page_title: Colorado River
section_title: Discharge
hierarchical_section_title: Colorado River / Discharge
caption_attribution_description: Graph showing annual flow volumes in the Colorado River, 1890–2010
context_page_description: The Colorado River is one of the principal rivers in the Southwestern United States and northern Mexico. The 1,450-mile-long river drains an expansive, arid watershed that encompasses parts of seven U.S. states and two Mexican states. Starting in the central Rocky Mountains of Colorado, the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border. After entering Mexico, the Colorado approaches the mostly dry Colorado River Delta at the tip of the Gulf of California between Baja California and Sonora.
  Known for its dramatic canyons, whitewater rapids, and eleven U.S. National Parks, the Colorado River and its tributaries are a vital source of water for 40 million people. The river and its tributaries are controlled by an extensive system of dams, reservoirs, and aqueducts, which in most years divert its entire flow for agricultural irrigation and domestic water supply. Its large flow and steep gradient are used for generating hydroelectric power, and its major dams regulate peaking power demands in much of the Intermountain West.
context_section_description: In its natural state, the Colorado River poured about 16.3 million acre feet (20.1 km³) into the Gulf of California each year, amounting to an average flow rate of 22,500 cubic feet per second (640 m³/s). 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 annually.  At Topock, Arizona, about 300 miles (480 km) upstream from the Gulf, a maximum historical discharge of 384,000 cubic feet per second (10,900 m³/s) was recorded in 1884 and a minimum of 422 cubic feet per second (11.9 m³/s) was recorded in 1935. In contrast, the regulated discharge rates on the lower Colorado River 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 (27.4 km³) in 1984 to a low of 3.8 million acre feet (4.7 km³) in 2002, although in most years only a small portion of this flow, if any, reaches the Gulf.
  Between 85 and 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 10 to 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 often contribute significant volumes of runoff. Most of the annual runoff in the basin occurs with the melting of Rocky Mountains snowpack, which begins in April and peaks during May and June before exhausting in late July or early August.
  Flows at the mouth of the river 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 Pacific Ocean. Irrigation, industrial, and municipal diversions, evaporation from reservoirs, natural runoff, and likely climate change, have all contributed to this substantial reduction in flow, threatening the water supply for the future. 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 of the Colorado at the northernmost point of the Mexico–United States border (NIB, or Northerly International Boundary) is about 2,060 cubic feet per second (58 m³/s), 1.49 million acre feet (1.84 km³) per year—less than a tenth of the natural flow—due to upstream water use. Below this point, the remaining flow is diverted to irrigate the Mexicali Valley, leaving a dry riverbed from Morelos Dam to the sea that is supplemented by intermittent flows of irrigation drainage water. There have been exceptions, however, namely in the early to mid-1980s, when the Colorado once again reached the sea during several consecutive years of record-breaking precipitation and snowmelt. In 1984, so much excess runoff occurred that some 16.5 million acre feet (20.4 km³), or 22,860 cubic feet per second (647 m³/s), poured into the sea.
  The United States Geological Survey (USGS) operates or has operated 46 stream gauges 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 gauges. River flows as gauged 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 
caption: Annual Colorado River discharge volumes at Lee's Ferry between 1895 and 2004
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