Lake Powell functions as a giant water bank for the states of the Upper Colorado River Basin. Its water originates as snow in the Rocky Mountains; the melt runs down through the upper basin, is stored behind Glen Canyon Dam, and is released downstream to meet obligations to the lower basin. That makes the lake's level a direct readout of two things at once - how much snow fell last winter, and how much water has had to be sent downstream. Physically, it is a reservoir of drowned sandstone canyons, valued as a recreation area for its rock formations, and ecologically it is oligotrophic: nutrient concentrations and algal production are low, which is why its water is characteristically very clear. Decades of drought, a poor snowpack in the last winter and extreme heat that accelerates evaporation have combined to push it to this record.

A falling reservoir does three separate kinds of damage, and the Lake Powell case shows all three at once. First, supply: the Colorado River serves tens of millions of people and a large irrigated agriculture, and storage is the buffer that carries that demand through dry years. Second, power: hydropower generation depends on head - the height of water above the turbines - so a reservoir can still hold a great deal of water and yet stop generating electricity once it drops below minimum power pool. At less than 30 feet of margin, the Glen Canyon Dam's eight turbines, which serve close to 500,000 households, are within one bad season of that threshold. Third, allocation politics: the Interior Department's mandated 20 per cent cut for Arizona, California and Nevada shows what happens when the physical supply falls below the volumes that legal allocations assume. That last point travels well to Indian examples, where inter-state river disputes similarly turn on allocations fixed in wetter periods than the ones now being lived through.