Channelization and Channel Modification Activities and Impacts
4.2.1.2 Environmental Effects
The removal of snags and debris reduces habitat diversity in the channel. Increased velocities allow
deposits of leaves, twigs, and fine grained sediments to be washed downstream. These deposits are an
important habitat for many benthic species and in channels with sandy, shifting substrates form the only
suitable habitat. Removal of the vegetative canopy from streambanks may result in decreased shade and
resultant high stream temperatures, decreased input of organic matter such as leaves, and increased
photosynthesis in the stream (Shields and Palermo, 1982). The removal of snags increases the mean
velocity of the stream, which may affect plankton production or erode away fine sediment that provides
substrates for specific kinds of benthic organisms. Impacts on the macro invertebrate community will
ultimately affect fish populations that depend on invertebrates for food. The change in food resources may
result in a fish population reduction or an undesirable change in species composition. Additionally, fish may
be adversely affected by the removal of snags that serve as cover and shelter.
Clearing large amounts of terrestrial vegetation can affect terrestrial communities. Populations of
mammals and reptiles that utilize streambank vegetation for shelter and feeding areas will decrease
accordingly. Studies in Vermont (Dodge et al., 1977) and Mississippi (Arner et al., 1976) found mammal
track counts along natural streams were almost twice as great as mammal track counts along streams that
had vegetation removed by snagging and clearing operations.
4.2.1.3 Remedial Practices
Adverse environmental effects may be greatly reduced with little loss in flood control by limiting the
type and amount of snags and vegetation removed and by using construction methods that create only
minimal disturbance (Nunnally and Shields, 1985). Specific obstructions are designated for removal while
environmentally valuable logs, snags, and vegetation that have little or no effects on flow capacity are left
in place. Planning and design of clearing and snagging operations should include an evaluation of the
importance of the canopy to the stream community. Specifications may be written to restrict the amount
and type of terrestrial vegetation to be removed. Additionally, the type of equipment used and the access
to the stream can be controlled by specification.
4.2.1.4 Operation and Maintenance of Snagging and Clearing Projects
Clearing and snagging operations require more frequent inspections than other flood control
projects, particularly in regions having long growing seasons. Regrowth of vegetation on cleared channel
sides and top banks can significantly increase resistance more than one or two years growing seasons.
Clearing and snagging projects should never be implemented on unstable streams with wooded banks
because of the high probability of future bank failures with the subsequent re-introduction of debris into the
stream.
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