Skip to main content
U.S. flag

An official website of the United States government

Geohydrologic framework of the Snake River plain regional aquifer system, Idaho and eastern Oregon

January 1, 1992

The Snake River Plain in southern Idaho is a major geologic
structure of uncertain origin. Surface geology is generally well
defined, but subsurface geology is poorly defined below about
500 feet. Rocks that underlie the plain form the framework for a
regional ground-water system that supplies large quantities of
water for irrigation and makes the plain nationally important in
terms of agricultural production.
The 15,600-square-mile Snake River Plain is a grabenlike
structure that formed in middle Miocene time. The graben may
have been formed by oblique extensional forces resulting from
interactions between the North American and Pacific tectonic
plates. The oldest known rocks underlying the plain, penetrated
in a 14,007-foot-deep test hole northwest of Boise, are of middle
Miocene age. Miocene volcanic rocks at the plain's margin that
dip toward and underlie the plain were highly faulted and se-
verely eroded before the plain was formed.
Faults along the margins of the eastern part of the plain are
not visible at land surface and have been defined chiefly by geo-
physical methods. However, well-defined fault systems bound
the western part of the plain.
The eastern plain is underlain predominantly by Quaternary
basalt of the Snake River Group, which is intercalated with
sedimentary rocks along the margins. Basalt crops out or is less
than 10 feet below land surface in the central part of the east-
ern plain and is usually less than 100 feet below land surface
elsewhere. Geophysical data and drillers' logs indicate that
Quaternary basalt in the central part of the eastern plain is as
much as 5,000 feet thick. A test hole about 10 miles northeast
of the Snake River near Wendell provided the first information
about deep subsurface stratigraphic relations in that part of the
plain. The stratigraphic sequence penetrated in the test hole is
similar to that in the north wall of the Snake River canyon be-
tween Milner and King Hill. In that area, basalt of the Snake
River Group thins toward the river and is underlain by sedi-
mentary rocks and basalt of the Tertiary and Quaternary Idaho
Group.
The western plain is underlain mainly by unconsolidated and
weakly consolidated Tertiary and Quaternary sedimentary rocks
as much as 5,000 feet thick. Basalt also is present in the west-
ern plain and is most extensive near Mountain Home.
Quaternary basalt of the Snake River Group, which composes
much of the Snake River Plain regional aquifer system, is highly
transmissive. In the eastern plain, a thick sequence of thin-
layered basalt flows yields large volumes of water to wells. Wells
open to less than 100 feet of the aquifer yield as much as 7,000
gallons per minute; yields of 2,000 to 3,000 gallons per minute
with only a few feet of drawdown are common. Transmissivity
commonly exceeds 100,000 feet squared per day and, in places, 1
million feet squared per day.
Large springs in the Snake River canyon between Milner and
King Hill issue at the contact between highly transmissive pil-
low lava and less transmissive underlying rocks. In 1980,
ground-water discharge between Milner and King Hill, largely
spring flow, averaged about 6,000 cubic feet per second.
In the western plain, coarse-grained sedimentary deposits are
thickest and transmissivity is highest along the northern mar-
gins. The percentage of coarse-grained sedimentary deposits de-
creases to the southwest, where lacustrine sedimentary deposits
predominate.
In most of the eastern plain, the upper part of the ground-
water system is unconfined. At depth and in much of the west-
ern plain, aquifers are confined.
Across most of the plain, Quaternary basalt aquifers overlie
aquifers in the Tertiary Idavada Volcanics and Banbury Basalt
of the Idaho Group. The older volcanic rocks are typically much
less transmissive than the Quaternary basalt. Faults and frac-
tures are permeable zones for water storage and conduits for
water movement. In places near the margins of the plain, the
Idavada Volcanics contains important geothermal aquifers.

Publication Year 1992
Title Geohydrologic framework of the Snake River plain regional aquifer system, Idaho and eastern Oregon
DOI 10.3133/pp1408B
Authors R.L. Whitehead
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Professional Paper
Series Number 1408
Index ID pp1408B
Record Source USGS Publications Warehouse
USGS Organization Idaho Water Science Center