Channelization and Channel Modification Activities and Impacts
of in-stream and riparian habitat for fish and wildlife. Clearing of banks along waterways has eliminated
in-stream and riparian habitats, decreased the quantity of organic matter entering aquatic systems, altered
the water temperature, and increased the movement of non-point source pollutants from the upper reaches
of watersheds into river systems and ultimately coastal waters. Excavation projects can result in reduced
flushing, increased suspended sediment loads, lowered dissolved oxygen levels, saltwater intrusion, loss
of riparian vegetation, accelerated discharge of pollutants, and changed physical and chemical
characteristics of bottom sediments in surface waters surrounding channelization or channel modification
projects. Reduced flushing, in particular, can increase the deposition of fine-grained sediments and
associated organic materials or other pollutants. Confining river channels to reduce overbank flooding
results in a reduction of sediment needed to nourish riverine and estuaries, wetlands and riparian areas and
accelerates the delivery of suspended sediments to coastal and near coastal waters during high flow events.
Construction activities that support channel modification projects can have adverse impacts on both river
processes and the environment. Clearing of terrestrial and riparian vegetation results in a loss of habitat and
can accelerate runoff and subsequent erosion of the banks.
Channel modification projects are designed and implemented to provide a benefit to the surrounding
areas, whether for preventive measures such as flood control or economic measures such as mining. The
adverse impacts associated with channel modification activities can be systematically addressed with
specific remedial practices to reduce or eliminate the severity of impacts.
4.1
CHANNELIZATION AND CHANNEL MODIFICATION PROJECT CATEGORIES
Channel modification projects will involve activities that alter channel parameters such as length,
width, depth, slope, discharge, sediment size, or sediment discharge. In Chapter 3, the concepts of channel
stability and system equilibrium were discussed. Lane's Balance was presented as a methodology for
qualitatively determining channel stability as discharge, sediment discharge, slope, and sediment size are
changed due to channel modification activities. The following project category descriptions provide a
broad overview of the need for projects, and the general activities that are implemented to accomplish the
project goals. The concepts embedded in Lane's Balance apply to channel modification activities that
involve changing the variables critical for channel stability.
4.1.1
FLOOD CONTROL AND DRAINAGE
Channel modification activities for flood control are designed and implemented to insure that flood
flows remain within acceptable levels. Methods are implemented to either convey, confine, or control flood
discharge. The projects reduce the channel resistance to flow, provide flood water storage, bypass the
flood flows, or artificially confine the flows within the original channel. This can generally be accomplished
by removing obstructions from the channel, straightening the channel, enlarging or deepening the channel,
or constructing impoundments, diversion channels, or levee systems. Although these are logical
methodologies to consider, and will improve flood defense, each can potentially alter the equilibrium of the
channel.
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