Document Type

Thesis

Date of Degree Completion

Spring 2024

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Lisa Ely

Second Committee Member

Breanyn MacInnes

Third Committee Member

Carey Gazis

Abstract

Relatively few studies have analyzed how the removal of small dams and re-engineering of the channel affect river channel processes. The low-head Nelson Dam was built in 1920 on the Naches River in central Washington, causing two miles of aggraded sediments. This resulted in upstream flooding and excessive downstream incision that led to ineffective irrigation diversions, and hindered fish spawning. Nelson Dam was removed in 2021 and replaced with a graded, roughened, nature-like channel and a newly engineered diversion that was completed in 2023. The research presented here quantifies the effects of the Nelson Dam removal and channel redesign on river channel processes. Documenting the locations and scale of the initial response were used to estimate longer-term changes to the fluvial system.

Aerial imagery, sediment-size surveys and elevation differencing software were used to determine upstream and downstream changes. Aerial imagery was used to map channel changes, and sediment-size surveys tracked sediment transport following the dam removal. Pairs of LiDAR images from 2000, 2005, 2008, 2013, 2019, 2022, and 2023 were differenced to quantify locations and amounts of erosion and deposition before and after the Nelson Dam removal.

Aerial imagery shows little downstream change leading up to and after dam removal, but frequent, major channel shifts have occurred upstream through time. Since dam removal, sediment-size surveys suggest up to a 13% increase of fine sediment; however, the coarse component of two downstream bars skewed slightly coarser. The differencing analysis indicates locations of recent bank erosion upstream and downstream that may continue. This process could help replace the present coarse sediment distribution with a more even distribution of coarse and fine sediment. Future downstream lateral channel migration and redistribution of impounded upstream sediment may return the downstream reach to a more natural state, but confinement of the river between bedrock and a highway may hinder this change.

This study provides a baseline description for future study in 5-10 years. The results of this study give understanding to the local region, but also supply knowledge toward similar rivers where communities want to remove the adverse effects of a dam while retaining some functionality.

Download

Share

COinS