Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Immediately after breaching Condit dam, absent any obstructions in the tunnel, the reservoir
would be drained during the first six hours. The dam would be breached in late fall so that most
of the reservoir sediment would be transported downstream during the fall-winter rainy season.
Immediately after the dam breaching sediment-laden waters would begin pouring out of the
reservoir at a rate of approximately 8,000 to 10,000 cfs, draining the reservoir in about 6 hours.
Suspended sediment concentrations in the White Salmon River during the initial spike could
briefly reach 250,000 ppm. During the first day, while the reservoir is draining and soft
sediments are sliding into the river, the average sediment concentration could be 150,000 ppm.
The elevated turbidity levels would extend from the dam downstream and into the Bonneville
pool and clay particles are expected to remain suspended all the way to the mouth of Columbia
River, where it discharges into the Pacific Ocean. These suspended sediment concentrations are
predicted to become episodic (e.g., pulses in response to streamflow and surface runoff) and fall
off rapidly to about 3,000 ppm in the White Salmon River after three months and 200 ppm after
6 months.
During the high flows immediately following the beaching of the dam, the White Salmon River
would compose approximately 7 percent of the average flow of the Columbia River (G&G
Associates 2004a and 2004b, PacifiCorp 2005). During the period immediately following the
breaching of the dam, suspended sediment concentrations would be relatively high. After
mixing with the Columbia River for about three miles downstream from the mouth of the White
Salmon River, water in the reservoir may have suspended sediment concentrations of 2,300 to
5,800 ppm; at Bonneville Dam, the concentrations may range from 1,100 to 2,600 ppm; and in
the lower Columbia River, the concentrations may range from 900 to 2,200 ppm (G&G
Associates 2004a and 2004b and PacifiCorp 2005). The range of estimated concentrations
reflects the potential variability of flow in the Columbia River and the uncertainty of the volume
of sediments that will be eroded initially following dam breaching and the amount of sediment
that will settle out in the White Salmon River near the in-lieu site and in the Columbia River. In
any event, the concentrations are predicted to generally decrease downstream as sediments are
deposited in the White Salmon River and in the Bonneville pool. Dilution would also occur
downstream of the Bonneville Dam as a result of tributary flows and longitudinal spreading of
sediment as it move downstream at different velocities (G&G 2004b).
After the initial six hours, the White Salmon River would return to a normal flow and make up
only approximately 0.6 percent of the Columbia River flow. This would dramatically reduce
suspended sediment concentrations as they mix with the Bonneville pool. The anticipated
concentrations would range from 200 to 1,700 ppm after complete mixing with the Columbia
River a few miles downstream from the mouth of the White Salmon River; from 100 to 750 ppm
at Bonneville Dam; and from 80 to 625 ppm in the lower Columbia River (G&G Associates
2004a and 2004b and PacifiCorp 2005). After approximately the first week, sustained
concentrations in the Columbia River would be at essentially background levels, with brief
spikes in concentrations occurring over the first month. Any contaminants in the sediment
would move with the sediment. The small amount detected in the samples and the overall
mixing and dilution would make the levels very small, and the consequences on water quality
from mobilizing the contaminants would be minor.
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