RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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Channel anabranching and avulsion
Channels with tight loops or with subchannels around numerous bars and
islands are liable to accumulate drifting ice and precipitate ice jam formation,
because their capacities are typically insufficient to convey the amount of
incoming ice. They may be too curved, narrow, or shallow to enable large quan-
tities of drifting ice pieces to pass. Jam formation may greatly constrict flow,
causing it to discharge along an alternate, less-resistant course. Prowse (1998)
and Dupre and Thompson (1979) suggested that ice-jam-induced avulsion plays a
major role in shifting the distributary subchannels of river deltas.
When an ice jam forms at a channel loop, upstream water levels may rise
enough to force the flow over the banks and across the neck of a meander loop. If
the meander neck is made of erodible sediment and the flow is of sufficient
scouring magnitude, a new channel can form through the neck, leaving the for-
mer channel largely cut off. A meander cutoff shortens and steepens a channel
reach, affecting the river both upstream and downstream from the cutoff reach.
The net effect of ice jams is to reduce channel sinuosity. Mackay et al. (1974)
cited examples of such events. For the Fort Peck reach, anecdotal accounts
describe a loop cutoff that occurred in 1979 at RM1603 after an ice jam.
If, on the other hand, the channel loop is wide and not easily eroded, an ice
jam may have the reverse effect. Rather than causing erosion through the mean-
der loop, the overbank flow may deposit sediment, raising the bank height and
reinforcing the meander loop. Eardly (1938) reported that ice jams caused sub-
stantial sediment deposition on the floodplain of the Yukon River. Anecdotal
accounts also described a similar event that occurred in the Fort Peck reach in the
early 1980s at RM 1632 (Vournas site).
Cover influence on thalweg alignment
An ice cover reduces the effective energy gradient of the flow, and it reduces
the flow's capacity to move sediment, which can alter the channel's shape.
Consequently ice cover formation can, over time, re-orient the thalweg
Figure 10 depicts the effect that the flow drag of a free-floating ice cover has
on water depth at a given discharge; in effect the ice-covered flow is equivalent
to a deeper and slower open water flow. For a constant flow rate, this influence is
equivalent to reducing the channel slope (or reducing the stream power). Figure
11 relates thalweg and channel sinuosity to channel slope (in effect, to energy
gradient and stream power). It suggests that thalweg sinuosity is relatively sensi-
tive to changes in the energy gradient, much more so than the overall channel
sinuosity is. For a given flow rate, sediment provenance, and bed sediment com-
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