of fluvial characteristics to design preferred hydraulic habitats is illustrated in two examples: (i) a walleye (Stizostedion vitreum vitreum Mitchill) spawning rehabilitation project undertaken in a stream channelized as a lowland drainage canal, and (ii) a trout (Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum) habitat-enhancement project to create additional holding and resting areas for adult fish in a stream paved with glacially deposited boulders modified by a road crossing.
• 4 In both examples the ‘soft engineering’ of the river channels enhanced the hydraulic fish habitat.

     

Rehabilitation of aquatic habitats in warmwater streams damaged by channel incision in Mississippi

Channel incision has major impacts on stream corridor ecosystems, leading to reduced spatial habitat heterogeneity, greater temporal instability, less stream-floodplain interaction, and shifts in fish community structure. Most literature dealing with channel incision examines physical processes and erosion control. A study of incised warmwater stream rehabilitation was conducted to develop and demonstrate techniques that would be economically feasible for integration with more orthodox, extensively employed watershed stabilization techniques (e.g., structural bank protection, grade control structures, small reservoirs, and land treatment). One-km reaches of each of five northwest Mississippi streams with contributing drainage areas between 16 and 205 km2 were selected for a 5-year study. During the study two reaches were modified by adding woody vegetation and stone structure to rehabilitate habitats degraded by erosion and channelization. The other three reaches provided reference data, as two of them were degraded but not rehabilitated, and the third was only lightly degraded. Rehabilitation approaches were guided by conceptual models of incised channel evolution and fish community structure in small warmwater streams. These models indicated that rehabilitation efforts should focus on aggradational reaches in the downstream portions of incising watersheds, and that ecological status could be improved by inducing formation and maintenance of stable pool habitats. Fish and physical habitat attributes were sampled from each stream during the Spring and Fall for 5 years, and thalweg and cross-section surveys were performed twice during the same period. Rehabilitation increased pool habitat availability, and made the treated sites physically more similar to the lightly degraded reference site. Fish communities generally responded as suggested by the aforementioned conceptual model of fish community structure. Species composition shifted away from small colonists (principally cyprinids and small centrarchids) toward larger centrarchids, catostomids, and ictalurids. Fish density and species richness increased at one rehabilitated site but remained stable at the other, suggesting that the sites occupied different initial states and endpoints within the conceptual model, and differed in their accessibility to sources of colonizing organisms. These experiments suggest that major gains in stream ecosystem rehabilitation can be made through relatively modest but well-designed efforts to modify degraded physical habitats. This revised version was published online in July 2006 with corrections to the Cover Date.     

The boundaries of river systems: the metazoan perspective

1. This overview of metazoans associated with the riparian/groundwater interface focuses on the fauna inhabiting substratum interstices within the stream bed and in alluvial aquifers beneath the floodplain. The objective is to integrate knowledge of habitat conditions and ecology of the interstitial fauna into a broad spatiotemporal perspective of lotic ecosystems. 2. Most aquatic metazoans of terrestrial ancestry, secondarily aquatic forms including insects and water mites (Hydracarina), are largely confined to surface waters (epigean), most of the time penetrating only the superficial interstices of the stream bed. 3. Primary aquatic metazoans include crustaceans and other groups whose entire evolutionary histories took place in water. Some species are epigean, whereas other members of the primary aquatic fauna are true subterranean forms (hypogean ) , residing deep within the stream bed and in alluvial aquifers some distance laterally from the channel. 4. The hypogean/epigean affinities of interstitial animals are reflected in repetitive gradients of species distribution patterns along vertical (depth within the stream bed), longitudinal (riffle/pool), and lateral (across the floodplain) spatial dimensions, as well as along recovery trajectories following floods (temporal dimension). 5. Fluvial dynamics and sediment characteristics interact to determine hydraulic conductivity, oxygen levels, pore space, particle size heterogeneity, organic content and other habitat conditions within the interstitial milieu. 6. Multidimensional environmental gradients occur at various scales across riparian/groundwater boundary zones. The spatiotemporal variability of hydrogeomorphological processes plays an important role in determining habitat heterogeneity, habitat stability, and connectivity between habitat patches, thereby structuring biodiversity patterns across the riverine landscape. 7. The erosive action of flooding maintains a diversity of hydrarch and riparian successional stages in alluvial floodplains. The patchy distribution patterns of interstitial communities at the floodplain scale reflect, in part, the spatial heterogeneity engendered by successional processes. 8. Interstitial metazoans engage in passive and active movements between surface waters and ground waters, between aquatic and riparian habitats, and between different habitat types within the lotic system. Some of these are extensive migrations that involve significant exchange of organic matter and energy between ecosystem compartments. 9. The generally high resilience of lotic ecosystems to disturbance is attributable, in part, to high spatiotemporal heterogeneity. Habitat patches less affected by a particular perturbation may serve as ’refugia ‘; from which survivors recolonize more severely affected areas. Mechanisms of refugium use may also occur within habitats, as, for example, through ontogenetic shifts in microhabitat use. Rigorous investigations of interstitial habitats as refugia should lead to a clearer understanding of the roles of disturbance and stochasticity in lotic ecosystems. 10. Development of realistic ’whole river ‘; food webs have been constrained by the exclusion of interstitial metazoans, which may in fact contribute the majority of energy flow in lotic ecosystems. A related problem is failure to include groundwater/riparian habitats as integral components of alluvial rivers. A conceptual model is presented that integrates groundwater and riparian systems into riverine food webs and that reflects the spatiotemporal complexity of the physical system and connectivity between different components. 11. Interstitial metazoans also serve as ’ecosystem engineers, ‘; by influencing the availability of resouces to other species and by modifying habitat conditions within the sediment. For example, by grazing on biofilm, interstitial animals may markedly stimulate bacterial growth rates and nutrient dynamics. 12. Although there has been a recent surge of interest in the role of interstitial animals in running waters, the knowledge gaps are vast. For example, basic environmental requirements of the majority of groundwater metazoans remain uninvestigated. Virtually nothing is known regarding the role of biotic interactions in structuring faunal distribution patterns across groundwater/riparian boundary zones. Interstitial metazoans may contribute significantly to the total productivity and energy flow of the biosphere, but such data are not available. Nor are sufficient data available to determine the contribution of groundwater animals to estimates of global biodiversity. 13. Effective ecosystem management must include groundwater/riparian ecotones and interstitial metazoans in monitoring and restoration efforts. Evidence suggests that a ’connected ‘; groundwater/riparian system provides natural pollution control, prevents clogging of sediment interstices and maintains high levels of habitat heterogeneity and successional stage diversity. River protection and restoration should maintain or re-establish at least a portion of the natural fluvial dynamics that sustains the ecological integrity of the entire riverine–floodplain–aquifer ecosystem. Keywords: groundwater/riparian ecotones, hyporheic habitat, epigean, hypogean, interstitial fauna, biodiversity, food webs     

Quantifying Macroinvertebrate Responses to In-Stream Habitat Restoration: Applications of Meta-Analysis to River Restoration

The assumption that restoring physical habitat heterogeneity will increase biodiversity underlies many river restoration projects, despite few tests of the hypothesis. With over 6,000 in-stream habitat enhancement projects implemented in the last decade at a cost exceeding $1 billion, there is a clear need to assess the consistency of responses, as well as factors explaining project performance. We adopted an alternative approach to individual case-studies by applying meta-analysis to quantify macroinvertebrate responses to in-stream habitat restoration. Meta-analysis of 24 separate studies showed that increasing habitat heterogeneity had significant, positive effects on macroinvertebrate richness, although density increases were negligible. Large woody debris additions produced the largest and most consistent responses, whereas responses to boulder additions and channel reconfigurations were positive, yet highly variable. Among all strategies, the strength and consistency of macroinvertebrate responses were related to land use or watershed-scale conditions, but appeared independent of project size, stream size, or recovery time. Overall, the low quality and quantity of pre- and post-project monitoring data reduced the robustness of our meta-analysis. Specifically, the scope and strength of conclusions regarding the ubiquity of macroinvertebrate responses to restoration, as well as the identification of variables controlling project performance was limited. More robust applications of meta-analysis to advance the science and practice of river restoration will require implementing rigorous study designs, including pre- and post-project monitoring replicated at both restored and control sites, collection of abiotic and biotic variables at relevant spatiotemporal scales, and increased reporting of monitoring results in peer-reviewed journals and/or regional databases.     

Habitat preference by grayling (Thymallus thymallus) in an artificially modified, hydropeaking riverbed: a contribution to understand the effectiveness of habitat enhancement measures

This paper describes a case study to rehabilitate habitat for adult European grayling (Thymallus thymallus L.) in a large river reservoir in northern Finland. A channelled river reach was restored by building small islands and reefs as well as cobble and boulder structures for grayling. The total area of the restored stretch was 1.0 ha. The physical habitat was mapped using an echosounder, Doppler device, tachometer and scuba diving, and modelled with a 2D hydraulic model. The mean water velocity in the modelled stream section was 0.28 m s^?1 during 110 m³ s^?1 flow and 0.43 m s^?1 during 300 m³ s^?1 flow. Twelve adult grayling, tagged with transmitters, were released into the area and tracked for a maximum period of 30 days. The grayling largely stayed in the restored area and tended to avoid the unchanged channel of the river. The range of daily movement was from stationary to 2700 m per day. The adult grayling preferred water velocities between 0.20 and 0.45 m s^?1, water depths between 0.20 and 1.55 m and coarse substrate. The study provides a small part of the information needed in habitat restoration for grayling.     

Estimation of transport distance of fine particulate organic matter in relation to channel morphology in tailwaters of the Lake Biwa and reservoir dams

Knowledge on transport and deposition of fine particulate organic matter (FPOM) from reservoir dams is increasingly required for habitat management and restoration of dam tailwater ecosystems. Variations in the transport distance of FPOM, however, have never been studied well, particularly in relation to channel morphology, due to channel size restrictions of artificial tracers such as corn pollen when applied to larger river channels. This study aims to show the relations between FPOM retention efficiency and channel morphology in dam tailwaters using lentic plankters as tracers. We estimated the mean transport distance, S _p, by calculating downstream reduction ratios of lentic tracer plankters and calculated the deposition velocity, v _dep. Suspended FPOM samples were collected in tailwaters of two river channels below reservoir dams and two artificial canals below Lake Biwa in the Yodo River system. The longest S _p (19.2 km) and the shortest one (2.2 km) were recorded in the deep canal and shallow canal, respectively, showing a positive correlation with channel hydraulic radius. The values of v _dep were 4.7–6.4 times higher in river channels than in artificial canals. These results indicate that increasing complexity of bed morphology can minimize S _p, whereas bed degradation and armored bed materials may lead to increased S _p. Advantages of lentic plankters as tracers for estimating distance ranges of reservoir dam impact on river ecosystems are also discussed.     

Disturbance and patch-specific responses: the interactive effects of woody debris and floods on lotic invertebrates

Disturbance may play an important role in generating patterns of abundance and distribution of biotic assemblages, particularly if its impact differs among habitat patches. Despite much speculation concerning the probable importance of spatial variation in the response of stream fauna to flooding, empirical work on patch-specific responses to spates is largely lacking. Floods typically reduce the abundance of lotic invertebrates dramatically in open-channel areas. We conducted a set of experiments to determine if faunal abundances are less affected in patches more sheltered due to the presence of woody debris dams. Specifically, we tested two hypotheses using chironomids and copepods living in a warmwater, 4th order stream: (1) the effect of flooding on the fauna varies between patches associated with debris dams versus the open channel, and (2) the absence of woody debris in a stream impedes faunal recovery throughout the channel following floods. We tested the first hypothesis by quantifying faunal abundances prior to, during, and following two floods in four patch types: mid-channel sandy patches distant from dams, coarse sediments associated with dams, fine sediments associated with dams, and leafy debris in dams. The second hypothesis was tested by removing all of the woody debris from two stretches of the stream and comparing the impact of a flood on fauna in debris-removed versus control stretches. Across all of the eight study dams, there were patchspecific faunal responses to two floods. Removal of woody debris from the stream did not prevent faunal recovery throughout the channel; however, the presence of woody debris dams did confer greater resistance of fauna to floods (as measured by no decrease in abundance during flooding) in two patch types. Abundances of chironomids and, to a lesser extent, copepods in the leafy debris of dams and in fine sediment patches associated with some dams either did not change or increased during floods, despite the fact that abundances in the dominant patch type of the stream (the sandy mid-channel) were reduced by 75–95%. All instances of faunal increase were limited to fine sediment patches associated with dams, thus entire dams cannot be labeled as flow refugia per se. Statistically, we distinguished fine patches which accumulated animals during floods from the other fine patches based on two physical attributes. Patches accumulating animals were all characterized by low water flux and nearbed flow, which likely contributed to the retention and/or passive deposition of animals. Whole dam attributes (e.g. dam size or complexity) were not useful in predicting which of the dams would accumulate animals in their fine sediments during flooding. Although structural complexity — here in the form of wood and leafy debris — is clearly important in generating biotic pattern in many ecosystems, our work underscores the need to understand what processes are responsible for the link between physical structure and biotic pattern.     

Re-Meandering of Lowland Streams: Will Disobeying the Laws of Geomorphology Have Ecological Consequences?

We evaluated the restoration of physical habitats and its influence on macroinvertebrate community structure in 18 Danish lowland streams comprising six restored streams, six streams with little physical alteration and six channelized streams. We hypothesized that physical habitats and macroinvertebrate communities of restored streams would resemble those of natural streams, while those of the channelized streams would differ from both restored and near-natural streams. Physical habitats were surveyed for substrate composition, depth, width and current velocity. Macroinvertebrates were sampled along 100 m reaches in each stream, in edge habitats and in riffle/run habitats located in the center of the stream. Restoration significantly altered the physical conditions and affected the interactions between stream habitat heterogeneity and macroinvertebrate diversity. The substrate in the restored streams was dominated by pebble, whereas the substrate in the channelized and natural streams was dominated by sand. In the natural streams a relationship was identified between slope and pebble/gravel coverage, indicating a coupling of energy and substrate characteristics. Such a relationship did not occur in the channelized or in the restored streams where placement of large amounts of pebble/gravel distorted the natural relationship. The analyses revealed, a direct link between substrate heterogeneity and macroinvertebrate diversity in the natural streams. A similar relationship was not found in either the channelized or the restored streams, which we attribute to a de-coupling of the natural relationship between benthic community diversity and physical habitat diversity. Our study results suggest that restoration schemes should aim at restoring the natural physical structural complexity in the streams and at the same time enhance the possibility of re-generating the natural geomorphological processes sustaining the habitats in streams and rivers. Documentation of restoration efforts should be intensified with continuous monitoring of geomorphological and ecological changes including surveys of reference river systems.     

Ontogenetic patterns of habitat use by fishes within the main channel of an Australian floodplain river

The ontogenetic patterns of habitat use by a community of fishes in the main channel of the Broken River, an Australian lowland river, was investigated. Stratified sampling was conducted fortnightly across six habitat types throughout the spring‐summer period within the main channel. As predicted by the 'low flow recruitment hypothesis', backwaters and still littoral habitats were important nursery habitats for most species. These habitats were found to be used by some species throughout all stages of their life cycle, while other species showed clear ontogenetic shifts in habitat preference. Only one species, Murray cod Maccullochella peelii peelii , was never found in backwaters. This study confirms the significance of main channel habitats in the rearing of larvae of some riverine fish species, and emphasizes the importance of considering the habitat requirements of all stages of a fish's life cycle in the management and restoration of rivers and streams.     

River beads as a conceptual framework for building carbon storage and resilience to extreme climate events into river management

River beads refer to retention zones within a river network that typically occur within wider, lower gradient segments of the river valley. In lowland, floodplain rivers that have been channelized and leveed, beads can also be segments of the river in which engineering has not reduced lateral channel mobility and channel-floodplain connectivity. Decades of channel engineering and flow regulation have reduced the spatial heterogeneity and associated ecosystem functions of beads occurring throughout river networks from headwaters to large, lowland rivers. We discuss the processes that create and maintain spatial heterogeneity within river beads, including examples of beads along mountain streams of the Southern Rockies in which large wood and beaver dams are primary drivers of heterogeneity. We illustrate how spatial heterogeneity of channels and floodplains within beads facilitates storage of organic carbon; retention of water, solutes, sediment, and particulate organic matter; nutrient uptake; biomass and biodiversity; and resilience to disturbance. We conclude by discussing the implications of river beads for understanding solute and particulate organic matter dynamics within river networks and the implications for river management. We also highlight gaps in current understanding of river form and function related to river beads. River beads provide an example of how geomorphic understanding of river corridor form and process can be used to restore retention and resilience within human-altered river networks.     

Stream channel geomorphology influences mussel abundance in southern Appalachian streams, U.S.A.

1. Studies of North American streams have shown that hydraulic parameters and stream geomorphology can explain unionid mussel abundance at both the reach and catchment scale. However, few studies have examined applicability of hydrogeomorphic variables across broader spatial scales, such as across whole catchments, or have elucidated conditions under which spates can affect mussel populations in streams. 2. We quantified freshwater mussel abundance and species richness and their physical habitat at 24 sites in eight streams in southern Appalachian catchments in 2000 and 2001. In addition, we modelled site‐specific hydraulic parameters during summer baseflow and bankfull stages to estimate high‐ and low‐discharge conditions, respectively. 3. Mussel abundance was related to stream geomorphology, whereas richness was related to stream size. Baseflow habitat parameters explained only minor variation in abundance or richness, and both measures were highly correlated with mean current velocity or stream size. Bankfull shear stress composed a relatively low proportion of overall mussel habitat variability, but it accounted for significant variation in abundance and richness. 4. Mussel abundance was highly variable at sites subject to low‐shear stress during spates, whereas abundance always was low at sites subject to high‐shear stress. These data suggest that habitat conditions during floods, rather than those at summer baseflow, limit the abundance of mussels in Appalachian streams. These data also suggest that mussel abundance and assemblage structure may be sensitive to any changes in channel geomorphology and hydraulic conditions that might result from land use in the catchment.     

Influences of Woody Debris on Flow Patterns and Channel Morphology in a Low Energy,Sand‐Bed Stream Reach

In lowland areas, such as the glacial landscapes of eastern Germany, sand‐bed streams are the most common stream type. They have low gradients and their hydrological regime is often subdued due to the frequent interruption by lakes. Very few is known about the influence of woody debris in these streams, since nearly all previous studies are from high‐gradient conditions, where streams have coarse bed sediments and harsh hydrological regimes. The research objectives of this study were first to assess the quasi‐natural quantity and quality of wood in a lowland sand‐bed stream and second to understand the influence of wood on the channel morphology and the flow patterns at base‐flow. The three‐dimensional stream bed relief was surveyed by electronic distance measurement. The position and the size of large woody debris was assessed by close‐up photography. An acoustic Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographic Information System. The standing stock of wood was 1.9 m³ and 39 woody elements per 100 m² of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed deflected flow and locally caused strong secondary current, high turbulence, and scour of the stream bed at baseflow. Wood resting directly on the stream bed, which contributed the majority of the wood inside the bank‐full channel, determined the roughness of the stream bed. Near‐bed flow patterns observed were isolated roughness flow and wake interference flow, which was registered inside the accumulations of wood. 68% of the stream bed had shear stress above critical. Hence, the secondary morphological structures of the sand‐bed were controlled at base‐flow by the flow which was determined by the woody debris distribution.     

Success of Instream Habitat Structures After a 50-Year Flood in Gold Creek, Montana

Gold Creek, in western Montana, lost complexity and diversity of fish habitat following riparian logging activities, removal of instream wood, and subsequent scouring. In the 4.8-km study area, the stream was almost totally void of large woody debris (4.2 pieces/km) and associated pools (1.3 pools/km). We constructed 66 structures made of natural materials (rock and wood) that resulted in 61 new pools in the study area in an attempt to restore salmonid habitat in the fall of 1996. An estimated 50-year recurrence interval flood occurred in the following spring. Of the original 66 structures, 55 (85%) remained intact and stable. Laterally confined reaches retained significantly more pools than laterally extended reaches. Owing to a history of anthropogenic impacts in forested streams in the intermountain west, restoration efforts are needed. If instream structures are tailored to specific morphologic channel types, fish habitat restoration can be successful and withstand major floods.