The only known exception is a small basin on the northwest end of the Sheep Range, ca. 100 km east of Yucca Mountain. The widespread paleohydrologic evidence for a drier ice-age climatic regime in the vicinity of Yucca Mountain contrasts with the central and northern Great Basin record of wetter climatic conditions. Other lines of evidence that indicate that the last pluvial climatic regime in the Yucca Mountain region was actually relatively arid are presented in the following discussion.

The data from two sites within the southern Great Basin are relevant to understanding the chronology of paleohydrologic changes: Searles Lake in southeastern California (Smith, 1979; Smith and Street-Perrott, 1983) and the springs of Las Vegas Valley in southern Nevada (Figure 3.7) (Haynes, 1967; Quade, 1986; Quade and Pratt, 1989). Artesian springs are end-points of a hydrologic system that differs from that of a pluvial lake. High stands of a pluvial lake largely reflect surface water runoff (Enzel et al., 1989), while spring discharge, or outflow, is affected by the amount of water entering the water table, known as aquifer recharge, chiefly from snowmelt in the high mountains. Both systems, however, should be sensitive to significant pluvial episodes. The Paleozoic carbonate aquifer of the Las Vegas Valley is a confined system, sandwiched between impermeable layers. Increased recharge in the highlands of the Spring and Sheep Ranges should increase the hydrologic gradient, which is the pressure difference between recharge and discharge areas, resulting in a rapid increase in outflow through springs at the end of that gradient. A persistent increase in rainfall should also result in increased lake levels. Thus, there is a reason for the apparent correlation of major “pluvial” episodes evident in a comparison of the record of lake-level fluctuations from Searles Lake with Las Vegas Valley spring records (Figure 3.7). It should be noted that Benson et al. (1990) have published an alternative chronology of Searles Lake to that shown in Figure 3.7. Unfortunately, they offer no guidance regarding which chronology is more reliable. In the absence of defensible arguments to the contrary, we rely on the chronology originally proposed by G. I. Smith (Smith and Street-Perrot, 1983; Benson et al., 1990).

Ancient packrat (Neotoma spp.) middens, or den deposits, provide much of the data discussed here. Descriptions of these deposits and the methods used in their analysis are offered by Betancourt et al. (1990). These middens, composed primarily of mummified plant fragments and fecal pellets encased in a matrix of crystallized packrat urine, are common in the hills of the region. The fossilized plants