Interbasin flow is a process by which groundwater moves from one topographic basin to another through an intervening structural or topographic barrier. For decades, interbasin flow has been the prevailing conceptual paradigm for groundwater movement in the arid southwestern United States wherever carbonate rocks are thought to be in continuous contact. This conceptual model of groundwater flow is especially relevant in the Death Valley region where water resources are scarce and where the U.S. government has conducted underground nuclear tests as well as the planned storage facility for spent nuclear fuel at Yucca Mountain, Nevada. Recent studies of large flux springs (˜10,000 L/min) in the Furnace Creek area of Death Valley, California (Figure 1), however, indicate that the concept of interbasin flow may be fundamentally flawed, or at least not as universally applicable as previously thought. Rather, it appears that aquifers supplying Furnace Creek springs were replenished locally during episodes of wet climate more than 9500 yr ago, a contention supported by extensive regional fossil spring deposits.