Ground Water Atlas of the United States -
Segment 1 California Nevada
GROUND-WATER FLOW SYSTEMS
The ground-water flow systems of the Basin and Range area are in individual
basins or in two or more hydraulically connected basins through which ground
water flows to a terminal discharge point or sink. Except for relatively small
areas that drain to the Colorado River, water is not discharged to major
surface-water bodies but is lost solely through evapotranspiration. Each basin
has essentially the same characteristics--the impermeable rocks of the mountain
ranges serve as boundaries to the flow system, and the majority of the ground
water flows through basin-fill deposits. In the area where carbonate rocks
underlie the basins, substantial quantities of water can flow between basins
through the carbonate rocks and into the basin-fill deposits, but this water
also is ultimately discharged by evapotranspiration. Most recharge to the
basin-fill deposits originates in the mountains as snowmelt, and, where the
mountain streams emerge from bedrock channels, the water infiltrates into the
alluvial fans and replenishes the basin-fill aquifer. Intense thunderstorms may
provide some direct recharge to the basin-fill deposits, but, in most cases, any
rainfall that infiltrates the soil is either immediately evaporated or taken up
as soil moisture; little water percolates downward through the unsaturated zone
to reach the water table in the valleys. In mountain areas underlain by
permeable carbonate rocks, most of the recharge may enter the carbonate rocks
and little water remains to supply runoff.
Because regional aquifers are not continuous within the Basin and Range area,
the individual basins, which are encircled by topographic drainage divides, have
been classified as one of four types based on similar recharge-discharge
relations (fig. 25 and table 1). The simplest type is the "undrained, closed
basin," a single valley in which the underlying and surrounding bedrock is
practically impermeable and does not allow interbasin flow, and all recharge is
discharged at a sink represented by a playa near the center of the basin. Basins
underlain by permeable bedrock commonly are hydraulically connected as multiple
valley systems. The "partly drained, closed basin" is underlain or surrounded by
bedrock that is moderately permeable and allows some ground water to flow out of
the basin. In this type of basin, some water is evaporated or transpired at the
upgradient side of a playa, but most of the water continues to flow past the
downgradient side of the playa and leaves the basin. The "drained, closed basin"
has a deep water table that prevents evapotranspiration. The bedrock is
sufficiently permeable to allow all recharge to flow through it and out of the
basin. The "terminal sink basin" is underlain or surrounded by bedrock that is
sufficiently permeable to conduct flow into the basin, and the playa in the
basin is the discharge point for recharge from several connected basins.
In some places, an existing or ancestral stream course connects several basins
that are not closed. The individual basins connected by such streams can also be
classified as partly drained, drained, or terminal sink.
Examples of the individual type basins follow, except for the drained closed
basin. With the exception that the water table in the drained closed basin is
far below the playa, the partly drained and drained closed basins are
sufficiently similar that discussion of each type is not warranted.
Back to "Basin-Fill Aquifers" - Forward
to "Single, Undrained, Closed Basin"
Back to Basin and Range aquifers "Contents"
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