Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Impacts
Sediment Transport
• The longer the sediment takes to exit the reservoir, the greater the impacts
on water quality and aquatic organisms.
Table 1-1 (Continued)
Summary of Impacts and Mitigation Measures
Geology, Soils, and Sediments (Continued)
Sediment Transport
Mitigation
• The dam would be breached in October to minimize the risk of harm to
seasonal fish runs and to take advantage of the rainy season when there
would be fewer adverse effects on aquatic life and recreation. The higher
seasonal flows would aid the transport of sediment from the reservoir.
• Dislodging unstable sediment and woody debris would help to ensure that
the reservoir sediment is transported downstream quickly, therefore within
the predicted three- to five-year period, and does not affect long-term
water quality. It might also help to mitigate downstream flooding related
to sediment transport and deposition.
• If possible, the Bonneville pool level would be lowered by the Corps of
Engineers when the dam would be breached to facilitate sediment moving
past the in-lieu site.
• Much sediment would be deposited in the in-lieu site. • As part of the Settlement Agreement, funds would be provided to the
tribes. These funds could be used for maintenance, including dredging, at
the “in lieu” site. Because of the natural flux of sediment that would be
transported downstream after the dam is removed, removal of sediment
from the “in lieu” site may need to be accomplished repeatedly after dam
breaching. Any dredging would be subject to a separate environmental
review and permit process.
Post-Removal Management
Upstream Sediment Management
• Unstable sediment slopes in the former reservoir area pose safety and
aquatic organism impact concerns.
Post-Removal Management
Upstream Sediment Management
• Sediment that remains in the former reservoir area would be evaluated for
stability in accordance with the Bank Stabilization Plan (PacifiCorp 2004),
and unstable sediment would be dislodged in a way that protects public
and worker safety and the environment.
• Use of mechanical means to modify unstable sediment slopes would
require building temporary access roads across the reservoir sediment.
Access roads across reservoir sediment would be removed after they are
no longer necessary.
• To avoid fish passage and erosion issues the delta at Mill Creek will have
a stable channel cut through it by equipment that is onsite for other
sediment management activities if fish passage does not develop naturally
by May 1 of the year following dam breaching.
1-14