What has been affected by upstream diversions and recervoir evaporation?
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.
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 feet (13.22 km3) per year, from 1921 to 2010. This figure has been heavily affected by upstream diversions and reservoir evaporation, especially after the completion of the Colorado River Storage Project in the 1970s. Prior to the completion of Glen Canyon Dam in 1964, the average discharge recorded between 1912 and 1962 was 17,850 cubic feet per second (505 m3/s), 12.93 million acre feet (15.95 km3) per year.
average discharge