Lil’wat Nation - Grandmother Slough Climate Adaptation Project

in progress

Grandmother Slough is a low-gradient, fish-bearing stream within the Lillooet River floodplain. The 3.5 km natural drainage path and interconnected wetlands flows through the community of Mount Currie, SLRD, and the Village of Pemberton. During high water events the Slough is highly susceptible to flooding, impacting farmland, residential properties, and multiple crossings of Highway 99. Because of its low gradient, once flooded, the Slough does not effectively drain; elevated water levels remain for a long duration of time, exacerbating and increasing exposure to flood hazards. To address susceptibility and exposure of the system to flooding and associated flood hazards and risks, proposed project components include construction, rehabilitation, and improvement in the following areas: 1. Reed canary grass management - removal of dense areas of invasive reed canary grass causing impediment to water flow and overgrowth of important fish habitat for 1500 m2 2. Improved culvert operations of two major crossings (4 culverts) to address drainage and backflow. 3. Improved hydrometric monitoring 4. Salmon habitat restoration - improve natural infrastructure for increased habitat and flood resilience. Project Works include: 1. Management of invasive reed canary grass to rehabilitate and improve both habitat and drainage for an area of approximately 1500m2. 2. Effective culvert infrastructure will be installed to manage flow capacity during high water events. There are two crossings requiring upgrades: Main Street and Highway 99 East. Improvements to culverts that can restrict backwater flow from the Birkenhead River would have a substantial impact on water levels in the Slough during a flood. 3. Improved hydrometric monitoring to include real-time access for decision-making during floods, and a tool to aid in planning for future works. 4. Review an overflow pathway to Grandmother Slough to determine viability for flood mitigation and fish habitat improvement.