STREAM CHANNEL INSTABILITY
Present Condition
Stream bank erosion is common in nearly every Vermont river basin. Streams are
constantly adjusting to find the right combination of width, depth, meander pattern, flood plain,
and slope to convey the water and sediment produced in their watershed. Roads, bridges,
buildings, and land uses that have constricted valleys or changed runoff patterns have often
caused stream instability. Streams throughout Vermont have been trying to adjust to these
changes for decades. On some streams, erosion is mostly related to lateral instability. These
streams are moving back and forth in their valleys, but have access to their floodplains and would
exhibit considerably less erosion with riparian vegetation holding their banks together. Other
streams have incised or cut down into their valleys, resulting in tremendous erosion of stream
beds and banks. These streams have lost access to their historic floodplains during the annual
flood by eroding downward. Having gone through this downward erosional process, these
streams are presently eroding outward or laterally, have over-widened considerably, and are
currently filling with sediment to develop new floodplains. The cumulative effect of erosion and
these channel adjustments along these rivers has resulted in tremendous loss of property, loss of
aquatic and wildlife habitat, decreased water quality, and greater risk of flood-related damage.
Over-Widened, sediment-filled White River main stem following 1998 flood
This fact sheet is one of a series on river resource issues written by the ANR, Department of Environmental
Conservation, Water Quality Division, River Corridor Management Section
10 North Building, 103 South Main Street, Waterbury,
Vermont 05671. (802)241-3770
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Reason for Condition
The stability of the river channel is based on maintaining a certain flow of water, shape
and slope of the channel, and sediment loads. When any of these change significantly, the river
channel must change, resulting in erosion of the stream bed or banks.
Channel Incision
Between the 1700's and the 1800's, the building of roads and railroads within the
floodplains, deforestation, and moving streams to accommodate agricultural fields resulted in
unstable river channels. Following the floods of 1973 and 1976, large-scale channelization
practices were employed to reclaim damaged lands. The 70's and 80's were also a period of
extensive gravel mining. Post-flood channel straightening and gravel mining of point bars have
had the effect of steepening the stream channels. A steep channel in a relatively flat valley may
initiate a bed degradation process referred to as “headcutting.” Once a stream begins to headcut,
it will typically erode its way through the five-stage channel evolution process depicted in Figure
1. until it has created a new floodplain at a lower elevation in the landscape.
Five foot lowering of stream bed resulting from channel straightening
and subsequent head cutting.
The bed erosion that occurs when a meandering river is straightened in its valley is a
problem that translates to other reaches. As shown in Figure 2, headcuts will travel upstream and
into tributaries eroding sediments from otherwise stable stream beds. These bed sediments will
move into and clog reaches downstream leading to lateral scour and erosion of the stream banks.
Channel evolution processes may take decades to play out. Even landowners that have
maintained wooded areas along their stream and river banks have watched those banks erode as
the river channel slopes have adjusted to match the valley slopes.
2
Q1.5
I
TERRACE 1
STABLE
FLOODPLAIN
II
Q10
INCISION
(Headcutting)
III
WIDENING
Q10
(Bank Failure)
IV
STABILIZING
V
STABLE
Q10
Q1.5
TERRACE 1
TERRACE 2
Figure 1. Five Stages of channel evolution showing headcutting that leads to
bed lowering and floodplain redevelopment
A.
Top of Bank
C.
Beginning
of headcut
Upstream erosion of
stream bed
Channel Bed
D.
B.
Headcut migrating
upstream
Downstream buildup of
sediment on stream bed
Figure 2. Longitudinal profile showing bed erosion from a headcut moving
upstream leading to channel filling downstream
It is difficult for streams to attain a new equilibrium where the placement of roads and
other infrastructure has resulted in little or no valley space for the stream to access or create a
floodplain. Landowners and government agencies have repeatedly fought to armor and berm
stream and river reaches to contain the flood waters in the channel. These efforts have proven to
be temporary fixes at best, and in some cases have lead to disastrous property losses and resource
degradation.
3