Ice Jams in River Confluences
ROBERT ETTEMA, MARIAN MUSTE, AND ANTON KRUGER
INTRODUCTION
Confluences are an integral connective feature of most watersheds. They com-
prise relatively short but complex reaches, in which two or more channels merge
and concentrate flows of water and sediment from the upper reaches to lower
reaches of a watershed. For many watersheds in cold regions, confluences also
may concentrate ice formed in the upper reaches of a watershed, which are then
conveyed downstream as ice runs. By virtue of their role in connecting channels
and thereby concentrating ice within a watershed, confluences are perceived as
locations especially prone to the occurrence of ice jams. Indeed, fairly numerous
accounts exist of jams in the vicinity of a confluence. As discussed in this report,
flow and ice concentration in a confluence may cause ice to jam within a conflu-
ence channel, within the confluence itself, or at some distance downstream of a
confluence. Various mechanisms may trigger jams occur in the vicinity of conflu-
ences. Those mechanisms constitute the subject of this report, which presents the
findings of a detailed investigation into ice jam formation at the confluence of two
channels.
The study was motivated by a gen-
eral concern that jams frequently
occur at confluences and by specific
concerns about ice jams formed at
the confluence of the Missouri and
Mississippi Rivers. A severe ice jam
formed at this particular confluence
Missouri
River
is illustrated in Figure 1. The pri-
mary concern regarding ice jams at
Mississippi
this confluence is their blockage of
River
winter navigation between the
Mississippi River and the Illinois
River, which is confluent with the
Mississippi River several kilome-
ters upstream of its confluence with
the Missouri River. In addition, ice Figure 1. Ice jam in the confluence of the Missis-
jams damage and disrupt towboat sippi and Missouri Rivers.
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