Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
sediment may be present. The water content of these sediments would make site access initially
difficult.
Flood Scouring
Once the White Salmon River has established a stable channel through the reservoir sediments,
successively higher flood levels would erode areas not previously reached by floods. This
process would continue over time, but the frequency and magnitude of the erosion events would
vary over time. Computer model simulations by G&G Associates using actual flow records for
the White Salmon River show that only two floodplain widening events are likely to occur
subsequent to the fifth year after breaching (PacifiCorp 2004).
Downstream Effects
As the water levels fall after dam breaching, coarser sediment would be stranded within
floodplain areas downstream of the dam. Finer sediment would be suspended and continue to be
transported downstream. Pools in the river might fill with some larger sediment present along
the current stream bank; however, reservoir and deltaic gravels would not yet have been moved
by the river unless very high river flows occur soon after breaching occurs. A large portion of
the reservoir sediment would be deposited in the lower 0.8 mile of the river, which is part of the
Columbia River Bonneville Dam pool. Approximately 0.6 million cubic yards of the predicted
1.6 to 2.2 million cubic yards of sediment eroded from the reservoir would be deposited in the
lower portion of the river known as the “in lieu site.” Finer suspended materials would form a
plume that extends into the Bonneville pool. The sediment concentration of this plume would
decline downstream because of settling in the water column, spreading of the plume, and dilution
with flow from tributaries. The mixing zone would extend approximately three miles
downstream from the mouth of the White Salmon River (PacifiCorp 2004). After the first six
hours, when the flow would return to normal, sediment concentrations in the Columbia River
would drop dramatically and continue to drop to background levels within approximately one to
two weeks. Brief spikes may occur over approximately the first two months.
As long as sediment retained behind the dam is subject to river transport, even on an intermittent
basis, turbidity levels in the White Salmon River would be affected. Computer model
simulations have been used in an attempt to address expected erosion of the reservoir sediments;
however, the actual rates of erosion depend on the unknown bedrock geometry, sediment sizes,
rainfall, and river flow conditions. These unknown factors may affect the model predictions and
require additional sediment management, as described below.
Dam and Appurtenance Removal
Impacts associated with dam and appurtenance removal are associated with the use of haul roads,
staging areas, and disposal sites. As part of removal of the structures associated with the dam,
the surge tank tailrace would be filled in.
The original cofferdam present beneath the reservoir sediment would be removed by blasting and
log yarding equipment with little impact to the geology, soils, and sediment. If removal by that
means is not possible, an access road would be required. The access road would require blasting
and would change the topography. After cofferdam removal, the sediment trapped behind the
cofferdam would be released.
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