"The Green River, a tributary of the Colorado that runs through Wyoming, Utah, and Colorado, is a typically acute case. Split between an upper and lower basin, its water is used by seven states. Those upstream in Wyoming attempt to pull out as much water as possible before the hungry downstream cities slurp up their share. The system has strict per-state allocations despite the variable nature of the supply. It runs on a “use it or lose it” policy: Either you take out water annually or your right to it disappears forever. There is no reward for conservation and many western states have no limit on how much water can be taken out of rivers in times of low rainfall, even to guarantee minimum flows for fish."