Hydrology of the Virgin River Basin

Located in the northwestern corner of Arizona, the Virgin River Basin contains a broad alluvial valley in the western half and the relatively high elevation Beaver Dam and Virgin Mountains in the south and east.  Principal aquifers are basin fill in the Virgin River Valley and Beaver Dam Wash, and the Muddy Creek Formation.  The mountainous portions of the basin are underlain by sedimentary and igneous rocks with little groundwater development.

The basin-fill aquifers are composed of a younger floodplain unit and an older underlying unit of semi-consolidated silts, sands, gravels and boulders. In the Virgin River Valley, the basin-fill aquifer contains floodplain and terrace alluvium southwest of Littlefield and includes alluvial-fan deposits from the Virgin Mountains.  Groundwater is unconfined and flows toward the southwest.  In Beaver Dam Wash, the basin-fill aquifer is largely isolated from other water bearing units in the basin and is also unconfined.  Groundwater flow is toward the Virgin River Valley. 

The Muddy Creek Formation consists of a series of siltstones, sandstones and conglomerates that is utilized as a water supply in the western part of the basin and by the City of Mesquite, Nevada adjacent to the basin along Interstate 15 (Black and Rascona, 1991).   It is several thousand feet thick in places and covers the land surface over much of the basin north of the Virgin River. The Muddy Creek Formation is underlain by saturated Paleozoic carbonate rocks.  South of the Virgin River, alluvial deposits from the Virgin Mountains overlie the Muddy Creek Formation. Fault and fracture zones in the formation control groundwater movement and may have groundwater development potential (Dixon and Katzer, 2002).

Between Littlefield and the Virgin River Mountains and south of the Virgin River, a shallow, basin-fill aquifer overlies a limestone formation known locally as the Littlefield Formation.  Few wells are completed in the shallow aquifer but a number of springs emanate from groundwater flowing over or through the Littlefield Formation (Black and Rascona, 1991).

Natural recharge is estimated at less than 30,000 AFA. Groundwater in storage is estimated to total 1.7 maf. Well yields range widely in the basin, as listed on Table 6.6-6, from a reported 10 gpm in the Virgin River basin-fill aquifer to over 5,000 gpm during a pump test in the Beaver Dam Wash basin-fill aquifer (Black and Rascona, 1991).  The median of well yields reported from 53 large diameter (>10 inch) wells completed in the basin is 650 gpm.  Water quality ranges from very good to poor, the latter due to elevated concentrations of arsenic, chloride, sulfate and total dissolved solids.  Salt concentrations in groundwater increase downstream in the floodplain area along the Virgin River. Water quality data collected between 1997 and 2002 listed in Table 6.6-7 show elevated concentrations of arsenic, nitrate and radionuclides.

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

Click for Table 6.6-6 Groundwater Data for the

Virgin River Basin

Click for Figure 6.6-7 Virgin River Basin Groundwater Conditions

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