OCR Text |
Show 6 - range from about 100 feet in the San Pedro drainage (Basins 5 and 6, pi. 1) to about 800 feet in the Lower Santa Cruz Basin (Basin 2, pi. 1). The younger alluvium also includes lake-bed and playa deposits but is commonly composed of materials ranging in grain-size from silt to gravel. The water-bearing properties of the older fill and younger fill are markedly different. Because of its cementation and finegrained texture, the older fill is believed capable of storing large volumes of water but is known to release it relatively slowly to wells. In many places, therefore, successful stock wells can be drilled in older fill, but yields in general are insufficient to permit pumping for irrigation. Local exceptions occur in parts of the San Pedro River Valley (Basins 5 and 6, pi. 1) and parts of the Lower San Simon-Gila River Basin (Basin 8, pi. 1) where artesian aquifers are present in the older fill. The younger fill, in contrast, is in many places uncemented and coarse-grained, yielding water readily to wells. The younger alluvium is the principal aquifer in most basins from which water can be pumped in quantities sufficient for irrigation. Reconnaissance studies of the geology of the basins indicate that in some of the basins such as the Upper Santa Cruz Basin (Basin 4, pi. 1) and the Lower San Simon-Gila River Basin (Basin 8, pi. 1), the younger fill receives water by slow percolation from those parts of the older fill that lie topographically higher than the younger fill. The outline of each ground-water basin shown on the accompanying map, plate 1, marks the approximate boundary between valley alluvium and mountains or mountain pediment. Mountain pediments are so thinly covered with alluvium that they are ineffective for the storage of ground water, but may be important as recharge areas. Major areas of mountains and pediment that are entirely within some of the |