Potential sedimentation impacts related to dam removal: Icicle Creek, Washington, U.S.A.

A series of small dams were built in Icicle Creek in 1937 to facilitate the operations of Leavenworth National Fish Hatchery. However, several of those dams have been abandoned spurring recent discussions among local watershed conservation groups, as well as state and federal agencies, about removing the dams and the potential impact to the lower reaches of Icicle Creek due to elevated sedimentation. The objective of this study was to measure the total volume of sediment trapped behind the Icicle Creek dams and estimate the potential sedimentation impacts for the lower portion of Icicle Creek should the dams be removed. Another objective was to assess the ability of the river to flush the sediments and naturally restore the system to as close to its predam condition as possible. A flow-competence approach was used to assess the restoration potential of the river to do the work of flushing sediments and reestablishing the predam stream channel characteristics. A sediment probe, a total station, a GPS and aerial photographs were used to map out sediment deposits and measure their depths to determine sediment volumes. Grain size distributions from bed sediments, bars, islands and stream banks were used to assess potential downstream sedimentation impacts. The total volume of sediment trapped behind the dams was estimated at 36,000 m3 (± 4000 m3). The river has sufficient stream power to flush these sediments over 90% of its natural discharge regime. Controlled flushing of the trapped sediments over several years poses very little threat to the water quality and spawning habitats in the lower Icicle Creek scaled against natural flux rates.