Lowering Capitol Lake for Estuary Design: Revealing Sediment and Data

Photo byWashington State Department of Enterprise Services

See what lies below Capitol Lake when the 73-year-old manmade lake is drained next week to help restore the 260-acre Deschutes Estuary, the Washington Department of Enterprise Services has announced.

Over four days, DES officials say the department will open the water gate on the 5th Avenue Dam to let water flow in from the lake to Budd Inlet to collect data on the lakebed's underwater features.

That includes the 5th Avenue Dam and fish gate, sediment deposits, outfall locations, and underground pipes used to disperse stormwater, DES reports.

A small, permitted drone will also be flown over the lake to record the conditions and topography of the lakebed, according to the department.

The Capitol Lake is expected to fall to its lowest levels between July 24 and July 25, which should reveal exposed sediments that make up the lakebed, the DES reports.

According to the department, the lake is regularly lowered for maintenance work and to reduce the risk of flooding during severe weather.

DES reports it's partnering with the Washington Department of Fish and Wildlife and the Squaxin Island Tribe throughout the estuary restoration design process, which is expected to help buffer the area from rising sea levels in Olympia. The restoration is early in the design stages of the restoration, DES reports.

While Capitol Lake is lowered, the pedestrian path around Deschutes Parkway and Capitol Lake will remain open, DES officials confirm.

The department is also discouraging visitors from stepping onto the exposed sediment of the lakebed to prevent the spread of invasive species. Capitol Lake was closed to the public in 2009 due to the presence of the New Zealand mud snail, an aquatic invasive species, according to the DES.

The Deschutes Estuary was transformed into Capitol Lake in 1951 when the state of Washington built the 5th Avenue Dam and tide gate, according to DES.

Over the years, ecological issues and poor water quality led to sediment build-up which, in combination with the arrival of the New Zealand mud snail, resulted in the state closing the lake to the public, the DES reports further.

The restoration of the estuary will also see the replacement of the 5th Avenue Dam with a new bridge equipped with vehicle, bicycle, and pedestrian lanes by 2026, according to the DES.

DES officials say they expect the lake to refill around Aug. 5 of this summer.

The department reports it is partnering with Floyd | Snyder and KPFF Consulting Engineers of Seattle, Washington for the project.