There are specific geomorphic factors limiting aquatic habitat recovery. These factors include bedrock-dominated reaches and high width to-depth ratios. We sought to make specific channel improvements to remedy these geomorphic limiting factors. The floodplain and some of the mainstem of Mosby Creek were mined for gravel for road building and maintenance. flashy flows, lack of structural components in the stream, and exposed bedrock are challenges addressed in the project. These result in habitat quality issues by limiting spawning and rearing habitat and allowing stream temperatures to rise above preferred levels for salmon.
We decided that in-stream structures could both enhance spawning and rearing habitats as well as potentially reduce peak water temperatures. We used boulders as the primary structural element to construct full spanning weirs and various other boulder arrangements, including clusters and barbs. These features are capable of capturing gravel, creating rearing habitat, and are unlikely to be swept out at high flows. Large wood was not used in the mainstem; rather it was strategically placed on high terraces. There were placements of over 1,200 structural pieces to create habitat for fish. Six channel-spanning rock weirs, numerous boulder clusters, and a few rock barbs were installed in the stream channel. A total of 1187 cubic yards of rock was used in the project. Logs were placed at three of the locations.
The project had two high flow events this winter. Initial observations suggest that the channel spanning weirs and boulder clusters survived and functioned very well during both events. Obvious aggradation of gravels upstream of the channel spanning weirs has begun and geomorphic effects from the structures are evident in the channel. Initial observations suggest the logs remained in place.