Channelization and Channel Modification Activities and Impacts
reported that while the reservoirs on the Columbia River basin did decrease the habitat for some mammals,
waterfowl habitats increased. Babcock (1980) reported that on the Arkansas River Navigation Project,
the environmental quality actually improved due to construction of the project. The water quality improved
with a reduction in suspended solids. Dam outflows generally are at a lower temperature than existing
channel flows. The lower water temperature may be suitable for specific species of fish such as trout and
deleterious for native warm water fish populations, and the fishery diversity may be permanently altered.
Aquatic and terrestrial habitats are impacted by a reduction of flushing flows through the dam. In periods
of low flow through the dam, fish and other aquatic organisms that depend on higher flows for food and
habitat are affected. Terrestrial habitat along the stream that experienced frequent overbank flows in pre-
dam conditions may be dry for prolonged periods of time, thus potentially displacing wildlife dependent on
a more wet environment. Additionally, flows through the dam will be based on needs such as hydropower,
flood control, and recreation. This will result in a change in the channel forming discharge that will alter
channel morphology and subsequent habitat features. Throughout the country, such as the Northwest, fish
passage around dams is a serious environmental concern. Dams block migrating fish such as salmon from
completing spawning runs.
4.2.7.3 Remedial Measures
The construction of dams can adversely impact downstream channel stability. Channel and
streambank remediation techniques may be required to reduce erosion and deposition of sediments resulting
from fluctuating flows and reduced sediment transport through the dam. Changes in dam operating
procedures can be made to accommodate environmental needs. Periodic flushing flows can be released
to enhance downstream aquatic and terrestrial habitats. Fishways or fish ladders can be used to allow
migrating fish to bypass dams. On the lower Snake River in Washington, salmon are bypassed around
dams using barges. In some cases, dam removal is advocated to restore a rivers natural and recreational
value.
4.2.7.4 Operation and Maintenance of Dam Projects
Dams require frequent inspections to insure structural integrity. Grade control structures and
streambank stability projects constructed to remediate channel and bank erosion require periodic inspection
to insure proper operation.
4.2.8
FLOW TRAINING STRUCTURES - DIKES
Dikes are free standing structures of stone, pile clusters, or pilings with stone fill placed within
waterways either parallel or transverse to the channel, and are generally constructed to constrict the channel
at a specific location for the purpose of concentrating flow in a narrower, deeper channel. The reduced
cross sectional area results in an increase in flow velocities thus increasing the sediment transport capability
of the stream. In navigable rivers the decrease in shoaling reduces dredging requirements. Dikes have been
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