Hydrology of the Grand Wash Basin

The Grand Wash Basin in the western part of the planning area is located along the boundary of the Colorado Plateau and Basin and Range physiographic regions. Groundwater is found in recent stream alluvium, basin fill, and sedimentary rocks of the Muddy Creek Formation and underlying Cottonwood Wash Formation.  The Muddy Creek Formation is composed of siltstones, sandstones and conglomerates with interbedded basaltic lavas in the northern part of the basin.  The Cottonwood Wash Formation is composed of sandstones and siltstones.

There is a relatively well-defined basin-fill aquifer interbedded with basalt flows between Grand Wash and Gyp Wash (located west of the Grand Wash Cliffs, see Figure 6.2-1).  This aquifer is underlain by the Muddy Creek Formation, which restricts the downward movement of water.  This area was identified as favorable for groundwater development in a geohydrologic reconnaissance study of Lake Mead National Recreation Area conducted by the USGS (Bales and Laney, 1992).

Data on groundwater flow direction, annual natural recharge rate and groundwater in storage is not available for the basin. Recharge from precipitation or local surface runoff is assumed to be small.  In the southwestern corner of the basin, surface water from Lake Mead has saturated adjacent rocks and deposits in quantities greater than pre-lake conditions.  This saturated zone is estimated to extend less than half a mile inland from the lake (Bales and Laney, 1992).

Only 12 wells are registered in the basin. A median well yield is not available. Well yields were estimated to range from 0-500 feet by Anning and Duet (1994). Two wells measured in the basin report water level depths ranging from about 20 feet to over 500 feet bls (see Figure 6.2-6).  Water quality is generally good although total dissolved solids concentrations equal or exceed drinking water standards at several springs (Table 6.2-4).

For information on surface water hydrology in the Grand Wash Basin see Colorado River Lees Ferry to Lake Mead Watershed

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