Ground Water Atlas of the United States -
Segment 1 California Nevada
Geohydrologic Setting
In all the basins, most of the freshwater is contained in aquifers that consist
of continental deposits of sand and gravel that might be interbedded with
confining units of fine-grained material, such as silt and clay. The aquifers
and confining units compose an aquifer system. Water enters a typical
coastal-basin aquifer in several ways. Runoff from precipitation in the
surrounding mountains infiltrates the permeable sediments of the valley floor
either at the basin margins or through streambeds where the water table is lower
than the water level in the stream. Precipitation that falls on the valley floor
provides some direct recharge, but in the coastal basins, most of the
precipitation evaporates or is transpired by plants. In a few basins that are
hydraulically connected to other basins, water can enter an aquifer system as
lateral subsurface flow from an adjacent basin. Of these methods of recharge,
runoff from the mountains and percolation through streambeds provide the largest
amounts of water to the ground-water system.
Natural movement of water in the aquifers is generally parallel to the long axis
of the basin (fig. 103) because of impermeable rocks that commonly form a
barrier between the basin and the sea. However, in a few coastal basins, most
notably in the Los Angeles--Orange County coastal plain, the coastal barrier is
absent, and the natural direction of flow is perpendicular to the long axis of
the basin or from the inland mountains to the sea. Before major development,
ground water in all the basins discharged directly into the ocean or into bays
connected to the ocean. After development, however, most or all the ground water
is withdrawn by wells in the basins.
Although all the coastal basins have similar hydrogeologic settings, each is
different in its geologic history and land- and water-use characteristics.
Because it is beyond the scope of this Atlas to describe all of the coastal
basin aquifers, only the basins with the largest ground-water withdrawals are
described in this section.
Back to "GEOLOGIC SETTING" - Forward
to "FRESH GROUND-WATER USE AND MANAGEMENT"
Back to Coastal Basins aquifers "Contents"
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