Work progresses on Hidden Lake and Boeing Creek

Hidden Lake is now just Boeing Creek
Photo by Keith McGlashan

What is Hidden Lake?

Hidden Lake was an artificial waterbody created by damming a section of Boeing Creek just upstream of NW Innis Arden Way, located partially within Shoreview Park. It was constructed by King County in 1996 as an environmental mitigation project and includes a sediment forebay to trap sediment as it moves into the lake.

What happened to sediment deposited in the lake?

Sediment deposition has occurred within Hidden Lake at a rate approximately six times greater than estimated by the facility’s designers; in order to maintain the lake as an open water feature from 2002 to 2013, the City’s Surface Water Utility spent over $600,000 to implement seven separate dredging projects which removed a total of nearly 13,000 cubic yards of material. No sediment removal has occurred since the summer of 2013.

Where does all the sediment come from?

Sediment moving into the lake primarily comes from upstream ravine slope failures. Most deposition within Hidden Lake occurs infrequently as a result of large storm events. Major development within the upper Boeing Creek basin, concentrated in the 1950s through 1970s and especially along Aurora Avenue N, has greatly increased stormwater peak flows, leading to erosion issues throughout the Boeing Creek ravine.

Why did the City stop dredging Hidden Lake?

The Hidden Lake Management Plan Feasibility Study analyzed alternative surface water management approaches for the Hidden Lake facility with the goal of reducing long-term costs to the utility while meeting other criteria. On September 8, 2014, City Council discussed the results of the study and authorized staff to cease dredging the lake and begin a phased approach to remove Hidden Lake Dam and re-establish Boeing Creek at Hidden Lake.

Boeing Creek is channeled into a naturally curving, rock-lined bed
Photo by Keith McGlashan

Did the Hidden Lake Dam block fish passage within Boeing Creek?

Yes, Hidden Lake Dam was a complete barrier to fish passage, one of four major barriers on lower Boeing Creek identified in the 2013 Boeing Creek Basin Plan (the other three barriers are the Seattle Golf Club diversion dam, riprap cascades below NW Innis Arden Way, and the NW Innis Arden Way culverts). 

The basin plan recommended taking advantage of any opportunities from Hidden Lake-related work to improve Boeing Creek fish passage (under Recommended Project BC-Hab-1). However, it is unclear which species may have been most affected by this loss of fish passage, other than resident cutthroat trout. 

In pre-developed “wild” conditions, the steepness of the channel slope and likely intermittent natural fish passage barriers created by old-growth treefall may have prevented upstream passage of the anadromous species (coho, chum, sea-run cutthroat) found in lower Boeing Creek for much of the channel upstream of the Seattle Golf Club Dam.

Boeing Creek still flows under the Hidden Lake Bridge
Photo by Keith McGlashan

How will these improvements be funded?

The 2018 Surface Water Master Plan Update prioritized the Hidden Lake Dam Removal Project alongside all other recommended surface water projects. This project and other priority projects were allocated funding from the Surface Water Utility Capital Improvements Budget.

The project has been awarded multiple grants, including a $300,000 Flood Reduction Grant from King County Flood Control District, a $448,000 Land and Water Conservation Fund (LWCF) grant from Washington State Recreation and Conservation Office (RCO), and a $50,000 King County WaterWorks Grant. We are seeking additional funding sources for this project to help offset the need for utility funds.

--Information from the City of Shoreline