The Colorado River is tapped out.

Another dry year has left the waterway that supplies 40 million people in the Southwest parched. A prolonged 21-year warming and drying trend is pushing the nation’s two largest reservoirs to record lows. For the first time this summer, the federal government will declare a shortage.

Climate change is exacerbating the current drought. Warming temperatures are upending how the water cycle functions in the Southwest. The 1,450-mile long river acts as a drinking water supply, a hydroelectric power generator, and an irrigator of crop fields across seven Western states and two in Mexico. Scientists say the only way forward is to rein in demands on the river’s water to match its decline.