water condition. These sites relate to one confluent channel having sluggish or
negligible flow velocities or covered by stationary ice. The seventh and eighth
sites selected do not involve sluggish flows at the confluence. One site is a lake
discharging into a river, which may be viewed as a series confluence of an ex-
tremely wide channel and a narrow channel. The other site is the confluence of
two rivers, the Missouri and Mississippi. As discussed subsequently in the present
report, the use of bendway weirs indeed would be effective in mitigating jam for-
mation at the confluence of those two rivers.
The problem of ice jam formation in the confluence of the Mississippi and the
Missouri Rivers was been investigated in three studies: USACE (1962), USACE
(1977), and Stevens (1978). The two USACE studies document conditions atten-
dant to jam events that occurred in 1962 and 1977. The focus of the study by Stevens
(1978) is on various fundamental aspects of ice-jam formation in generic channels,
rather than on the confluence of the Mississippi and Missouri Rivers per se.
The general mechanisms causing ice jams at the confluence of two channels are
not discussed in Tuthill and Mamone's report. It focuses mainly on the eight con-
fluences selected for consideration of structural mitigative measures. Nonethe-
less, a major proportion of jam problems at confluences may be hypothesized to
occur for two reasons:
Ice from one channel discharges into a channel that has a sluggish flow (a lake
is a limiting example of this situation) or has a stationary ice cover.
Bathymetric irregularities in confluence geometry retard ice discharge and
initiate jams.
On the basis of Tuthill and Mamone's survey, situations where ice jams result
from the merging of ice discharged from two confluent channels seem to be un-
common. For many large watersheds, the joint probability of ice discharging si-
multaneously from two channels into a confluence likely is relatively small. There-
fore, the frequency of these jamming situations likely is small for rivers draining
watersheds in significantly different hydrologic regions. Nonetheless, rivers drain-
ing closely adjoining hydrologic regions may experience ice runs at about the same
time. Also, somewhat of an exception is ice discharge through confluent branches
of a braided-meandering channel or a river channel that initially bifurcates around
an island (e.g., the Upper Niagara River bifurcating then merging around Grand
Island) or a large bar. Jamming of confluent ice discharges may be more likely for
freezeup situations, when channels in an entire watershed form and convey ice,
than in breakup situations, which arguably are more haphazard in their occur-
rence because of the greater diversity of factors affecting cover breakup.
The survey conducted by Tuthill and Mamone (1997), together with the infor-
mation contained in the other ice jam literature reviewed, directed the present
study toward addressing the following two aspects of ice jam formation at conflu-
ences:
Determine the conditions needed for two confluent ice discharges to jam.
Determine how confluence bathymetry may hamper ice movement through
confluences.
In summary, ice jam literature reveals that confluences play important, but quite
varied and inadequately understood, roles in the formation of ice jams in water-
sheds. Ice in confluences increases further the complexity of the flow, not only
from the hydrodynamic point of view, but because thermal and mechanical phe-
8
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