Channelization and Channel Modification Activities and Impacts
4.2.11.2 Environmental Effects
Activities and practices implemented for stream restoration should have a positive impact on stream
ecology. The introduction of artificial habitats into modified channels provides the diversity necessary to
support a wide variety of aquatic organisms and fish in otherwise unsuitable habitat. Modifications to
channel morphology in terms of restoring stream meander or sinuosity must be carefully planned to avoid
creating channel stability problems. The examples presented above on restoring sinuosity were for low
energy channels that under natural conditions do not actively migrate. Additionally, if watershed changes
occur that alter the sediment and water discharge of the original watershed, attempts to alter channel
morphology may disrupt the equilibrium of the restored channel.
4.2.11.3 Operation and Maintenance of Channel Restoration Projects
The incorporation of artificial structure such as boulders and sills into a modified channel design will
required a periodic inspection plan to insure that the structures remain effective. Habitat enhancement
features should never be placed in an unstable channel subjected to cyclical sediment erosion and
deposition processes. Periodic channel inspections upstream and downstream of the project are required
to evaluate system stability and determine potential future project maintenance problems.
4.3
SUMMARY
Channels are modified from their natural state for beneficial uses such as flood control, navigation,
and water supply. Additionally, channel modifications are required to treat the impacts of channel
instability (bed degradation and excessive sedimentation) resulting from changes in channel and basin
sediment and water discharge capacities. Modifications can result in adverse impacts to channel and
riparian ecology. The primary environmental impact of channel modification is the reduction of habitat
diversity. Straight, shallow channels with homogeneous substrates that are devoid of vertical relief, such
as pools and riffles, do not provide the necessary food, cover, and resting areas for fish and other aquatic
organisms. The destruction or elimination of riparian and riparian vegetation reduces the habitat for
mammals and birds, eliminates plants that provide necessary shade for the stream, and accelerates the
erosion of streambanks. The goal of this chapter was to present general descriptions of channel
modification projects, activities, and practices along with associated impacts on channel stability and
ecology. More specific information on design and implementation of environmental restoration practices
can be obtained from the references cited in this chapter, most notably the references of Shields and
Palermo (1982), Nunnally and Shields (1985), Shields (1983), and Brookes (1988).
Alluvial streams left in a natural state will strive to attain an equilibrium condition for which the
energy available in the stream (water discharge and stream gradient) is proportional to the energy required
to transport a given sediment quantity and size. This qualitative relationship as described by Lane (1947)
provides the basis for qualitatively describing channel and bank stability problems associated with channel
modification activities. Each of the examples of channel modification discussed in the above sections can
103
Previous Page
