Lake
Sakakawea
Ice Bridges
Trenton
Lake
N
Trenton
East
Bottom
2
1563.2
1564.48
Middle
Bottom
156
Buford
5.44
West
Bottom
Zero
5
15
9.4
7
Bottom
4.1
7
15
6
1E
1570.30
1F
1584.17
03
1578.
1583.07
Figure 2. Details of the BufordTrenton Irrigation District. The cross sections are the locations used by the
Omaha District to monitor sediment aggradation. The locations where ice bridged across the river on 7 March
1992 are also shown.
or even the exact dates when ice cover breakup
Table 1. Missouri River discharge
and ice jamming have occurred in the past are
frequencies below the Yellow-
unknown. In the absence of more detailed histor-
stone River confluence.
ical information, spring breakup flows were tak-
Discharge†
Return interval
Discharge*
en to be the first major peaks in flow occurring
(years)
(cfs)
(cfs)
during March or early April. These events may
2
95,000
90,000
not accurately represent the date of actual ice cov-
5
130,000
120,000
er breakup events and their discharge magni-
10
160,000
150,000
tudes, but they will provide a conservative esti-
25
210,000
190,000
mate for use in further analysis.
50
260,000
251,000
100
340,000
300,000
The estimated breakup discharge frequencies
500
440,000
400,000
presented in Table 2 were developed by ranking
these combined flows, plotting them on log-prob-
* After USACE (1978).
† Based on data provided by Roger L. Kay,
ability paper and fitting a curve by eye. The peri-
Omaha District, June 1992.
od of record ranged from 1966, after Lake Sakaka-
wea reached its maximum normal pool elevation,
through 1990. For comparison, dischargefre-
the month of March contained in the 1978 report
quency values for peak flows in March devel-
(USACE 1978) will be used later in the report.
oped by the Omaha District (USACE 1978) are
While these discharge values are appropriate
for determining open-water flood flows, the anal-
also included. The March values are somewhat
ysis of spring-breakup-related flooding requires
higher than the estimated breakup discharges
information on flow magnitudes during past
since they consider the maximum discharge in
breakup events. Unfortunately the actual flows
the calendar month rather than the first signifi-
3
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