and to determine the likely effective-
ness of bar reduction in improving
the ice conveyance capacity of the
confluence. In addition, the model-
ing sought to determine whether the
use of bendway weirs would contrib-
ute inadvertently to other problems
for ice movement through the con-
fluence. The modeling builds on the
findings of the small-scale diagnos-
tic model described above, translat-
ing their significance to the situation
of an actual confluence of two rivers.
A precursory investigation was
conducted of the weather and river
flow factors to obtain background in-
formation concerning the flow and
ice conditions typically prevailing
when ice jams form in the conflu-
ence. Those conditions were simu-
lated in the model.
Figure 33. Light ice run in the confluence of
Mississippi and Missouri Rivers (9 Feb 1989).
Weather and flow conditions
associated with jamming
A study of the conditions associated with ice jam formation at the confluence of
the Mississippi and Missouri Rivers leads to the following findings:
The Missouri River is the major source of ice contributing to the jam. Because
of the ice retention action of the navigation structures along the Mississippi
River, only a relatively small amount of ice enters the confluence from the
Mississippi River.
The ice from the Missouri River comprises mainly pans and floes of frazil ice
generated during a freezeup of the Missouri River. Therefore, the confluence
jam is a freezeup jam.
A period of frigid air temperatures precedes and sometimes accompanies the
jams.
The jams occur when flow rates are low in both the Missouri and Mississippi
Rivers. At low flow rates, the bar in the confluence is exposed and, relatedly,
the surface area available for ice passage through the confluence is constricted.
The constriction is further aggravated by the growth of border ice along the
bar and in regions of sluggish flow in the confluence.
Table 2 lists ice events at the confluence. The list is too brief to provide reliable
statistics on the frequency of ice jams. Jams sometimes have occurred in a succes-
sion of years; otherwise, they have not occurred for periods of about 5 to 10 years.
Tuthill and Mamone (1997) suggest a tentative return period of 10 years for jams
at the confluence.
The occurrence of jams can be linked directly to low flow rate through the con-
fluence and frigid air over the rivers upstream of the confluence. For example, this
linkage can be illustrated for ice jams that formed during the winters of 197778,
197879, and 198081. Table 3 briefly summarizes the main characteristics of the
42
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