Impact/Purpose:

To characterize sediment amounts and particle size distribution in a one mile section of Icicle Creek in Chelan County, WA, and to determine sediment fate if historical flows were returned to the channel.

Description:

Most of the flow of Icicle Creek, Chelan County, WA has been diverted into a canal at the Leavenworth National Fish Hatchery since 1938, reducing flows in a 1.1 mile reach. The Hatchery has also constructed five dams that currently impede fish passage. The US Fish and Wildlife Service is now considering removing some or all of the dams and restoring unregulated flows to the historical channel. Residents downstream of the Hatchery, where the stream gradient is less, were concerned that sediments that have collected in the historical channel would flush out and settle downstream, affecting angling and fish spawning. EPA funded Washington Trout (WT), to study the stream. WT's researchers measured sediment amounts and size distributions, mapping their sampling locations to sub-meter accuracy using GPS. After determining amounts and sizes of sediment in the Hatchery reach, they determined that restored flows would be sufficient to mobilize and transport sediment out of the Hatchery reach. WT then compared the amount of trapped sediment with the estimated average annual load of the basin and concluded that the stream has sufficient power to mobilize sediments in the Hatchery reach and move it through the downstream reach into the Wenatchee River. Restoration of flows in the historical channel is unlikely to accelerate downstream sedimentation. USFWS used this information in an Environmental Impact Statement to help evaluate different alternatives to restore fish passage and the historical reach. Without it, assessing the effects of removing the structures on downstream reaches would have been based on conjecture.

Record Details: