The Snake River Plain and tributary valleys south of the Snake River between
Twin Falls and Pocatello, Idaho (here called the South Side area), contain about
180,000 acres of irrigated land, of which 145,000 acres is irrigated with surface
water and 35,000 is irrigated wholly or partly with ground water. The area also
contains more than 200,000 acres of arable land that is idle or used only for grazing
because it lacks irrigation water. Most of the surface-water supply is already
used or reserved, and some land now irrigated needs supplemental water.
The climate of the area ranges from semiarid on the Snake River Plain to
subhumid on higher mountains. The average annual precipitation at lowland
stations ranges from about 9 to 12 inches.
The principal sources of ground water are extrusive volcanic rocks of silicic
to intermediate composition, basalt, and sand and gravel. Ground water occurs
commonly under artesian conditions in the silicic to intermediate volcanic rocks
and in sand and gravel tongues and lenses in lake beds. Basalt and alluvium
commonly contain unconfined water.
The area of this report is divided into 13 roughly defined ground-water districts,
some of which are further divided into subdistricts. The known geologic
and hydrologic factors of each area are summarized and a preliminary appraisal
is made of the ground-water resources in relation to land resources and to the
regimen of streams. The current state of development, proposed new developments,
and ground-water potential of each division are discussed.
The Dry Creek district is the most intensively irrigated area in Idaho in which
wells furnish the water supply. Ground water occurs under both artesian and
water-table conditions. More than 53,000 acre-feet of ground water was pumped
in 1954. There are large areas of undeveloped arable land in the district, but
pumping in some parts of the district currently is approaching or surpasses the
perennial yield of the ground-water reservoirs.
The Golden Valley district contains considerable arable land but, owing to
the relatively great depth to water and the generally poor yield of wells, the
prospects for extensive ground-water development are not promising.
In the Oakley district ground water is pumped from alluvium to supplement
surface water and to bring new land into production. The ground water will be
fully exploited within a few years if the present rate of development by individual
landowners continues. The total area of nonirrigated land far exceeds the amount
that could be irrigated with indigenous ground water.
Both artesian and unconfined water occur in the Burley district. Most existing
wells tap unconfined water in the southern part where there are still large
tracts of idle arable land. Pumping lifts are rather high.
The South Walcott district contains a considerable acreage of arable land and
is underlain by excellent aquifers. The effect that heavy pumping would have
on the flow of the Raft and Snake Rivers and on seepage from Lake Walcott is
Taot well understood. Presumably substantial pumping would be feasible without
direct deleterious effects.
The Raft River basin, including the Elba and Almo-Yost subbasins, is the
largest district in the South Side area. Ground water occurs in both unconfined
and artesian aquifers. Possibly as much as 150,000 acres of dry land is irrigable,
but the ground-water supply presumably is sufficient to irrigate only a few thousand
acres in addition to the approximately 40,000 now irrigated with surface
and ground water. Pumping of wells at some locations would deplete the base
flow of the Raft River and would be competitive with surface-water use.
The United States Bureau of Reclamation has started construction of the
Michaud Flats Irrigation Project in the Western Michaud Flats district. The
adopted reclamation plan is to irrigate about 10,000 acres, using surface water
pumped from American Falls Reservoir and ground water pumped from wells.
Ground water in part of the district is tributary to the reservoir. Withdrawals
of ground water will be compensated in part by the return of waste water to the
reservoir and to the Snake River.
The Eastern Michaud Flats district contains more arable land and has better
aquifers than the Western Michaud Flats district, but pumping might reduce
noticeably the discharge of ground water to the American Falls Reservoir. The
Bureau of Indian Affairs plans to develop about 13,600 acres of Indian land with
water stored in Palisades and American Falls Reservoirs.
Virtually nothing is known about ground-water conditions in the Arbon and
Rockland Valleys and in several small areas such as the Basin district, the
Albion basin, and along the northern border of the Sublett Range. Preliminary
studies have been made in three areas, the Dry Creek, Raft River, and Western.
Michaud Flats districts. None has been studied comprehensively. The available
data for each district are summarized in tabular form.
Further investigations in the area are needed and should include accurate
hydrologic mapping. Studies are needed of the sources and amounts of groundwater
recharge, of the effects of ground-water withdrawals on the total water
supply, and of numerous related problems.