RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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REVIEW OF RIVER ICE INFLUENCES ON CHANNEL
MORPHOLOGY
Currently it is possible to describe only in conceptual terms how ice might
influence alluvial channel bathymetry. No quantitative evidence exists that ice
hastens or slows large-scale changes, such as the migration of a series of meander
loops. Such evidence would be hard to obtain, since ice is only one of several
factors that influence the dynamic balance between flow, slope, and sediment in
an alluvial channel. It is useful, nonetheless, to consider the possible ways in
which river ice may influence alluvial channel bathymetry.
Changes in channel geometry (e.g., width, statistical properties of meander
wavelengths, and radii) may occur in response to diverse changes in flow rate
and sediment supply. Some evidence exists that ice may influence midscale
features of alluvial channels. For example, ice jams may lead to meander loop
cutoffs. However, at this scale, ice effects are still largely unknown. At the local
(or site) scale, it is possible to identify several mechanisms by which ice may
hasten bank erosion and channel shifting. Two such mechanisms are flow con-
centration beneath an ice cover and bank or bed gouging by an ice run. Yet
questions remain as to whether these mechanisms prevail over other processes
and conditions and exactly how they work.
No prior study seems to have examined the full extent to which the seasonal
appearance and disappearance of river ice perturbs the bathymetry, and thereby
the stability, of alluvial channels subject to frigid winters. The literature regard-
ing ice impacts on alluvial channels is sparse and inconclusive.
Factors affecting river ice
Several factors enable river ice to influence alluvial channel bathymetry.
Most of them are explainable in terms of a functional relationship between a
dependent variable, such as hydraulic radius of flow R, and the typical set of
independent variables for alluvial channels:
R = fR (Q, Qs, ρ, v, d, σg, ρs, g∆ρ, B, So)
(1)
where Q
=
inflow rate of water
Qs
=
inflow rate of bed sediment
ρ
=
water density
ν
=
kinematic viscosity
d
=
bed sediment diameter
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