Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Immediately after the dam is breached, sediment-laden waters would begin pouring out of the
reservoir at a rate of approximately 8,000 to 10,000 cfs. Approximately 75 percent of the
accumulated reservoir sediment is in a size range that would readily be suspended in the river
and transported downstream to the mouth of the White Salmon River and ultimately into the
Columbia River. Sediment would erode from the reservoir through river channel formation by
the White Salmon River and its tributaries in the reservoir area, surface erosion of the reservoir
sediment, and floodplain development.
River Channel Formation
Immediately after dam breaching, river channel formation would be a dominant factor in the
erosion of reservoir sediments trapped by the dam. Initially, a narrow slot would be cut in the
sediments. As that slot deepens and widens, sediment would slough into the new river channel
and be removed through the breach in the dam. Woody debris likely present in the reservoir
sediments may interfere with draining the lake through the drain tunnel; however, provisions
have been made before and after breaching to remove debris using blasting techniques or a crane.
As the river channel widens and erodes down to bedrock, reservoir sediment would continue to
erode.
In addition to direct erosion and transport by the White Salmon River, tributary streams would
erode the reservoir sediment adjacent to those channels. Where deltas have been built into the
reservoir by the tributaries, those tributary streams would erode their former deltas when
streamflow is sufficiently high to mobilize the sediments. Erosion of deltaic sediments would
proceed more slowly than the erosion of the finer reservoir sediments, because of the larger size
of these materials. This delayed erosion may present a barrier to fish passage until a stable
channel through the delta is formed. Coarser sediment in both the deltas and the upstream portion
of the reservoir (more than 7,500 feet upstream) would be transported primarily at the bottom of
the stream channel (bed load) and would likely be retained by the cofferdam thought to be
present in the bottom of the channel near the dam. However, the cofferdam may be full of
sediment that cannot be eroded by the flows at the time the dam has been removed and before the
cofferdam is removed. In this case, the bed load sediments would be transported over the top of
the cofferdam and likely would fill in downstream pools in the White Salmon River.
Although it may take longer than three to five years to move accumulated gravel and cobbles
downstream, flows that move these materials would occur, and eventually the downstream pools
in bedrock would be filled with gravels. This delay would help to avoid the deposition of fine
reservoir sediment in the space between gravel particles. The movement of these gravels and the
time it takes to achieve a stable channel configuration are of concern because of the role that
gravels without fine sediment play in providing fish habitat. Carving graded channels in the
deltaic materials with construction equipment as soon as they are accessible would accelerate the
channel stabilization process and decrease the time that is needed to evacuate sediments
downstream. This would prevent the deltaic materials from acting as a barrier to fish passage
into Mill Creek.
Surface Erosion
Surface erosion and transport of reservoir sediments that are not eroded and transported by the
White Salmon River or its tributaries would continue until vegetation is fully established.
Because of the steep underlying bedrock topography, steep unstable slopes on the reservoir
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