Channelization and Channel Modification Activities and Impacts
The benefits for flood control are increased conveyance of floodwaters. For navigation, a straight
channel reduces transit time and the need for dredging point bars adjacent to bends in the river. Channel
realignment may be necessary to protect an infrastructure located near or on the stream bank.
4.2.3.1 Hydraulic Effects
Changes to a fluvial system, whether man made or natural, tend to be absorbed by the system
through a series of channel adjustments (Simon and Hupp, 1987). Realignment of channels by creating
cutoffs generally reduces the sinuosity and increases the slope. According to Lanes's Balance, if the slope
increases and the water discharge and median grain size remains constant in the stream, the sediment
transport capability of the stream increases. To approach equilibrium, the additional sediment must be
obtained from either bed or bank degradation. As the bed continues to degrade, the zone of increased
slope will migrate upstream. The additional sediment load transported through the realigned reach will then
be deposited in lower reaches where the slope was not increased. Channel erosion migrates upstream in
the form of a headcut, which is a vertical discontinuity in the streambed. The headcutting process is
described in Chapter 3. Bank erosion in the steepened reaches and aggradation in the lower reaches tends
to increase the width/depth ratio. This sequence is the classic response to cutoffs described by Lane
(1947).
4.2.3.2 Environmental Effects
The environmental impacts of realigning channels include many of the impacts of channel
enlargement and snagging and clearing. Overall, the habitat diversity is reduced in the channel as well as
on the banks due to access problems with heavy equipment and clearing of vegetation. The major
problems unique to channel realignment are increased channel slopes due to reduction in channel length and
the reduction in habitat diversity caused by creating cutoff meanders. The increased channel slope results
in an increased sediment transport capacity. The additional sediment requirement is met by degradation
of the bed and stream bank. The degradational process increases sediment loads and turbidity levels that
are detrimental to both benthic and in-stream aquatic organisms. Sediment deposition downstream of the
unstable reach may smother benthic organisms. Unstable, shifting substrates are not conducive to
maintaining macro invertebrate populations. Because of the decrease in light penetration in turbid waters,
photosynthesis is reduced and plant populations are impacted. Fish populations are directly impacted by
the loss of food resources.
Channel realignment activities can result in a significant loss of aquatic habitats. Cutoff meanders
resulting from channel straightening activities are a significant backwater habitat. If the meanders are not
maintained, these will become isolated from the main channel due to sediment deposition at the confluence
with the main channel. The resulting oxbow lake will eventually fill with runoff sediment and become
terrestrial habitats. If the realigned channel is maintained, new meanders will not form to replace the lost
aquatic habitat. A large scale reduction in aquatic habitats will reduce the productivity of the system and
may impact the diversity and population of native aquatic organisms.
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