Pierce County will likely see more invasive species in streams, ponds and lakes throughout the county. The county is also likely to see an increase in toxic algae in lakes. Higher temperatures could impact the quality of near shore habitat, shellfish growing areas and increase the likelihood of vectors like the West Nile virus. Surface Water Management has historically taken the lead of managing response to West Nile virus. SWM currently has staff monitoring and inspecting water quality best management practices (BMP’s) for parameters such as temperature, nutrients, and toxicology. It is likely these obligations would increase and there would be a need to create a lakes management program.

Sediment loads in the Puget Sound rivers are expected to increase, as declining snowpack and glacial recession expose more unconsolidated soils to severe weather. Increased sediment in our rivers systems decreases the flood carrying capacity and increases risk of channel migration. SWM may have to create a sediment management program to manage the amount of sediment coming down the mountain and into the rivers and streams. Our current plans call for building more setback levees which helps capture sediment loads. In order to respond to a rapidly changing risk, the length or number of planned setback levees may have to be increased.  Alternative methods for sediment management or setback levee construction would require additional staff support and funding to address this issue. Increase sediment can have a negative impact on salmon productivity.

With Pierce County experiencing higher than average summer temperatures, this will cost more money to maintain plantings during the first years of establishment. Where wildfires occur there will be increased runoff and risk of flooding and sedimentation.

Glaciers are expected to continue to shrink due to warmer temperatures in the Puget Sound watersheds. Severe low flows in the summer could impact the salmon runs in rivers and streams. With glaciers receding, there will be more loose material exposed in the upper basin for rapid erosion causing greater sediment loads in rivers. Additionally, glaciers help to hold snowpack in the mountains, often acting as a sponge. With less glaciers, there is the potential for higher flows throughout the river system.

The risk of flooding is expected to increase with the anticipated increase in heavy precipitation events. Atmospheric rivers are the source of all major river flooding in the county and are expected to increase in frequency, intensity, and duration. This could have major impacts on managing flood risk. The size of facilities would need to be modified or relocated to maintain current levels of service.  Increase flow and sediment will lead to more channel migration and degradation issues. Major flooding may pressure agriculture to move out of the floodplain, and SWM would see an increase in the demand for property acquisitions. Increases in rainfall would lead to more road closures as stormwater conveyance systems are overwhelmed. Undersized stormwater conveyance systems can also be a barrier to fish migration. This will also increase the potential for groundwater flooding, increased septic system failures, and the demand for SWM to mitigate for these changes. SWM has constructed multiple setback levee projects and has identified several additional potential locations for setback levees.

Ocean acidity is expected to continue to increase in the Puget Sound. SWM is part of Clean Water Partners, a partnership organization formed in 2006 between Pierce County, Pierce Conservation District, and Tacoma-Pierce County Health Department. SWM and Clean Water Partners will work to prevent shellfish bed closures due to poor water quality and help ensure that shellfish harvested is safe to eat.

Increases in precipitation and duration will lead to more winter landslides. This will be exacerbated where wildfires have caused a loss of land cover.  Landslides can cause increase sedimentation loads in river systems leading to greater risk of channel migration. Marine bluff landslides can lead to loss of eel grass habitat and near shore fish habitat degradation.  Landslides can also damage built stormwater infrastructure.