WSP09281

• File Number: 8407.500
• Description: Platte River Basin - River Basin General Publications - Missouri River
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 5/1/1983
• Author: MBSA
• Title: Missouri River Basin Hydrology Study - Final Report - Missouri Basin States Association
• Water Supply Pro - Doc Type: Report/Study

UJH73

WATER USE AND AVAILABILITY

As described in a previous section of this report,
water use is accounted for by net depletions to stream-
flow. Water availahility is accounted for by monthly
adjustment of net depletions to historic streamflow.
The following sections descrihe streamflow depletions
and water availability on a subbasin basis.

Upper Missouri Subbasin

The Upper Missouri Subbasin covers 82,755 square
miles in the extreme northwest part of the Missouri
River Basin, Figure 14 (page 23). Here the principal
tributaries of the Missouri, the .Jefferson, Madison,
and Gallatin Rivers meet at Three Forks, Montana to
form the Missouri River. These rivers originate in
mountainous areas and then flow through level to
gently rolling plains. Elevations in the basin range
from 1,860 to 11,293 feet. The topography is of two
types: a series of intermountain valleys descending
from the Rocky Mountains in the west., and a large
table land prairie, broken by deeply entrenched, nar-
row, meandering streams in the northern and eastern
parts of the subbasin.

The majority of the subbasin area is located in the
State of Montana with small areas also in Wyoming
and North Dakota. A small percent of the basin lies
in the southern Canadian provinces of Alberta and
Sflskatchewan.

Grassland is the predominate ecosystem with areas
of cropland dispersed throughout the subbasin.
Ranching and farming, with production of livestock
and small g-mins is the predominate economic activity
in the subbasin. Mining, tourism, and forestry are also
important sectors of the economy.

The ground. water resource in this subbasin is lim-
ited to small alluvial aquifers near streams. While gen-
erally sufficient for domestic use, this supply does not
provide for extensive irrigation development. Overall,
ground water is not an important source of water in
this subbasin.

The majority of the surface water in this subbasin
originates as mountain snow in the Rocky Mountains.
As a result. high flows occur in the spring and early
summer months of May and June. The surface water
outflow point selected for this suhbasin is the USGS
gage number 0618,';500 located on the Missouri River
near Culbertson, Montana. The drainage area above
that point is 91,557 square miles, For the 1944 to 1978
period of record, the average recorded streamnow at
thai point was about 10,872 c.f,s. or 7.88 million acre-
feet annually.

Water imported or exported can have a substantial
impact on water supply availability, particularly if the
quantities involved constitute a significant part of the
total now at a given point. In this subbasin there is
an import of water from the Saint Mary's River basin.
In addition, one of only two exports of water from the
Missouri River hasin occurs in this subbasin. That
export is for water supply to the City of Butte, Mon-
tana from the Big Hole River. Imports and exports of
water generally are tied to a specific use, therefore,
the quantities may vary from year to year depending
on climatic or other conditions. Figure ]5 shows t.he
annual volume of water imported via the St. Mary
Canal near Babb, Montana. As shown, imports over
the 35 year time period have varied from a low of
80,000 acre-feet to a high of 220,000 acre-feet an-
nually.

The annual amount of t he Big Hole River export
to the Butte water supply was constant at about 13,500
acre-feel per year (Figure 16).

In the Upper Missouri Subbasin, there are four ma-
jor reservoirs, Figure 14. Fort Peck is the largest with
a normal capacity of 15.4 million acre-feet. This res-
ervoir is the oldest and the furtherest upstream of the
six Missouri River main stem reservoirs operated by
the Corps of Engineers. The Bureau of Reclamation
constructed two major reservoirs in this subbasin,
Canyon Ferry and Tiber with normal capacities of
1,947,000 and 967,300 acre-feet, respectively. Holter
Dam with a normal reservoir capaciy of 245,000 acre-
feet is the largest private reservoir development in this
subbasin.

Evaporation of water from the surface uf man-made
reservoirs is a major depletor of water. As shown in
Figure 17, the average monthly evaporation from the
Tiber reservoir during the peak evaporation months
of July and August is approximately 5,000 acre-feet.
During July and August, for the Canyon Ferry res-
ervoir, average monthly evaporation is approximately
15,000 acre-feet, Figure 18.

Because of the reservoir depth, peak evaporation at
Fort Peck reservoir occurs at a somewhat later period
of the year, Figure 19. In September, it is estimated
that the average monthly net evaporation at Fort Peck
reservoir exceeds 100,000 acre-feet. Net evaporations
for the six main stem reservoirs were obtained from
the Corps of Engineers reservoir operations. Net evap-
oration for main stem reservoirs, as shown in Figure
19 for example, takes into consideration the heat en-
ergy stored in the reservoir. Evaporation for the trib-

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