The east-central Great Basin near the Utah-Nevada border contains two great
groundwater flow systems. The first, the White River regional groundwater
flow system, consists of a string of hydraulically connected hydrographic basins
in Nevada spanning about 270 miles from north to south. The northernmost
basin is Long Valley and the southernmost basin is the Black Mountain area, a
valley bordering the Colorado River. The general regional groundwater flow
direction is north to south. The second flow system, the Great Salt Lake Desert
regional groundwater flow system, consists of hydrographic basins that straddle
the Utah-Nevada border, with a length of about 150 miles from north to south.
The general regional groundwater flow direction is from south to north towards
the Great Salt Lake Desert.
For 15 years with support from the Southern Nevada Water Authority (SNWA),
hydrologists, geologists, and geophysicists studied the basin connections and
the groundwater resources in these and adjacent flow systems over an area of
about 25,000 square miles. A major first part of the SNWA study was
constructing a 3-dimensional digital hydrogeologic framework based on
geologic maps and cross sections at 1:250,000 scale. This framework
documents the presence of three major aquifers: (1) Paleozoic carbonate
rocks, (2) Eocene to Miocene volcanic rocks, and (3) Miocene to Holocene
basin-fill sediments, as well as confining units that constrain flow. We
interpret that movement of most groundwater through and across basins is by
fracture-dominated flow along faults/fractures, yet in most places flow is
prevented or retarded across faults, so mapping structures gives a first
approximation to conduits and barriers to flow.
The most important structures by far are high-angle normal faults of the
basin-range episode of east-west extensional deformation. This event
began at about 20 Ma, although most deformation and the formation of the
present topography took place between 10 Ma and present. This topography
consists of north-trending basins (mostly grabens) that alternate with north-
trending ranges (mostly horsts); erosion of the ranges filled the basins with
clastic alluvial basin-fill deposits.
Geophysics provides data on the third dimension (cross sections) of the
hydrogeologic framework. Audiomagnetotelluric profiles and gravity
inversion located faults and enabled us to estimate thicknesses of basin-fill
deposits. To this framework, hydrologic studies addressed precipitation,
surface water, and springs, as well as groundwater levels, volumes,
geochemistry, water budgets, and monitoring. At nearly the same time as
our study, the Utah Geological Survey (UGS) and U.S. Geological Survey
(USGS) addressed the same issues in many of the same areas, and publication
of the efforts by all three agencies reveals a surprising similarity of conclusions,
with some critical exceptions, which therefore demonstrates the great value of
many scientists independently studying the same complex scientific problem.
The differences in conclusions include directions and volumes of some ground-
water flow paths, such as one proposed by the USGS of unlikely groundwater
flow from Steptoe Valley to southern Snake Valley, and another proposed by the
UGS of unlikely significant groundwater recharge flow from the Snake Range to
the Fish Springs complex.