Criteria for Geomorphic Response from Large Dam Removal

Context/Purpose:

For the removal of Elwha and Glines Canyon Dam (WA), criteria were needed to describe when the sediment impact signature of dam removal was muted such that quasi-equilibrium river conditions had been achieved. Potential sediment erosion was predicted to reach millions of cubic meters accounting for multiple decades of the dams trapping large volumes of sediment. The criteria were used to guide type, extent, and duration of monitoring and inform adaptive management decisions on the rate of dam removal.

Methods:

We utilized comparisons of topography, sediment data, river stage, field observations, and aerial photography during and after dam removal.

Results: Reservoir criteria were river incision to the pre-dam valley floor, occurrence of at least one flood (>5-yr), lateral terrace erosion limited to floods, annual erosion less than natural variability of background sediment loads, exponential decay of net erosion relative to natural background loads, and vegetation development. Water quality criteria focused on suspended sediment concentration fluctuations occurring in response to river discharge rather than as a response to dam removal. Downstream river criteria included transporting the majority of released sediment to the coastal delta and river channel migration limited to flood periods. Another criteria was increased surface water connectivity of side channels and floodplain indicating restored sediment storage in the downstream river.

Interpretation:

The criteria provided effective measures for the geomorphic response of dam removal and justification for continuation of monitoring. Criteria were not met until post-dam removal floods had occurred and sediment erosion and geomorphic response magnitudes reduced to levels expected in a naturally functioning river. Additional criteria for fish was ensuring no physical barriers remained at the former dam sites. Vegetation criteria focused on minimizing invasive species in the newly exposed reservoir landscape.

Conclusion: Criteria for geomorphic response to dam removal helped guide monitoring efforts, adaptive management decisions, and determination of whether post-dam removal the Elwha River was analogous to a naturally functioning river.