King County Road Services - The road to salmon recovery

Whitney Hill Bridge Replacement Project

The bridge replacement, completed in summer and autumn of 2000 provides two wider travel lanes and shoulders and increases the height of the bridge to improve clearance and driver safety.

Habitat improvements

The previous bridge had a narrow span that constricted the stream channel. This constriction caused gravel bars to form both upstream and downstream of the bridge as water either backed up behind the bridge or spread out across the stream channel below the constriction.

The design of the new bridge provides a wider span across the stream increasing the risk that the gravel bars could move and potentially cover salmon nests. To minimize this, boulders and rootwads (tree stumps or wood debris) were placed in several bands or belts across the stream. These bands were constructed to create a series of steps to prevent the movement of large quantities of gravel and sediment, thus protecting salmon habitat and nests. In addition to stream channel modifications, native plant species were planted in areas disturbed by construction activities adjacent to the stream to improve riparian (streamside) habitat.

Monitoring during construction was conducted to limit harmful impacts and ensure safe passage of fish through the area affected by construction activities. Staff carefully monitored water quality and presence of fish. For the past three years, post-construction monitoring was used to evaluate the health of the project area.

Results

Chinook salmon have been observed spawning under and around the bridge during the last three spawning seasons. The in-water structures have controlled the movement of the gravel bars creating steps, pools and spawning habitat used by Chinook salmon.

For more information

Erick Thompson
Senior Engineer Environmental
King County Department of Transportation
Road Services Division
Engineering Services Section
206-296-8747

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