Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Water Resources
Significant unavoidable adverse impacts identified with respect to surface water include massive
turbidity and sediment transport as part of the dam breaching and removal. Total suspended
solids (TSS) within the six hours after the dam breach could range from 100,000 to 250,000 Parts
per million (ppm) and turbidity values could range from 50,000 to 127,000 nephelometric
turbidity units (NTUs). Elevated TSS and NTU are expected through the first year following the
dam breach as bank and river channel stabilization occurs. These turbidity spikes are predicted
to near background levels within 3 to 5 years. Elevated turbidity levels are expected in the
Bonneville pool, where the waters of the Columbia River and the White Salmon River mix. Clay
particles will likely remain suspended in the Columbia River, thus temporarily increasing
turbidity, all the way to the mouth of the Columbia River.
Significant unavoidable adverse impacts were not identified with respect to groundwater.
Aquatic Resources
All fish and aquatic macroinvertebrates within the White Salmon River channel downstream of
the dam will likely be killed or displaced by the load of suspended solids that will occur during
dam breaching. While the actions having the effects will be short-term in duration and will
diminish as the level of suspended sediments is reduced over time, the effect on populations of
macroinvertebrates will likely take several years to fully reestablish.
One potential year-class of the few naturally spawned chum salmon imprinted to return to the
White Salmon River is expected to be lost due to the high concentrations of suspended and
deposited sediment and their inability to access stream habitat above the dam or cofferdam.
Chum salmon spawners that pass Bonneville Dam will not enter the White Salmon River during
the fall and winter months following dam removal and will spawn in other Columbia River
tributary or mainstem habitat. Run size during the years the lost year-class would be expected to
return as mature spawners will be reduced and composed entirely of spawners from other yearclasses.
This impact will be long-term (potentially several generation cycles for chum salmon).
In addition, it is likely that the spawning substrate necessary for their reproduction will be
impaired by fine sediment during the second year (and not fully recovered for 1 to 3 years after
that). New gravel recruited from upstream may not reach the lower 2.6 miles during that time.
The result will be essentially a loss of several year-classes of chum salmon. The small number
of chum salmon spawners currently documented to occur in the White Salmon are likely strays
from a population below Bonneville Dam and do not represent a viable population. The NMFS
Biological Opinion (NMFS 2006) provides for the incidental take. The long-term increase in
available chum salmon spawning habitat is expected to increase chances of successful
recolonization of the White Salmon River basin by chum salmon.
During the period immediately following the breaching of the dam, suspended sediment
concentrations entering the Bonneville Pool will be relatively high and the discharge of the
White Salmon River will make up approximately seven percent of the Columbia River flow.
Columbia River fish may be displaced from the most sediment-laden portions of the plume until
it has completely mixed with the Columbia River, approximately three miles downstream from
the mouth of the White Salmon River (PacifiCorp 2005). Beyond this point, the plume may
briefly interfere with foraging behavior and predator-prey relationships through the Bonneville
Pool and downstream of Bonneville Dam (PacifiCorp 2005, Korstrom and Birtwell 2006).
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