Channelization and Channel Modification Activities and Impacts
value can be assigned were for flood control, hydropower generation, irrigation, and recreation. For the
Columbia River dam projects the adverse environmental impacts were primarily due to the dams blocking
the salmon migration routes. Because of the multipurpose nature of some dams, it is difficult to optimize
the beneficial aspects of each use. From the flood control viewpoint, it is necessary to reduce flood peaks
downstream. This practice may result in inadequate flows for power generation and disrupt fish spawning.
Dams change the flow and sediment transport characteristics of the river. The back water extends
upstream of the dam, acting as a sediment retention basin. Regulated flows through the dam along with
reduced sediment transport below the dam may affect downstream channel stability.
4.2.7.1 Hydraulic Effects
The primary effect of dams on system stability is to reduce peak discharges and sediment supply
to the downstream channel. Upstream effects of a dam and associated reservoir include delta formation,
gradual raising of stream levels in the backwater zone, and a more pronounced meandering (USACE, EM
1110-2-1418, 1994). Downstream effects result from flow control through the dam and retention of
sediment. A reduction in peak discharge often reduces bank instability downstream by inducing deposition
at the channel margin in the form of berms. The channel adapts to a lower channel forming discharge by
shrinking. Reducing peak discharge and lowering the flowlines in the downstream channel may also induce
tributary instability by lowering their effective base level. Channel degradation in the form of a head cut
advances up the tributaries and ultimately increases the sediment supply to the main river. However,
reducing the sediment supply to the stream through reservoir retention also often induces channel
degradation downstream, which can actually lead to mass instability of the banks by increasing bank
heights. This may trigger a reversal of main channel response and lead to eventual aggradation due to
increased sediment supply from tributaries (Biedenharn, 1983). System response to flow control and
sediment retention aspects of dams are very complex and cannot be easily predicted or generalized.
Factors affecting channel response:
b. Degree of sediment retention;
c. Downstream controls such as geologic outcrops, man-made structures, armor layers and
backwater from another lake or river;
d. Reduced sediment transport capacity of the channel as a result of slope reduction due to
channel degradation;
e. Sediment input from tributaries and bed and bank erosion;
f. Vegetation and vegetative encroachment; and
g. Tributary response.
4.2.7.2 Environmental Effects
The construction of dams results in a decrease in terrestrial habitat through backwater flooding.
However, case studies of dams on selected river basins presented by Peterson (1986) indicate that
reservoirs have had a lesser impact on wildlife than urbanization and agriculture. Green and Eiker (1983)
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