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1998 Mojave Region Water-Level Study

REGIONAL WATER TABLE (1998) AND GROUNDWATER-LEVEL CHANGES IN THE MOJAVE RIVER AND MORONGO GROUNDWATER BASINS, SAN BERNARDINO COUNTY, CALIFORNIA by Gregory A. Smith and M. I. Pimentel

Groundwater Levels

Static water-level measurements collected during the spring of 1998 by the U.S. Geological Survey (USGS), the Mojave Water Agency (MWA), and local water districts from about 620 wells were used to construct a regional water-table map, which shows the altitude of the water table and general direction of groundwater movement in the Mojave River and Morongo groundwater basins. Water-level contours from the 1996 water-level map (Mendez and Christensen, 1997) were used as a guide to interpret the 1998 water-level contours in areas where 1998 water-level data were not available. In addition to being available on the interactive map (1998 water-level contours on interactive map), the 1998 water-level data and contours are shown on a PDF formatted plate. Historical water-level data were used in conjunction with data collected from this study to construct 37 water-level hydrographs to show both long-term (as early as 1930) and short-term (beginning in 1990) water-level changes in the Mojave River and Morongo groundwater basins. Water-level changes between spring 1996 and spring 1998 (on interactive map) were determined by comparing water levels measured in the same well during both periods.

Water-Level Changes

Historical water-level data were used in conjunction with data collected during this study to determine both long-term and short-term water-level changes in the Mojave River and Morongo groundwater basins. Long-term water-level changes are depicted by 23 water-level hydrographs (PDF formatted plate, hydrographs) that include data collected beginning in the 1930's. Selected hydrographs include data from more than one well and show water-level changes for periods greater than the period of record of an individual well. During 1998, water-level trends remained relatively unchanged in most of the wells, including several wells in the Yucca Valley in which water levels continued to rise. Artificial recharge, which began in the Yucca Valley area in 1995, has reversed the trend of declining water levels in the Warren subbasin. For example, the water level in well 1N/5E-36K2 rose 85 ft between 1996 and 1998.

Fourteen short-term hydrographs were constructed from data collected in 1996 and 1998 along the Mojave River (PDF formatted plate). These short-term hydrographs show the effects of seasonal recharge and discharge. Recharge to the flood-plain aquifer occurs mainly from infiltration of stormflows in the Mojave River. Stormflows usually occur during January through March; however, in 1998 stormflow occurred as late as May. As a result, groundwater levels in the flood-plain aquifer, which usually peak in February and March, peaked as late as May in many of the wells. The effects of this recharge were most significant in the Alto and Centro subareas, where water levels rose as much as 30 ft in some wells; however, water-level changes in the Transition zone between the Alto and Centro subareas were smaller.

In the Mojave River groundwater basin, a comparison of groundwater levels measured in spring of 1996 and spring 1998 (PDF formatted plate, water-level change in the Mojave River groundwater basin on interactive map) indicates that changes in the regional aquifer system generally were less than or equal to 5 ft. In the southwestern part of the Centro subarea, south and north of the Mojave River, water levels declined as much as 13 ft in wells tapping the flood-plain aquifer and the regional aquifer. Water levels measured in the flood-plain aquifer showed the greatest change. In the Alto subarea of the Mojave River groundwater basin, the water levels of the flood-plain aquifer rose as much as 12 ft; however, farther downstream in the Centro and the Baja subareas, water levels in the flood-plain aquifer declined as much as 25 ft.

Results of a comparison of groundwater levels measured in the Morongo groundwater basin in spring 1996 and spring 1998 (PDF formatted plate, water-level change in the Morongo groundwater basin on interactive map) indicate that most of the wells in this basin had water-level changes of 5 ft of less. Wells in the Lucerne and Warren subbasins showed the greatest water-level change. Water levels in the northern part of the Lucerne subbasin declined as much as 22 ft in the regional aquifer. The Warren subbasin has received artificial groundwater recharge since February 1995. This recharge has resulted in measured water-level rises as great as 149 ft in the regional aquifer.