Ten Mile River Habitat Enhancement Project Mainstem Phase 2 and Mill Creek

Ten Mile River Habitat Enhancement Project Mainstem Phase 2 and Mill Creek

The project encompasses roughly 69 acres of the Mainstem Ten Mile River and at Mill Creek (a tributary of the Mainstem Ten Mile River). It will construct salmonid habitat enhancement features that build on the recent success of similar projects in the lower watershed. The project will enhance habitat and support salmonid populations by increasing salmonid access to off-channel /floodplain habitats and by installing large wood structures to provide refugia and instream cover. Maximizing rearing and foraging habitat of the Ten Mile River Watershed is a key component of coho population recovery. Out-migrating smolts visit nutrient-rich, seasonally flooded wetlands to bulk up before heading out to sea, and young-of-the-year juveniles need accessible floodplains and low velocity environments to survive winter high flows. Large wood structures help create and maintain deep, complex pools that provide winter base flow and summer low flow rearing habitats. With climate change bringing more frequent winter droughts and high summer temperatures, the lower reaches of this coastal watershed have become critical refugia zones where riparian vegetation and coastal fog maintain cool water temperatures and perennial springs sustain streamflow. Biological, topographical, hydrologic, and geomorphic conditions informed specific elements of the designs. The overall project includes two work areas: Mainstem Ten Mile River and Mill Creek. Mainstem Ten Mile River Phase 2 includes both in-stream and off-channel enhancements including side channels at three locations and installation of large wood through the reach. It also includes construction of a pond at one additional site. Mill Creek – includes berm removal and construction of side channels to create a wetland complex. The sites within the Mainstem Ten Mile project area were selected to 1) take advantage of existing features and unused valley land, or 2) to enhance low-value, in-channel habitat areas. The intention is to provide significant ecological uplift and increase Coho salmon rearing capacity in the 0.8-mile reach. The site elements will function together to provide high-value in- and off-channel habitats that can be utilized by salmonids and other aquatic species across the full spectrum flow conditions. The proposed features were designed to slow down the water, add habitat cover and diversity, create high-flow refugia, and establish extensive riparian wetlands. Project elements include a seasonally-flooded wetland with alcoves providing deep summer pools, 19 complex wood structures for cover and channel geomorphic work, and accelerated recruitment-type log jams that utilize downed alders and whole redwood trees. The proposed work at within the Mill Creek project area will reconnect the creek to its adjacent right-bank floodplain by removing portions of a manmade berm currently separating the channel and floodplain and creating constructed side channels through the floodplain. Restoring streamflow to the naturally occurring floodplain will create complex and connected habitats for juvenile salmonids to use for winter high flow refugia, rearing, and foraging.

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Mendocino