Hydrologic response to channel reconfiguration on Silver Bow Creek : science to inform the restoration process

Abstract

Hydrologic residence time in streams is rarely considered as a response variable for assessing restoration design strategies. However, residence time is a useful index of hydrologic controls on ecosystem processes that may facilitate or limit the achievement of project goals. Interactions between the physical structure of streambeds and the patterns of flow through the channel determine hydrologic residence time and largely control solute transport and exchange among the various physical and biological components of the stream ecosystem. The influence of reach-scale channel reconfiguration on these complex interactions are not well characterized despite well-documented linkages between individual channel features, hydrologic retention, water quality, and in-stream habitat quality. This study documented changes in solute transport and variation in channel water velocity prior to and immediately following large-scale channel realignment along Silver Bow Creek in southwestern Montana. Channel restoration increased water residence time in the channel by increasing sinuosity, decreasing channel slope, and introducing frequent slow-moving pools. However, channel realignment yielded a reduction in the fine-scale variation in streambed topography. Therefore, post-realignment channel water velocities were more uniform, yielding a reduction in transient storage within the system, which could offset some of the beneficial effects of slower advective velocities. Restoration actions may be more effective at recovering normative hydrologic function if planning and design efforts consider the hydrologic effects and ecological benefits of fine-scale topographic variation and the bio-geomorphic processes that create and maintain such fine-scale variation over time.