Table 2. Missouri River breakup
discharge relationships at critical points along the
discharge frequencies below the
river. These relationships are used to establish de-
Yellowstone River confluence
sign elevations for irrigation control structures. In
(USACE 1978).
a report dealing with the design of channel blocks
to prevent Missouri River water from backing up
March
into the main drainage ditches of the Irrigation
Return Interval
Discharge
discharge
(years)
(cfs)
(cfs)
District (USACE 1978), the Omaha District estimat-
ed that the long-term sediment inflow for the Mis-
2
32,000
36,000
souri River at Culbertson, Montana, averaged
5
67,000
80,000
10
92,000
125,000
13,500,000 tons per year, and for the Yellowstone
25
130,000
170,000
River at Sidney, Montana, the average was
50
160,000
220,000
41,500,000 tons per year. The suspended sediment
100
195,000
280,000
at the Culbertson gaging site averaged 45% sand,
50% silt and 5% clay. For the Yellowstone River at
Table 3. Area flooded by differ-
Sidney, the percentages were 35, 60 and 5%, re-
ent water stages for West Bottom
spectively. Bed material at Culbertson had a mean
and Middle Bottom. Missing val-
grain size (D50) of 0.28 mm, while at Sidney the
ues were not reported in the origi-
D50 was 0.25 mm. Based on an average deposition
nal reference.
density of 70 lb/ft3, the measured sediment inflow
Elevation
West Bottom
Middle Bottom
rate between 1964 and 1975 was 260,000 acre-ft, or
(ft msl)
(acres)
(acres)
about 23,600 acre-ft/yr.
1858
--
0
The aggradation analysis contained in the
1859
--
50
USACE (1978) report is being updated based on
1860
0
1450
surveys of sediment deposits made through 1989
1861
--
1950
and a review of sediment transport data. Prelimi-
1862
200
2400
nary results of a study by the U.S. Geological Sur-
1863
--
2800
1864
450
--
vey on the Missouri River at Bismarck, North Da-
1865
700
--
kota, indicates that the procedures previously
1866
1000
--
used by the Corps of Engineers for collecting and
1868
1800
--
analyzing sediment load data at Bismarck, and
1870
3500
--
possibly elsewhere, may have overpredicted sedi-
ment loads by an average of 30%. The updated
cant discharge peak. Further, the March dis-
aggradation analysis, described in a draft report
charge frequencies were developed using a log-
(USACE 1992), found that approximately 486,000
Pearson Type III distribution rather than a
acre-ft of sediment were deposited in the reach be-
log-normal distribution, and the period of record
tween the confluence and Tobacco Garden Creek
analyzed covered 19291975. As discussed previ-
(river mile 1512) between 1956 and 1988, or about
ously, when the all-season dischargefrequency
15,200 acre-ft/yr.
relationship was extended to cover the period
The deposition of sediment in the calmer head-
from 1929 to 1984, the predicted ten-year dis-
waters of Lake Sakakawea has resulted in a pro-
charge value was reduced by about 5%.
gressive loss of channel capacity and an upward
In their 1978 analysis of flooding problems
shift in the stagedischarge relationship for the
within the BufordTrenton Irrigation District
Missouri River in the BufordTrenton area. The
(USACE 1978), the Omaha District estimated the
Omaha District (USACE 1992) found that from
area flooded by different water stages for the
1965 through the mid-1970s, Missouri River
West Bottom and Middle Bottom areas. These
stages for a discharge of 40,000 cfs had shifted up-
were based on stagedischarge rating curves at
wards by 2.5, 3.3 and 5.0 ft at water level gages 5A,
HEC-2 cross sections 1563.5 and 1570.0 and topo-
6 and 7, respectively. Water level gage 5A is located
graphic maps of the area. The results of this
just below the Yellowstone River confluence, near
analysis are presented in Table 3.
HEC-2 cross section 1581.31. Water level gage 6 is
adjacent to the West Bottom at cross section
Sedimentation
1576.38, while gage 7 is adjacent to the Middle Bot-
The Omaha District investigated channel
tom at cross section 1566.39.
aggradation in the Missouri River at the upper
Since the mid-1970s, aggradation in the vicinity
end of Lake Sakakawea to predict future stage
4
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