Cadillac Desert: The American West and Its Disappearing Water

by Marc Reisner

The 17.5-million-acre-foot yield that the Compact negotiators had ascribed to the Colorado River was based on about eighteen years of streamflow measurement with instruments that, by today’s standards, were rather imprecise. During all of that period, the river had gone on a binge, sending down average or above-average flows three out of every four years. Not once had the flow dropped below ten million acre-feet, as it had repeatedly during the Great Drought of the 1930s. But all it takes to make statisticians look foolish is a few very wet or very dry years. In San Francisco, precipitation records have been kept for more than a hundred years—a log which, one might think, is good enough for a highly accurate guess. But 1976 and 1977, two unprecedented drought years, lowered the average rainfall figure from 20.66 to 19.33 inches. In a marginal farming region such as the Great Plains, an inch less precipitation can mean all kinds of trouble. In a desert region such as the Southwest, utterly dependent on one river, a difference of a couple of million acre-feet can spell disaster.