The floodplain in Marymoor Park, which links Lake Sammamish to the Sammamish River, will get a restoration treatment by the King County Flood Control District in coming years.

The district has hired a consultant to complete preliminary engineering designs by March 2019 for improvements to the floodplains and along a 1,500-foot stretch of river, known as the transition zone, to improve river flow and drainage from Lake Sammamish.

The project was launched in 2013 to address flood control and habitat issues at the area where the lake funnels into the river. The rock-lined transition zone was built in 1964 by the U.S. Army Corps of Engineers to help control flooding downstream in Redmond by bottlenecking the lake’s outflow. The rocks lining the zone slows the water and provides valuable habitat for the region’s native fish, including kokanee and Chinook salmon. To help these fish further, the county later planted willow trees along the transition zone to provide shade and food.

However in 2010, following several springs with record rainfalls, Lake Sammamish’s water levels were found to be higher and the water flow passing through the transition zone had decreased. In addition to higher lake levels that were causing problems for lakeside homeowners, the slower flow raised water temperature in the river, which is dangerous to salmon.

In response, the Flood Control District decided to modify the transition zone to allow for necessary flow while preserving flood protection. More details will become available when the preliminary designs are completed next spring but the project will include improving the transition zone as well as an adjacent wetland located in Marymoor Park.

The county will build a small side channel from the mouth of Lake Sammamish and the Sammamish River that winds through the wetlands. This will let more water drain from the lake during heavy rains and direct it into the wetland where it will be slowed and absorbed to prevent flooding downstream. This would create 3,400 feet of secondary channel habitat for fish. Vegetation and wood would be placed along the main channel fringes to provide habitat and shade for fish.

In addition, 2.5-acres of new wetlands would be created in the floodplain and the existing 15 acres of wetland would be enhanced by controlling invasive species. Some 20 acres of riverside forest would be replaced too. In total, the project was expected to cost $8.2 million according to a 2015 plan.

Much of the Sammamish River used to wind through extensive wetlands before draining into Lake Washington, but the river was straightened and the wetlands drained and built on beginning in the early 20th Century through the 1960s. In recent decades, the number of salmon, especially kokanee, has dropped to extremely low levels, in part due to development around the lake and river.

While there are many possible reasons for the decline in kokanee, one of the biggest killers is likely increased water temperatures in Lake Washington. Salmon avoid warm water, so when surface water temperatures rise, it drives the fish lower in the lake, sometimes into regions that don’t have enough oxygen. This has contributed to only 19 salmon returning to the lake to spawn in 2017. Warm water in rivers is also dangerous as salmon will opt to stay put when water heats up, waiting until it cools down.