Linking geomorphic changes to salmonid habitat at a scale relevant to fish

The influence of geomorphic change on ecohydraulics has traditionally been difficult to quantify. With recent improvements in surveying technology, high-resolution, repeat and topographic surveys have become a common tool for estimating fluvial sediment budgets and documenting spatial patterns of net erosion and net deposition. Using a case study from a spawning habitat rehabilitation (SHR) project on California's Mokelumne River, some new DEM-differencing analytical tools and ecohydraulic models were used to test whether hypotheses about pool-riffle maintenance mechanisms used in designing SHR projects were producing self-sustaining spawning habitat when subjected to competent flows. Following peak flows associated with the spring snow-melt, a total of 999.6 m3 of erosion and 810.1 m3 of deposition were recorded throughout the study area, with a net loss of 196.2 m3. Using an ecohydraulic spawning habitat suitability model to segregate the sediment budget, over 53% of the area in which gravel was placed in a 2005 SHR retained the same habitat quality characteristics, and 22% improved. The response to the flood was generically characterized by shallow deposition associated with areas of divergent flow over riffles and scour associated with areas of convergent flow in pools. Areas where habitat remained stable generally experienced only low-magnitude elevation changes, and accounted for only 19.5% of the total volumetric change. Areas where habitat quality degraded (primarily pool exit slopes) were dominated by larger magnitude erosion and made up 46% of the total volumetric change. By contrast, areas where habitat quality improved (primarily constructed riffle) accounted for 34.5% the total volumetric change, and were dominated by shallow, low magnitude deposition. The results support hypotheses about pool-riffle maintenance mechanisms used to design the rehabilitation projects, while also highlighting some simple but powerful techniques for linking ecohydraulic and geomorphic field monitoring data at a salmon-relevant spatial scale.

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