Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Sediment Transport
• The dam will be breached in late autumn to take advantage of the rainy season
when there will be fewer adverse effects on aquatic life.
• Dislodging unstable sediment and woody debris will help ensure that the reservoir
sediment is transported downstream over the predicted three- to five-year period
and does not affect long-term water quality, pool depths, or spawning gravels.
• Heavy equipment should be used to cut channels through tributary lake sediment
delta at Mill Creek to hasten the creation of a stable stream channel and prevent
fish passage blockage by the sediment.
• PacifiCorp has proposed to capture and transport to a hatchery the fall Chinook
returning to the White Salmon River before the dam is breached in October to
prevent the loss of a Chinook year-class.
• PacifiCorp will take measures to coordinate with managers and protect the
USFWS fish rearing facility at RM 1.4 from high flows and reservoir sediments.
Dam and Appurtenance Removal
• Use of BMPs will avoid or minimize impacts associated with the use of haul
roads, staging area, and disposal sites, and filling the surge tank tailrace.
• Cofferdam removal will either occur as soon as possible after dam removal and be
accomplished by blasting while suspended sediment levels excluded upstream
migrating fish, or mechanical means will be used rather than blasting. The
cofferdam will be removed by May following dam breaching so that steelhead
returning to the river can pass quickly upstream to less turbid areas of the stream
or its tributaries.
Post-Removal Management
Upstream Sediment Management
• After the initial dam breaching, sediment management will be conducted above
the dam until all unstable slopes have been stabilized and areas of bare sediment
in the former lakebed are revegetated.
• Heavy equipment should be used to cut through the delta and lake sediments
overlaying the Mill Creek (RM 4.0) channel to avoid barriers to fish passage
forming at head-cuts and to shorten the time required to stabilize the stream
channel.
• If blasting is used to stabilize slopes or remove debris, it should be confined to
daylight hours when salmonids are least likely to be actively moving. This will
reduce the number of fish exposed to hydrostatic shock from blasting activities.
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