Solutions for Habitat-Critical Side Channel Loss
* How can habitat-critical side channel complexes be rehabilitated following the loss of hydraulic and hydrologic connectivity to the main channel?
Side channels provide critical spawning and rearing habitat for endangered fish species as well as for fish species with a high recreational value. With the construction of Reclamation dams on many river systems in the Western United States, side channel habitat has been lost due to a varied combination of channel degradation, decreased flow magnitudes, loss of sediment supply, and vegetation encroachment. Solutions are needed to rehabilitate side channels that have been progressively abandoned downstream of Reclamation structures and to prevent further losses of side channel habitat.
Need and Benefit
This research is an investigation to develop feasible solutions for the rehabilitation of abandoned side channel complexes. Conditions in side channels provide the habitat complexity along with the conditions in the main channel to optimize fish survivorship. Not only do side channels provide critical habitat for juvenile salmonids, they also provide refuge for fish during floods and important habitat for riparian vegetation and wildlife.
Previous studies on the downstream effects of large dams document decreases in flood peaks, sediment concentrations and suspended load as well as degradation of channel beds, channel armoring and increases in vegetation along river channels (e.g., Williams and Wolman 1984). These changes tend to concentrate flow in the main channel while side channels are dewatered or receive flow only seasonally during larger flow releases, which may or may not be the corresponding time of year for spawning and rearing.
The Bighorn River in south-central Montana has experienced side channel loss following the construction of Yellowtail Dam and Afterbay. Local stakeholders have noted the progressive abandonment of several side channels downstream of the dam, marked by progressive shallowing at side channel connections to the main channel, dewatering at low flow releases, and eventual abandonment. High flows since 1990 appear to have intensified channel degradation in the reach downstream of Yellowtail Dam and Afterbay. Due to the high productivity and recreational use of the trout fishery downstream of the dam, habitat for rainbow and brown trout is of critical concern.
Limited studies exist that document the observed side channel loss on the Bighorn River downstream of Yellowtail Dam and Afterbay. The only study focused on this reach of the Bighorn River was conducted as part of the Yellowstone Impact Study by Peter Martin. From 1939 to 1974, Martin documented an increase in bank riparian area, loss of river area, loss in the number of vegetated islands, island gravel bars, and lateral gravel bars on the Bighorn River between Yellowtail Dam and Afterbay and the confluence with the Yellowstone River (Koch et al. 1977). These channel changes were most pronounced in the upstream reaches, closest to Yellowtail Dam and Afterbay.
Solutions for reconnecting abandoned side channels to the main stem include, but are not limited to, in-stream structures, flow releases, gravel augmentation, and mechanical lowering of side channel entrances. While these techniques have been applied on various rivers, there is still much that needs to be learned about the most effective method for a specific river system and particularly for river systems that have been impacted by the construction of dams.
At the completion of this study, we will have obtained a comprehensive history of side channel formation and loss and associated channel changes before and after the construction of Yellowtail Dam and Afterbay. We will have quantitative information on the magnitude of flows needed to inundate side channels at depths and velocities suitable for salmonid habitat. We will also have documented what alternatives are possible and out of these alternatives, which are most feasible to pursue for side channel rehabilitation considering Reclamation's responsibilities for water delivery and many other factors.
This research has broad application across Reclamation regions. Many Reclamation dams have been built on river systems with side channel complexes that are an integral component of the ecosystem and that provide optimal habitat for salmonids. The research will benefit Reclamation in a variety of regions by providing a case study of side channel abandonment and the solutions developed to address the loss of critical habitat downstream of a Reclamation structure. The solution or set of solutions can be applied to other river systems that have experienced loss of side channel habitat as a result of dam construction.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Bighorn River side channel investigation: geomorphic analysis (final, PDF,
By Jeanne Godaire
Report completed on February 18, 2010