Salmon Creek Chute-Pool Channel: Design and Performance

In 2008 a 650-foot reach of Salmon Creek in southwestern Washington was reconstructed as part of a project to replace the aging Highway 99 Bridge. The previous bridge, with piers founded on shallow spread footings, was in danger of collapse due channel incision. The reconstructed channel was designed with the goals of rehabilitating and stabilizing the channel and restoring fish passage. Since its construction the channel has been monitored to document physical changes as well as its success in providing fish passage.

The project reach of Salmon Creek has a long history of channel stability problems, culminating in 1996 with a breach into a downstream off-channel gravel pit that lowered the channel bed, and sent a headcut 1,400 feet upstream to the Highway 99 Bridge. The headcut was arrested by a sheet-pile structure constructed just downstream of the bridge in about 1950, resulting in a 6 to 8 foot drop across the failing sheet pile wall. This drop created a barrier to upstream fish migration, including adult and juvenile salmonids.

Removal of the headcut and stabilizing the channel was carried out as part of the bridge replacement project. The reconstructed channel consists of a series of rock chute structures separated by pools to dissipate energy and provide fish resting areas. The structures were designed with large boulders to provide stability during large floods and a mixture of smaller material to create hydraulic diversity for fish, while preventing excess piping and flow loss into the bed during low flow.

A 10-year monitoring program, including fish capture and tagging by Washington Fish and Wildlife (WDFW) biologists, is being carried out to document the stability of the channel, note changes to the chute-pool structures, and to evaluate fish passage. Immediately following construction, the channel appeared to perform well with good flow conditions across the structures and no observed piping or flow loss. The design was tested during the first winter season with an approximately 10-year flood event on January 2, 2009. Post-flood monitoring showed moderate damage to the rock chutes with movement of some of the larger boulders and removal of much of the fines between the boulders. With only moderate floods since that time, the channel has seen only minor additional changes. Monitoring of fish passage by WDFW demonstrates that adult salmonids are migrating upstream, with successful spawning of both coho and steelhead upstream of the project.