Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Impacts
Sediment Transport
• Transport of the mass of sediment from the reservoir downstream to the
Columbia River will kill most aquatic organisms. The longer the high
concentrations continue, the longer or more difficult it will be for fish to
recolonize the reach of the White Salmon River below the Condit Dam site
or anadromous fish to migrate up the White Salmon River.
• Salmon trying to enter the White Salmon River to spawn while the mass of
sediment is passing would be killed and no reproduction of anadromous
fish will occur until levels of suspended fall below lethal levels and
migration upstream above the upstream end of the reservoir to suitable
spawning gravels becomes possible.
• Sediment will likely fill the fish-rearing channels of the U.S. Fish and
Wildlife Service’s fish rearing facility at river mile 1.4.
Dam and Appurtenance Removal
• The old cofferdam in the reservoir upstream of the dam is expected to be a
barrier to upstream migration by anadromous fish.
Post-Removal Management
Upstream Sediment Management
• Unstable or erodible sediments could continue to adversely affect the
substrate habitat for aquatic organisms and slow recovery of habitable
substrate.
• The delta at the mouth of Mill Creek is likely to be a barrier to access by
fish from the river.
Table 1-1 (Continued)
Summary of Impacts and Mitigation Measures
Aquatic Resources (Continued)
Sediment Transport
Mitigation
• The dam will be breached in October to minimize the risk of harm to
seasonal fish runs. The timing would also take advantage of the rainy
season when there will be fewer adverse effects on recreation and aquatic
life. The high flows of the season will aid in transporting sediment from
the reservoir.
• Dislodging unstable sediment and woody debris will help ensure that the
reservoir sediment is transported downstream quickly, therefore within the
predicted 3–5 year period, and does not affect long-term water quality,
pool depths, or spawning gravels.
• 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 protect the fish rearing facility from high
flows and reservoir sediments.
Dam and Appurtenance Removal
• Cofferdam removal will either occur as soon as possible after dam
removal and be accomplished by blasting while suspended sediment levels
exclude 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.
• If the delta is a barrier on May 1 at the year following dam removal, heavy
equipment would 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.
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