Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Post-Removal Management
Upstream Sediment Management
Unstable slopes, high angles of repose, and lateral channel migration until the river and its
tributaries downcut to their original bedrock channels would create temporary pulses of
suspended sediments that can have physiological and behavioral impacts to fish that can
result in injuries, stress, reduced foraging efficiency, increased predation rates, etc. (Bash et
al. 2001).
Coarse sediment and large woody debris released during dam removal may create debris
dams that create fish passage issues. The banks of the channel created by the river downcutting
through lakebed sediments are likely to be deep and have a very steep angle of
repose. Deep drawdowns for dam maintenance operations on dams in the Boise River
watershed of Idaho created extensive flats of mud and sand through which the river downcut,
creating steep banks that slough continually as the river migrated laterally (Salow 2004).
Bank sloughs can have lethal consequences for fish as radio tagging studies have shown.
During a study by Salow (2004) stationary tags often had to be dug out of a bank slough
along with the tagged fish that was buried during the collapse of the streambank (Salow
2004). This could be a problem for returning fish, especially during the first year after
breaching the dam.
Blasting to stabilize slopes or remove debris jams during the migration of winter- and
summer-run steelhead in the spring and summer following dam breaching would expose
steelhead to hydrostatic shock, and some fish mortalities are likely to occur in proximity to
blasts.
Any fish occupying lacustrine habitat at the time of dam removal would probably be flushed
through the tunnel and downstream to the Bonneville pool. Many will be killed by high
concentrations of suspended solids suffocating them or direct physical injury, but those that
survive the trip down to the Bonneville pool would probably resume a lacustrine or
lacustrine-adfluvial life history in the pool.
Downstream Sediment Management
Immediately after breaching of the reservoir, sediment would be deposited in the floodplain
areas at progressively lower levels as the river flow subsides. This sediment would be
transported downstream during natural storm and flood events. Woody debris trapped
behind the dam would be washed downstream and accumulate at various points along the
river, creating log-jams. If woody debris in the river does not interfere with downstream
transport of large quantities of bedload or normal development of streambanks, it would help
to store gravel in the river channel to eventually be used as spawning gravel. If blasting is
required to redistribute large woody debris, any fish in the proximity of blasts would likely
be killed by hydrostatic shock. Attempts to scare fish away from blasting are unlikely to be
successful unless some form of chemical can be used as a repellent (perhaps the scent of a
predator).
4.3-25