Colorado River - Discharge

Discharge

In its natural state, the Colorado River poured about 15.7 million acre-feet (18.63 km3) into the Gulf of California each year, amounting to an average flow rate of 21,700 cubic feet per second (610 m3/s). However, its flow regime was not at all steady – indeed, "prior to the construction of federal dams and reservoirs, the Colorado was a river of extremes like no other in the United States." Once, the river reached peaks of more than 100,000 cubic feet per second (2,800 m3/s) in the summer and low flows of less than 2,500 cubic feet per second (71 m3/s) in the winter on an annual basis. A maximum historical discharge of 384,000 cubic feet per second (10,900 m3/s) in 1884 and a minimum of 422 cubic feet per second (11.9 m3/s) in 1935, were recorded at Topock, Arizona, about 300 miles (480 km) upstream from the mouth of the river. In contrast, the regulated discharge rates on the lower Colorado below Hoover Dam rarely exceed 35,000 cubic feet per second (990 m3/s) or drop below 4,000 cubic feet per second (110 m3/s). Annual runoff volume has ranged from a high of 22.2 million acre-feet (27.38 km3) in 1984 to a low of 3.8 million acre-feet (4.69 km3) in 2002, corresponding to average annual flows of 30,700 to 5,200 cubic feet per second (870 to 150 m3/s).

Between 85–90 percent of the Colorado River's discharge originates in snowmelt, mostly from the Rocky Mountains of Colorado and Wyoming. The three major upper tributaries of the Colorado – the Gunnison, Green and San Juan – alone deliver almost 9 million acre-feet (11 km3) per year to the main stem, mostly from snowmelt. The remaining 15 percent comes from a variety of sources, principally groundwater base flow and summer monsoon storms. The latter often produces heavy, highly localized floods on lower tributaries of the river, but does not contribute significant volumes of runoff. Most of the annual runoff in the basin occurs with the melting of Rocky Mountains snowpack beginning in April, which peaks during May and June before exhausting in late July–early August.

Flows at the mouth have steadily declined since the beginning of the 20th century, and in most years after 1960 the Colorado River has run dry before reaching the sea. Irrigation and municipal diversions, evaporation from reservoirs, and likely climate change have all contributed to this substantial reduction in flow. For example, the Gila River – formerly one of the Colorado's largest tributaries – contributes little more than a trickle in most years due to use of its water by cities and farms in central Arizona. The average flow rate at the northernmost point of the Mexico–United States border (NIB, or Northerly International Boundary), below major diversions such as the All-American Canal and Colorado River Aqueduct, is about 2,060 cubic feet per second (58 m3/s), 1.49 million acre-feet (1.84 km3) per year. In 1984, record-breaking precipitation and snowmelt greatly exceeded the storage capacity of reservoirs, and the Colorado River's average flow at the NIB reached 22,860 cubic feet per second (647 m3/s), 16.5 million acre-feet (20.35 km3), in that one year.

The United States Geological Survey (USGS) operates or has operated 46 stream gages to measure the discharge of the Colorado River, ranging from the headwaters near Grand Lake to the Mexico–U.S. border. The tables at right list data associated with eight of these gages. River flows as gaged at Lee's Ferry, Arizona, 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. The average discharge recorded there was approximately 14,800 cubic feet per second (420 m3/s), 10.72 million acre-feet (13.22 km3) per year, from 1921 to 2010. Prior to the closure of Glen Canyon Dam and extensive engineering development in the upper basin, the average discharge recorded between 1912–1962 was 17,850 cubic feet per second (505 m3/s), 12.93 million acre-feet (15.95 km3) per year.