The water-table contours on this map were constructed from water-level control derived from an inventory of springs and wells in the area, U.S. Geological Survey topographic maps, and from data presented by Rogers (1967). Because of the available water-level control and the rough terrain throughout the area, the delineation of the water table was limited to 20-foot contour intervals. On the basis of recent river-stage and precipitation records, this delineation should represent near or slightly below average conditions.

In addition to the alluvium, several other geologic formations crop out in the areas. These formations are capable of producing small to large quantities of water to wells, but the current (1981) pumping demand is small. The pressure heads in each of these formations appear to be in continuity, and the contours indicate that no barriers restrict ground-water movement from one formation to another. Several water-producing horizons may be present at many locations. The water-table map was constructed by using the head in the horizon that is nearest the land surface. The configuration of the water table closely approximates the topography. The water table slopes from the highland areas toward the Colorado River and small creeks.

The delineation of the water table in the alluvium of the Colorado River is fairly well defined, and 10-feet contour intervals may be interpreted with confidence in the area called ' potential lignite-mining area. ' The water table in the bedrock aquifers is more difficult to delineate with the available data; therefore, the contours are only estimates of the position of the water table in the hilly bedrock area adjacent to the Colorado River alluvium.