Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
4.0 AFFECTED ENVIRONMENT, IMPACTS, MITIGATION
MEASURES, AND SIGNIFICANT UNAVOIDABLE
ADVERSE IMPACTS
4.1 GEOLOGY, SOILS, AND SEDIMENTS
This section describes the affected environment and the effects of dam breaching and removal
activities and processes on geology and soils. It also describes sediment movement and potential
sedimentation that could occur from the dam removal and other removal activities. For a
discussion of the impacts of sedimentation on water quality, please see Section 4.2 (Water
Resources). Mitigation for sedimentation effects on the environment is described below, along
with significant unavoidable impacts.
4.1.1 Affected Environment
The affected environment for geology, soils, and sediments includes Northwestern Lake (Condit
Dam reservoir), the area immediately surrounding Condit Dam, access roads to work areas and
staging areas, the tailrace from the power plant, the corridor occupied by the wood stave pipe and
the power line, and the spillway from the surge tank to the White Salmon River. It includes the
White Salmon River and its floodplain from the dam down to and including part of the pool for
the Bonneville Dam on the Columbia River.
The project site is located on the White Salmon River in an area of rugged topography
approximately three miles upstream from the confluence of the White Salmon and Columbia
Rivers (USGS 1994). Soils in the White Salmon River Basin are generally characterized as
loams and silt-loams derived from the regional basaltic bedrock, andesite, and pumice. Near
Condit Dam, these soils are shallow and typically underlain by an intermediate gravel unit above
the bedrock. Bedrock outcrops are common in areas of steeper topography along the river
(FERC 1996).
Immediately west of the dam, the geology is dominated by colluvial deposits, capped by coarse
glacial outwash material at the rim of the canyon. East of the dam, the topography is somewhat
less severe, supporting a thin veneer of soil over the basaltic bedrock. North of the dam, the soils
along the shores of Northwestern Lake are composed principally of the Hood loam (USDA
1990). Bedrock is present over much of the channel of the White Salmon River below the dam.
At the time of dam removal, an estimated 2.7 million cubic yards of sediment (2.3 million
according to Finley 2006), primarily silt and sand with some gravel, will have accumulated in
Northwestern Lake since the construction of Condit Dam in 1913. More than 7,500 feet
upstream from the dam and primarily upstream from Northwestern Lake Bridge, the size of
reservoir sediments is dominantly in the sand (.075-4.75 mm) and gravel (4.75-75 mm) range.
Closer to the dam, the sediments are primarily sand and silt (.0039-.075 mm), and finally silt and
clay (