Introduction:
Climatic fluctuations have pro-found effects on water resources in the western United States (fig. 1). In the arid and semiarid Southwest, climatic fluctuations affect many hydrologic characteristics of water-sheds, including the quantity of base flow, the occurrence of large floods, and the timing of snowmelt runoff (Cayan and others, 1999; Stewart and others, 2004). Since the start of a persistent drought around the year 2000, inflows to Lake Powell on the Colorado River have been below average, leading to drawdown of Lake Mead and Lake Powell (fig. 2), the primary means of storage on the river. Although the factors that caused and have sustained this drought have not been positively identified, it is possible to describe the broader climatic context within which it is occurring. This fact sheet provides a perspective on the current drought occurring in parts of the western United States, particularly in the Colorado River basin.

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