Hydrologic response to channel reconfiguration on Silver Bow Creek, Montana

Hydrologic residence time in streams is rarely considered as a response variable for assessing restoration design strategies. However, residence time is an important control on ecosystem processes such as the biotic uptake and processing of excess nutrients and other pollutants in streams. 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 various channel habitats and the hyporheic zone. The influence of reach-scale channel reconfiguration on residence time is not well characterized despite documented linkages between streambed topography, channel pattern, hydrologic retention, water quality, and in-stream habitat. This study documents changes in hydrologic 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 increasing pool frequency. However, channel realignment yielded a reduction in the fine-scale variation in streambed topography. Water velocity profiles in post-realignment channels, thus, exhibited greater uniformity at short spatial scales. As a result, and possibly due to loss of hyporheic exchange, transient storage within the system declined after channel realignment, offsetting some of the increase in residence time associated with slower advective velocities. We conclude that restoration actions may be more effective at recovering normative hydrologic function if goal setting, planning and design efforts consider the hydrologic effects and ecological benefits of fine-scale (cm–m) topographic stream-bed variation and the bio-geomorphic processes that create and maintain such fine-scale variation over time.