Lifeblood of the Desert

The Mojave River has long been the "lifeblood" of the Mojave Desert. Water from the Mojave River was the critical element to survival for the Indigenous people that inhabited the Mojave River Basin prior to European Settlement. River water and the vegetation that grew on the floodplain also were vital to the Spanish explorers of the late 1700s, the American trappers of the early 1800s, and the thousands of pioneers who later followed the Spanish Trail and Mormon Road (Walker, 1986). By the 1880s, boosted by the Calico mining boom and completion of a transcontinental railroad, Euro-American settlement along the Mojave River was expanding (Earle, 1992). Because of the heavy reliance on river water and natural subirrigation, agriculture along the river was devastated by the drought of 1894-1904. With the advent of combustion engines (and later, electric motors) for powering pumps, a "new" source of water became available. Instead of having to rely on the flow of the river and rainfall, users obtained large quantities of water by building wells that tap into sand and gravel deposits beneath the floodplain (floodplain aquifer). It seemed that the problems associated with an unpredictable water supply had been solved.

The Mojave River about half a mile below Camp Cady, November 21, 1919. (Photograph Credit: U.S. Geological Survey)

However, it eventually became apparent from the fluctuation of water levels in wells that the replenishment of the floodplain aquifer was dependent on seepage from the river (Lines, 1996); thus, this new supply of water was limited. As groundwater pumping increased, the deep connection between the river and aquifer became even more apparent. Reaches of the river that historically had flowed year-round soon flowed only in response to strong storm runoff, a condition that still persists to this day.

The Mojave River is the primary source of surface water for the region and is normally dry along most of its length. Large flows extending the entire length of the river from the mountain front to Afton Canyon occur on average once every 5 to 7 years as a result of runoff from mountain areas during wet winters (Lines, 1996; Stamos and others, 2001, Seymour, 2016). As of 2021, groundwater discharges maintain year-round streamflows between the Upper and Lower Narrows at Victorville and in Afton Canyon.

The Mojave river is an unpredictable source of water, and residents of the area now rely almost entirely on groundwater for their water supply. This reliance on groundwater has resulted in overdraft conditions that have caused water-level declines, changes in the amount and location of recharge from the river, and loss of riparian habitat.

Water Supply

Surface water in the area is minimal and normally is limited to streamflow during winter and spring storms. Groundwater from wells is the sole source of water for public supply in the basin. In the southern part of the basin, water is pumped primarily for municipal, industrial, and agricultural uses. In the northern and northeastern areas, groundwater is pumped primarily for agricultural irrigation.

The Mojave River’s intermittent streamflow usually occurs during January through March and is the primary source of groundwater recharge. The area also receives recharge via sporadic releases of imported water from the  California State Water Project  at 13 managed recharge sites across the basin. Streamflows occur more frequently near the mountain front and less frequently farther downstream.

The Mojave River and the associated aquifer system are important water supplies in the Mojave Desert and are hydraulically connected in many areas.  Groundwater levels have declined  in most areas due to groundwater pumping exceeding recharge. Infiltration of streamflow from the Mojave River can rapidly recharge the floodplain aquifer (Lines, 1996; Stamos and others, 2001; Seymour, 2016).

Understanding the complex geologic controls on heavily used groundwater basins is important for anticipating water needs and making informed water-management decisions.

Stormflow in the Mojave River (16 February 2019). Water rises quickly and causes large increases in streamflow when a major rainstorm hits the watershed. Thus, during a storm, much more water flows in a few hours than flows during the entire year where base flow still occurs. (Video Credit: Mojave Water Agency)

Groundwater

Groundwater is water that exists underground in saturated zones beneath the land surface. The upper surface of the saturated zone is called the water table. Groundwater is the source of about 40 percent of water used for public supplies and about 39 percent of water used for agriculture in the United States (U.S. Geological Survey, 2021).

Contrary to popular belief, groundwater does not form underground rivers. It fills the pores and fractures in underground materials such as sand, gravel, and other rock, much the same way that water fills a sponge. Groundwater exists in rock units called aquifers that yield usable quantities of water; the aquifer types in the southwestern Mojave Desert are basin-fill aquifers that consist of sand and gravel deposits. 

Groundwater moves slowly, typically at rates of 3-25 inches per day in an aquifer. As a result, water could remain in an aquifer for hundreds or thousands of years. Chemical analyses show that much of the groundwater from the aquifers in this area have ages greater than about 10,000 years before present (Izbicki and Michel, 2004).

How groundwater occurs. (Credit: U.S. Geological Survey, 1999)

The pumping of wells can greatly influence water levels below ground. If water is withdrawn from the ground at a faster rate than it is replenished, either by infiltration from the surface or from streams, then the water table can become lower. Depending on geologic and hydrologic conditons of the aquifer, water table elevations can decline by a small amount or many hundreds of feet, the impact of which can be short lived or last for decades. Excessive pumping can result in undesirable effects such as:

  • drying up of wells 
  • reduction of water in streams and lakes 
  • deterioration of water quality 
  • increased pumping costs 
  • land subsidence 
  • saltwater intrusion in coastal zones

Groundwater Use

The Mojave River Basin is about 30 miles northeast of the Los Angeles metropolitan area. This proximity to Los Angeles has led to rapid population growth and increased demand on the groundwater basins in the area.

The lack of significant surface-water resources has resulted in the use of groundwater as the primary source for private, agricultural, and municipal supply. The groundwater system in the Mojave River Basin consists of an underlying regional aquifer surrounding a floodplain aquifer.

The floodplain aquifer is more productive and composed of permeable, young river deposits of Holocene age (less than 11,650 years ago) and older river deposits of Pleistocene age (11,650 to 2.58 million years ago). This aquifer is as much as 200 feet thick and yields most of the groundwater pumped from the Mojave River Basin. The regional aquifer is more widespread and is composed of unconsolidated, older alluvium and fan deposits of Pleistocene to Tertiary (201.3 to 251.9 million years ago) age (Stamos and others, 2001).

Areas most responsive to recharge from streamflow in the Mojave River include parts of the floodplain aquifer along southern and northern parts of the river (Seymour, 2016).

Conceptualized geologic section of the aquifer system in the Mojave River groundwater basin, southern California (Stamos and others, 2001).

Geologic Makeup of the Mojave River Aquifer System

The geology of an aquifer plays a role in the amount, quality, and movement of groundwater that is pumped for supply. The different deposits and sediments that make up an aquifer have different porosity and permeability characteristics, which means that water does not move around the same way in all aquifers. The geology of an aquifer also affects water quality. The exposure of aquifer materials to water can release trace elements into the groundwater, some of which (such as arsenic, chromium, uranium, and vanadium) pose potential public-health hazards if consumed.

The unconsolidated sedimentary deposits of the floodplain are less than 2.5 million years old, and the sedimentary deposits of the regional aquifer are as much as 5 million years old. The Tertiary volcanic rocks generally are non-water bearing. The Tertiary sedimentary rocks contain water-bearing strata, but such deposits typically yield only small quantities of poor-quality water to wells (Stamos and others, 2001).

Water Studies in the Mojave Region

The USGS, in cooperation with the MWA, has been collecting, documenting, and interpreting many types of data to monitor the water resources of the Mojave Desert since the early 1950s. Since 1890, the USGS has collected data from more than 5,200 wells in the MWA's management area; between 2017 and 2021, the MWA and the USGS have collected data from about 760 wells. These data can be accessed through the  USGS National Water Information System web service (NWISWeb)  (U.S. Geological Survey, 2021). NWISWeb serves as an interface to a data repository of site information, including current and historical groundwater, surface-water, and water-quality data collected from locations throughout the United States and elsewhere. Some of the many published investigative reports about the Mojave Desert can be found on the California Water Science Center’s  Mojave Groundwater Resources website .

The USGS and the MWA continue to work together to monitor the valuable water resources in the region and to help understand the stresses and the long-term effects on groundwater flow and quality. 

U.S. Geological Survey hydrologic technicians use a cable car over the Lower Narrows portion of the Mojave River in February 2019 to collect streamflow data. (Photograph Credit: Mojave Water Agency)


References

Cox, B.F., and Hillhouse, J.W., 2000, Pliocene and Pleistocene evolution of the Mojave River, and associated tectonic development of the Transverse Ranges and Mojave Desert, based on borehole stratigraphy studies near Victorville, California: U.S. Geological Survey Open-File Report 00-147, 66 p.

Earle, David, 1992, Overview of the history of the upper Mojave River area, in Natural resources inventory of the Mojave River corridor: Riverside, California, Herra Madre Consultants, Inc., p. 15-72.

Izbicki, J.A., and Michel, R.L. 2004, Movement and age of ground water in the western part of the Mojave Desert, southern California, USA: U.S. Geological Survey Water-Resources Investigations Report 03-4314, 42 p.,  https://pubs.er.usgs.gov/publication/wri034314 .

Lines, G.C., 1996, Ground-water and surface-water relations along the Mojave River, southern California: U.S. Geological Survey Water-Resources Investigations Report 95-4189, 43 p.  https://pubs.er.usgs.gov/publication/wri954189 

Matti, J.C., and Morton, D.M., 1993, Paleogeographic evolution of the San Andreas Fault in southern California: A reconstruction based on a new cross-fault correlation, in Powell, R.E., Weldon, R.J., II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution: Boulder, Colorado, Geological Society of America Memoir 178, p. 107–159.

Meisling, K.E., and Weldon, R.J., 1989, Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California: Geological Society of America Bulletin, v. 101, p. 106–128.

Seymour, W., 2016, Hydrologic and geologic controls on groundwater recharge along the Mojave River floodplain aquifer. Master’s Thesis San Diego State University, Department of geography San Diego, California, 73 p.  https://digitallibrary.sdsu.edu/islandora/object/sdsu%3A1599  

Stamos, C.L., Martin, Peter, Nishikawa, Tracy, and Cox, B.F., 2001, Simulation of ground-water flow in the Mojave River Basin, California: U.S. Geological Survey Water-Resources Investigations Report 01-4002, 129 p.  https://pubs.usgs.gov/wri/wri014002/ 

Thompson, D.G., 1929, The Mohave Desert region, California, a geographic, geologic, and hydrologic reconnaissance: U.S. Geological Survey Water Supply Paper 578, 759 p.  https://doi.org/10.3133/wsp578/ 

U.S. Geological Survey, 1999, Groundwater is the saturated zone of soil/rock below the land surface: U.S. Geological Survey web page, accessed June 30, 2021 at  https://www.usgs.gov/media/images/groundwater-saturated-zone-soilrock-below-land-surface .

U.S. Geological Survey, 2021, USGS water data for the Nation: U.S. Geo­logical Survey National Water Information System data­base,  https://doi.org/10.5066/F7P55KJN .

Walker, C.J., 1986, Back Door to California: The Story of the Mojave River Trail: Barstow, CA, Mojave River Valley Museum Association, 337 p.

The Mojave River about half a mile below Camp Cady, November 21, 1919. (Photograph Credit: U.S. Geological Survey)

How groundwater occurs. (Credit: U.S. Geological Survey, 1999)

Conceptualized geologic section of the aquifer system in the Mojave River groundwater basin, southern California (Stamos and others, 2001).

U.S. Geological Survey hydrologic technicians use a cable car over the Lower Narrows portion of the Mojave River in February 2019 to collect streamflow data. (Photograph Credit: Mojave Water Agency)