Tensions Are Rising Between States That Rely on the Colorado River | A prolonged drought means the nation’s largest reservoirs are dwindling, and litigation over access to water could lie ahead.

About 40 million people and 5.5 million acres of cropland depend on the Colorado for drinking water and irrigation, but its flow has gradually diminished over the past two decades as the climate becomes warmer and more arid across the West. Now the arcane system of water rights governing the river entitles each state and Mexico to far more water than is actually available. The rules prioritize the longest-established uses of water, in many cases dating to the 1850s and 1860s.