Channelization and Channel Modification Activities and Impacts
provide needed protection and placing artificial structures such as boulders, gravel, or sills into the channel
to provide the needed channel bed diversity for aquatic organisms to thrive.
National policy requires full consideration of recreation as a project purpose during the planning
of water resources development projects (Office, Chief of Engineers (OCE), 1982). Incorporation of
recreational features in a channel modification plan not only provides project benefits that offset any adverse
impacts, but also generates positive public perception of the project. Many recreational facilities on channel
modification projects are cost shared with local sponsors. Recreational activities associated with channel
modification projects include creation of lakes or reservoirs for water sports such as boating and fishing,
nature trails or study areas, and campgrounds. Channel improvements may be initiated to improve fisheries
and recreational boating. Channels should be designed to provide adequate access, suitable low flow
depths, and as few obstructions as possible for recreational navigation (Nunnally and Shields, 1985).
FLOW CONTROL FOR WATER SUPPLY
Channel modification activities associated with water supply include the creation of impoundments
and diversion canals with associated flow control structures. Dams and reservoirs have multiple uses such
as municipal water supply, flood water storage, sediment storage, and recreation. Diversion canals supply
water for irrigation, municipal water supply, and divert flood waters.
The construction of dams and associated reservoirs interrupts the natural sediment discharge of
the pre-dam river system. Dams alter the flow and sediment regime that can result in significant
morphological changes in downstream reaches. The bulk of incoming sediment is stored in the reservoir.
Thus, the sediment discharge downstream is minimal. Additionally, a reduction in discharge can occur
during dam operations to reduce flood peaks downstream. In accordance with Lane's Balance (Chapter
3), assuming that the flow and sediment size remains constant, the reduction in sediment discharge results
in a decreasing channel slope. The reduction of water discharge, however, will allow a steeper slope to
exist. Therefore, predicting the response of a downstream channel to dam flow control is extremely difficult
due to these complexities (USACE, GDM-54, 1990a). Additionally, water discharged through the dam
typically is at a lower temperature, thus possibly limiting the habitat acceptable for native aquatic species.
Diversion canals can also impact the stability of the river system by diverting water and sediment
from the main channel. Impacts include degradation of the bed and associated bank erosion and failure
and a reduction in habitat diversity. Morphological impacts occur for both the diversion channel and the
source stream. A reduction in flow in the main stream due to the diversion results in a decreased sediment
discharge, thus deposition occurs downstream of the diversion, assuming that the diversion does not change
the bed material load in the main channel. If a substantial bed material load is diverted without a