Channelization and Channel Modification Activities and Impacts
4.2.11 CHANNEL RESTORATION
After channel modification projects have been constructed, adverse environmental impacts can be
mitigated through channel enhancements and restorative methods. The primary impact on stream ecology
from channel modification is the reduction in habitat diversity. Channel clean out, enlargement, or
straightening practices may result in removal of the existing substrate, pool and riffle areas, and riparian
vegetation and canopy. The resulting effect on the aquatic environment is a reduction in the diversity of
aquatic life as well as population densities. The goal for restoration activities is to accelerate biological
recovery of the stream through the use of various techniques and methodologies without impacting the
stability of the stream.
In-stream structures are used to increase habitat diversity by altering flows, changing channel
morphology and substrate, and providing cover. Artificial structure such as boulders, boulder clusters, or
concrete can be randomly placed in the channel to provide zones of reduced velocity and scour holes
downstream of the boulder. Sills can be constructed across the waterway to create pools above and
scour holes below the structure. Sediment scoured from below the sill may redeposit some distance below
to form a riffle area. A series of sills installed in the stream will develop a pool and riffle sequence that is
highly desirable for providing feeding and resting areas for fish and aquatic organisms.
Channel modification usually results in poorly sorted, finer, less stable bed material (Shields and
Palermo, 1982). A study reported by Arner et al. (1976) indicated that fine, poorly sorted sediments in
a modified segment of the Luxapalila River, Mississippi resulted in a reduction in the quality and quantity
of aquatic organisms. The replacement of natural bed sediments following project completion may speed
the biological recovery. This is more successful when well sorted gravels replace unsorted sediments
(Hjorth and Tryk, 1984). Substrate reinstatement was used to speed the biological recovery of a stream
relocated to allow for coal mining. Gore and Johnson (1980) reported that material excavated from a coal
mining operation was used to line the relocated channel with layers of topsoil, gravel, and cobbles. Benthic
organism populations were rapidly established in the channel by colonization from undisturbed stream
reaches.
Low flows in enlarged channels may be too shallow to support fish and be devoid of pools.
Shallow channels can be excavated within modified channels to convey low flows and provide the
necessary depth for supporting fish and other aquatic organisms. A study conducted by McCall and Knox
(1978) described the environmental benefit of utilizing a low flow notch design in a modified channel for
Rock Creek in north-central Indiana. One year after completion, 23 species of fish were found in the low
flow channel, compared to 16 species collected from the natural channel upstream of the low flow channel
section.
Grade control structures such as weirs and drop structures obstruct fish movement and migration
in the channel. Additionally, culvert and shallow channel sections in which the flow is too slow or swift
impede the natural movement of fish. Fishways or fish ladders are designed to allow fish to either by-pass
or pass through channel obstructions.
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