Water-quality assessment of the upper Snake River Basin, Idaho and western Wyoming — Environmental setting, 1980-92

The 35,800-square-mile upper Snake River
Basin is one of 20 areas studied as part of the
National Water-Quality Assessment (NAWQA)
Program of the U.S. Geological Survey. Objectives of NAWQA are to study ground- and
surface-water quality, biology, and their relations
to land-use activities. Major land and water uses
that affect water quality in the basin are irrigated
agriculture, grazing, aquaculture, food processing,
and wastewater treatment. Data summarized in
this report are used in companion reports to help
define the relations among land use, water use,
water quality, and biological conditions.
The upper Snake River Basin is located in
southeastern Idaho and northwestern Wyoming
and includes small parts of Nevada and Utah. Total
population in the basin was about 425,000 in 1990.
Major urban areas are Idaho Falls, Pocatello,
Rexburg, and Twin Falls, Idaho, which make up
10, 11,3, and 6 percent of the total population,
respectively. Climate in the basin is mostly
semiarid and mean annual precipitation ranges
from 8 to more than 60 inches. The eastern Snake
River Plain is the major geologic feature in the
basin and is delineated mostly by Quaternary and
Tertiary basalt flows. It is about 55 to 62 miles
wide and 320 miles long and bisects the basin in a
northeast-southwest direction.
The Snake River is the dominant surface-water
feature and flows about 453 miles from the
southern border of Yellowstone National Park in
Wyoming to King Hill, Idaho, where it leaves the
basin. The Snake River flows through five reservoirs that provide a total storage capacity of more
than 4 million acre-feet. Gravity-flow diversions
are predominant in the upper part of the basin and
totaled 8.8 million.acre-feet in 1980. Pumped
diversions occur mainly in the lower part of the
basin and totaled 408,500 acre-feet in 1980.
The Snake River Plain aquifer is the predominant ground-water feature in the upper Snake
River Basin and underlies the eastern Snake River
Plain. The upper 500 feet of the aquifer may store
200 to 300 million acre-feet of water. Ground-water resources that supply agricultural lands are
sustained by recharge from surface-water irrigation, precipitation, and tributary inflow. Major
ground-water discharges are at springs and seeps
or from ground-water pumpage for irrigation.
Water use in the basin is dominated by irrigated agriculture, which is the largest consumptive
water use in the basin. Major crops in the basin
include potatoes, wheat, sugar beets, hay, and
barley. Most irrigation needs are supplied from
surface-water sources through a series of canals
and laterals. In 1990, about 2.5 million acres were
irrigated with more than 14.2 million acre-feet of
surface and ground water. About 21 percent of the
basin is agricultural land and 50 percent is
rangeland.
Idaho leads the Nation in trout production
for commercial sale. Combined mean annual
discharges from 12 aquacultural facilities in the
basin (1985-90) were about 787,000 acre-feet.
These facilities are clustered in a reach of the
Snake River between Milner Dam and King Hill
where ground-water discharge is from many seeps
and springs that provide sufficient quantities of
good-quality water. Other facilities that release
effluent to the Snake River include 13 municipal
wastewater treatment plants and 3 industrial facilities.