Climate Impacts to Groundwater Ponding and Salinity - Stillaguamish and Snohomish

Climate Impacts to Groundwater Ponding and Salinity - This engineering and geomorphology team collaborated with the Snohomish Conservation District (SCD) to assess localized and regional climate impacts to groundwater in the lower Stillaguamish and Snohomish River basins. The work contributed to a larger land use and community planning framework seeking solutions that promote both salmon and agricultural resiliency. Results of the assessment were presented to the local community via a series of sessions intentionally designed to facilitate climate change communication and education between the conservation district, flood control districts, Snohomish County, farmers, conservationists, scientists, and engineers. Each session began with presentations from scientists and engineers and ended with collaborative mapping tasks that identified habitat restoration and infrastructure improvement opportunities that could mitigate flood risk, address declining salmon populations, and adapt to rising sea levels and changing hydrology. The climate impacts assessment focused on the effects of localized relative sea level rise (RSLR) and predicted extents of rising groundwater tables and increased salinity intrusion to crop root zones. Tidal attenuation into the groundwater table was predicted using conductivity and water table elevation time series from current and historical groundwater monitoring wells. Increased groundwater ponding due to RSLR was predicted for two aquatic environments (groundwater tables dominated by tidal versus upstream fluvial inputs) via two different methods resulting in summer groundwater ponding maps and spring agricultural cropping access delay maps for existing conditions and future years 2050, 2080, and 2100. Salinity intrusion effects due to RSLR were extrapolated spatially and temporally for the future. Decision-relevant results were synthesized then presented for interpretation by county, farmer, tribal, and other stakeholder groups. Implications for coastal systems, shoreline management, and transportation infrastructure were focused locally on the lower Stillaguamish and Snohomish Rivers.