The western United States has low overall rainfall and snowfall levels, few rivers, and many deep groundwater basins. Small Native American populations once lived within the restraints of aridity by seeking harmony with nature. But owning land in such an arid region means little or nothing without a supply of fresh water. Instead of limiting population growth in the face of scarce and unpredictable rainfall, however, the west’s aridity challenged the newcomers to redirect water supplies and make the rich desert soils bloom. The region’s localized precipitation, generally doled out on boom-and-bust schedules, has made water “the most essential and fought over resource in the western United States.” Raising a lone voice of warning in 1893, western explorer John Wesley Powell foresaw that irrigating western lands would pile up “a heritage of conflict and litigation over water rights for there is not sufficient water to supply the land.”2 That Powell was right about conflicts goes without saying, for the west’s bitter heritage of water wars speaks for itself.3 Invading Americans used legal doctrines of first appropriation and “beneficial use” to take water from Indians’ lands and then turned to taking it from each other, oblivious to the effects on wildlife and natural habitats. Today’s depleted river flows and overpumped groundwater basins indicate that Powell probably was right about water supply limits, too. Expanding populations and increasing water contamination have strained supplies of fresh, clean water, even as per capita water demands decrease. By the 1970s, degraded natural settings, rising water pollution, and disappearing native fauna had lowered the quality of western life and built a constituency for environmental protection. But the 1970 National Environmental Policy Act and 1973 Endangered Species Act simply pitted environmental groups and courts against irrigators, cities, and states. In an ironic reversal, recently enriched Native Americans are poised to exercise their primary legal claims to many western rivers.