structural:scholar_edition
Discharge

The unimpaired annual runoff of the Colorado River at Lees Ferry, Arizona is estimated at 14.7 million acre-feet (18.1 km), or an annualized discharge of 20,300 cu ft/s (570 m /s), for the 1906–2023 period. River flows at the Lees Ferry stream gauge, about halfway along the length of the Colorado and 16 miles (26 km) below Glen Canyon Dam, are used to determine water allocations in the Colorado River basin. Flows originating above Lees Ferry represent the majority of Colorado River runoff. Tributaries between there and Imperial Dam near the US–Mexico border contribute a further 1.1 million acre-feet (1.4 km), and the Gila– Salt River system historically accounted for another 1.7 million acre-feet (2.1 km). While much of this water was lost to evaporation in the deserts of the Lower Basin, anywhere from 10 to 20 million acre-feet (12 to 25 km) per year reached the Colorado River Delta prior to the early 20th century.

Up to 90 percent of the river's flow originates as snowmelt from the Rocky Mountains and other smaller mountain ranges of the Colorado Plateau. Natural seasonal flows at its mouth varied widely, often exceeding 100,000 cu ft/s (2,800 m /s) in the spring melt and falling as low as 2,500 cu ft/s (71 m /s) in winter. Snowmelt typically begins in April, peaks in May or June, and can extend into August in wet years. Monsoon rainstorms often occur in the Lower Basin from August through October, contributing much of the flow in tributaries below Lees Ferry and often causing flash flooding. Since the initial closure of Hoover Dam in 1934, lower Colorado River flows have been greatly moderated, with monthly means below the dam ranging from 16,400 cu ft/s (460 m /s) in May to 11,200 cu ft/s (320 m /s) in October. Upstream of the massive Hoover and Glen Canyon Dams, inflows to Lake Powell still experience distinct spring highs and winter lows, though overall volumes have been reduced due to Upper Basin water diversions. Flow regimes in a few Upper Basin tributaries, such as the Yampa River, remain almost completely unaltered.

By the 1950s, the Colorado delta was regularly drying up in fall and winter, though spring high flows often still made it to the ocean. After the initial closure of Glen Canyon Dam in 1963, these seasonal flows were almost completely eliminated, with the exception of a few very wet years, such as 1983–1987. In 1984, 16.5 million acre-feet (20.4 km) of excess runoff reached the ocean. Flows have further declined due to evaporation from reservoirs and, particularly after 2000, reduced snowpack with warming winter temperatures in the Rockies. Water diversions have effectively removed some tributaries entirely, including the Gila River, once the Colorado's largest tributary below Lees Ferry. Most Colorado River water is diverted in the United States, with only about 2.8 million acre-feet (3.5 km) per year reaching Mexico since 1950. At Morelos Dam, nearly all of the remaining flow is diverted to irrigate the Mexicali Valley, leaving the Colorado River Delta almost entirely dry except for small amounts of agricultural wastewater.