| Summary: | Large amounts of ground water have been pumped from alluvial deposits in southern Arizona basins since the late 1940's. Significant declines of ground-water levels have occurred in some of the basins. Ground subsidence and earth fissures, believed to be related to the large declines, have been observed. Striking examples of the earth fissuring phenomenon occur in the Stewart area of the Willcox basin. Water levels have declined more than 100 ft (30.5 m) in the past 30 years due to agricultural pumping. Subsidence of 1 to 2 in (3.3 to 6.6 ft) has been recorded near the area of maximum water-level decline. Earth fissures associated with the water-level declines and subsidence have appeared at the basin floor margins near the Winchester Mountains, the Circle I Hills, and the Spike E Hills. The fissures occur in areas where alluvial sediments come into contact with Pleistocene lacustrine clays. Dense mesquite forests, with some unusually large members (some of which seem to be dying), commonly mark the boundaries. Two types of fissure patterns, semipolygonal to polygonal and semicurved to linear, are found intermixed in the Stewart area. The fissures in polygonal patterns appear narrower and shallower than linear fissures. The polygonal patterns suggest that some fissures may be due to horizontal contraction of clayey sediments. Some linear fissures may be the result of differential subsidence.
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