ECOLOGICAL FUNCTIONS OF FISH BYPASS CHANNELS IN STREAMS: MIGRATION CORRIDOR AND HABITAT FOR RHEOPHILIC SPECIES

The introduction of weirs into stream ecosystems resulted in modifications of serial continuity and in the decline of riverine fish species. Successful river restoration requires information on the ecological functionality of fish bypass channels that are considered an ecological improvement according to the European Water Framework Directive. In this study, we compared the functionality of three nature‐oriented fish passes as compensatory habitats and migration corridors for fishes. Fish passes differed significantly from upstream and downstream reaches of the weirs, revealing higher current speed, lower water depth, smaller channel width and greater habitat variability. Following these structural differences, they provided key habitats for juvenile, small and rheophilic fishes that are typically underrepresented in highly modified water bodies. All fish passes were used as migration corridors, with increased fish movements during high discharge and at spawning periods. Because river stretches with high variability of current speed and water depth are scarce in highly modified water bodies, fish passes can play an important role as compensatory habitats and should thus be considered more intensively in habitat assessments and river restoration. Ideally, fish bypasses should mirror the natural discharge dynamics and consider all occurring fish species and sizes. Copyright © 2011 John Wiley & Sons, Ltd.     

MODELLING CHANGES IN SALMON HABITAT ASSOCIATED WITH RIVER CHANNEL RESTORATION AND FLOW‐INDUCED CHANNEL ALTERATIONS

The River2D two‐dimensional hydraulic and habitat model was used to simulate fall‐run Chinook salmon (Oncorhynchus tschawytscha) spawning and fry and juvenile rearing habitat of the first phase of a stream channel restoration project on Clear Creek, California. Habitat was simulated for a range of stream flows: (1) before restoration; (2) based on the restoration design; (3) immediately after restoration; and (4) after one and two large flow events. Hydraulic and structural data were collected for three sites before restoration, and prerestoration habitat was simulated. Habitat simulated for these sites was extrapolated to the prerestoration area based on habitat mapping. The topographical plan for the restoration was used to simulate the anticipated habitat after restoration. Although the restoration increased spawning habitat, it was less successful for rearing habitat. Channel changes associated with high‐flow events did not entirely negate the benefits of the restoration project. The results of this study point out the need for models that can simulate the changes in channel topography associated with high‐flow events, which could then be used to simulate habitat over time. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

SEDIMENT TRANSPORT AND FLOW DYNAMICS AROUND A RESTORED POOL IN A FISH HABITAT REHABILITATION PROJECT: FIELD AND 3D NUMERICAL MODELLING EXPERIMENTS

Few studies have examined sediment transport patterns around in‐stream structures used to enhance fish habitat despite the importance of this variable in the successful design of stream restoration schemes. The objective of this study is to examine interactions between the (excavated) pool morphology, flow and sediment transport in a restored reach of the Nicolet River (Quebec, Canada). Bedload transport was investigated using passive integrated transponder (PIT) tagged particles that were followed from positions upstream of a pair of current deflectors which were designed to maintain the excavated pool downstream. Three‐dimensional numerical simulations of the flow field at various flow stages (with emerged or submerged deflectors) were used to relate near‐bed velocity and bed shear stress to transport patterns and to assess the impact of varying the pool location and geometry on the flow field and water surface profiles. Results show that from 2005 to 2008, of the 117 pit‐tagged particles that fell in the pool, only 27 are known to have exited. None of the 30 largest rocks entering the pool escaped. Bed shear stress values simulated at high and peak flow (slightly above bankfull level) are not sufficient to move the largest rocks in the pool exit zone. Simulations also reveal a complex water surface topography when flow is above the height of deflectors, with negative water surface slope in the pool zone. When modifying the pool geometry so that the deepest zones are close to the apex of the in‐stream structures instead of in the centre of the channel, both water surface slope and near‐bed velocity patterns are greatly modified. Understanding the interactions between excavated pools, bedload and 3D velocity patterns around in‐stream structures is essential for long‐term success of fish habitat restoration projects, and using 3D models to test various designs of artificial pools is a promising approach. Copyright © 2011 John Wiley & Sons, Ltd.