Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
dam was built. Initially, pools in the White Salmon River below the dam would be filled by
coarser sediment from the upper portion of the reservoir and from erosion of the tributary deltas.
Over the longer term, natural sediment flux would periodically fill these pools. Alternate
scouring and filling of these pools based on the ability of the river to erode and transport coarser
sediments is a natural process and a consequence of the river being returned to a more natural
state. Deposition of sediment in the Bonneville pool may be similar to that at the month of the
Klickitat River assuming relative sediment loads are similar. 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 of mobilizing it
minor.
4.1.3 Mitigation Measures
Pre-Dam Removal Activities
Haul Roads and Staging Areas
• Areas where soil has been disturbed would be revegetated in accordance with the
Plan for Revegetation of Reservoir Area and Other Areas Disturbed by Construction
Activities (PacifiCorp 2004).
• Fill placed in support of temporary roads on the reservoir sediments would be
removed once the roads are no longer needed.
• Erosion control for construction activities outside the dam and reservoir area as
described in the Upland (non-reservoir area) Stormwater and Erosion Control Plan
(PacifiCorp 2004). This plan provides for erosion stabilization for all roads, storage
or construction staging areas.
• Compliance monitoring would be independent from construction activities.
• At the conclusion of the proposed action all work sites would be regraded and
revegetated in accordance with these plans. This work is expected to stop erosion and
sediment release to the White Salmon River associated with these activities.
Northwestern Lake Bridge
• As described in the Sediment Assessment and Management Plan (PacifiCorp 2004),
silt curtains would be used during sheet pile installation to minimize silt entrainment
in the water, or construction would be performed as the water level is being lowered.
In either case, silt entering the water column would be minimized (PacifiCorp 2004).
• Any work near the river would use erosion control mats and silt fencing to protect the
river from sediment release.
Dam Breaching and Removal
Drain Tunnel
• The drain tunnel would be constructed with a slight bell shape with the largest
diameter downstream to prevent large woody debris from clogging the tunnel.
4.1-6