Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Because listed fish would be in the Bonneville Pool at that time, they could be displaced by the
heavy sediment plume, which has been considered a “take” under the Endangered Species Act
(NMFS 2006).
There would be an unavoidable short-term impact to available thermal refuge in the White
Salmon River until sediment deposited in: (1) pools between river mile (RM) 0.5 and RM 3.3;
and (2) the lake bed between RM 3.3 and RM 5.0 is transported to below RM 0.5 and a channel
forms below RM 0.5. However, new thermal refuge habitat will be available above RM 5.0 as
soon as passage is possible past the dam and cofferdam sites.
Blasting during the removal of Condit Dam, the cofferdam, sediment slopes, or woody debris
jams would create hydrostatic shock waves that cause direct mortalities to any fish in the vicinity
of a blast. A short-term unavoidable adverse impact to local fish populations would occur due to
the mortality of fish in the proximity of in-water blasting activities (if blasting activities occur
when fish are present).
Sediments flushed out of the reservoir would bury and kill any adult California floater mussels, if
they are present in the river below RM 3.3. If any adult California floaters are present in
Northwestern Lake, they could be flushed downstream and deposited in pools. California
floaters that are deposited near the surface of the substrate in appropriate habitat may survive,
while those that are buried or deposited in fast riffles and runs are unlikely to survive.
Depending on the presence of adult California floaters upstream of the reservoir or the
reestablishment of a population from the migration of host fish into the river reach below RM
5.0, a short- or long-term unavoidable adverse impact may occur if California floaters are present
in the White Salmon River below RM 5.0.
After dam breaching, sediment accumulations with an average depth of approximately 5 feet will
occur in the Columbia River downstream from the mouth of the White Salmon River. This area
will extend into the Columbia River channel about 1,500 feet and downstream for about 1 mile,
and cover about 100 acres (PacifiCorp 2005). The Bonneville pool is about 4,000 feet wide at
this location and sediment depth is expected to be zero in the navigation channel. Benthic
macroinvertebrates, such as crustaceans, aquatic insects, and freshwater mussels will be
physically buried (PacifiCorp 2005). With the exception of mussels, recolonization should occur
within 6 months to a year. Mussels have longer life-spans and are relatively slow growing and
will take more time to recolonize new substrates.
Wetland Resources
Unavoidable adverse wetland impacts include the loss of approximately 2.8 acres of lake fringe
wetlands. These impacts are expected to be mitigated by the establishment of riverine and slope
wetlands within 1 to 5 years of dam removal.
Terrestrial Resources
There will be no significant unavoidable adverse impacts.
Transportation
With the implementation of the identified mitigation measures, no significant unavoidable
adverse impacts are expected to occur to transportation or traffic.
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