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Decadal-scale evolution of Elwha River downstream of Glines Canyon Dam : perspectives from numerical modeling De Rego, Kathryn Grace


The goal of this study is to understand the legacy of dams on river channel evolution. Most major world rivers are dammed, and these features have pervasive impacts on downstream geomorphology. Dam removals have become a popular restoration technique. However, little is known about how rivers respond to dam removal on long timescales, especially with regards to sediment exchanges between the channel and floodplain. We examine how dam emplacement and removal have affected channel stability and migration along Elwha River, a cobble-bedded wandering stream. Two dams were built on the river in the early 20th century, blocking sediment supply to the reaches below them. The dams were removed between 2011 and 2014. A numerical model, MAST-1D, is adapted to simulate channel evolution on the set of reaches between the two dams. New representations of bank erosion, vegetation encroachment, and avulsion are developed to make the model suitable for cobble-bedded streams. In the model, channel width and migration oscillate between a range of values, increasing after avulsions due to reorganization of channel geometry. The model is successful at simulating channel change during the sediment-starved period following dam emplacement. While it replicates the general pattern of channel change following dam removal, the simulations underestimate the competence of the system to export the initial pulse of sediment from the former reservoir deposit. Constraining the volume and caliber of sediment supply from the reservoir is crucial for predicting sediment deposition and storage downstream. Model simulations indicate that dam emplacement results in channel armoring, which reduces the competence of the flow to undercut bank toes, reducing the migration rate and leading to net channel narrowing. Both field and model data show that activation of floodplain channels via avulsion and, to a lesser extent, bank erosion, were responsible for increased levels of channel-floodplain exchange during the post-removal period. We predict that in the future, Elwha River will be more laterally unstable than it was in the 20th century, both due to the legacy of the dam removal and because of climate change.

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