Exposed to the beating sun and hot dry air, more than 10% of the water carried by the Colorado River evaporates, leaks or spills as the 1,450-mile (2,334-kilometer) powerhouse of the West flows through the region’s dams, reservoirs and open-air canals.

For decades, key stewards of the river have ignored the massive water loss, instead allocating Arizona, California, Nevada and Mexico their share of the river without subtracting what’s evaporated.