RIVER RESPONSE TO THE REMOVAL OF A LOW-HEAD DAM AND REPLACEMENT WITH A ROUGHENED CHANNEL ON THE NACHES RIVER, WASHINGTON

Resource managers are increasingly recognizing the role of dam removals in reestablishing the natural channel complexity required to sustain native aquatic and riparian species. The 8-ft high Nelson Dam on the Naches River in central Washington accumulated a 3-km wedge of sand and gravel that led to upstream overbank flooding and lateral channel erosion, while downstream the dam created channel incision, floodplain isolation and coarse sediment armoring. The dam was removed in 2021 and replaced with a graded, roughened channel that was completed in 2023. We quantified the effects of the dam removal and replacement on the sediment transport, sediment-size distribution, channel pattern and floodplain connectivity of the river. The locations and scale of the initial geomorphic responses provided a basis to anticipate longer-term changes to the fluvial system that could be relevant to aquatic and riparian habitats.

Aerial imagery and sediment surveys were used to map channel changes and track sediment transport. Paired LiDAR images from 2000, 2005, 2008, 2013, 2019, 2022, and 2023 were differenced to quantify locations and amounts of erosion and deposition. Before the dam removal, frequent channel shifts occurred upstream of the dam, while downstream channel change was minimal. Sediment upstream of the former dam site remained finer than downstream throughout the study period, although overall sediment size has decreased since 2015. Sand and silt increased both upstream and downstream in the year after the dam removal, but no significant sediment pulse downstream of the dam was detected. The channel and adjacent gravel bars aggraded during and after the dam removal, which reversed the tendency toward incision of the downstream reach in most previous years. Locations of lateral bank erosion persisted throughout the channel, and the bank erosion rate increased upstream after the dam removal.

The Naches River may be undergoing initial changes in response to the Nelson Dam removal, but longer-term monitoring is required to definitively distinguish post-dam changes from the natural variability of the river system. Continuation of the observed trends in bank erosion, sedimentation and migration of gravel bars could increase channel complexity, replace coarse downstream gravel with a more varied sediment-size distribution, and improve habitat diversity.