One of the major rivers of the western United States, the Colorado River flows for some 1,500 mi (2,415 km) from Colorado to northwestern Mexico. Dropping over 2 mi (3.2 km) in elevation over its course, the Colorado emptied into the Gulf of California until human management reduced its water flow. Over millions of years the swift waters of the Colorado have carved some of the world's deepest and most impressive gorges, including the Grand Canyon.

The Colorado River basin supports an unusual ecosystem. Isolated from other drainage systems, the Colorado has produced a unique assemblage of fishes. Of the 32 species of native fishes found in the Colorado drainage, 21–66%, are endemic species—species that arose in the area and are found nowhere else.

Major projects carried out since the 1920s have profoundly altered the Colorado. When seven western states signed the Colorado River Compact in 1922, the Colorado became the first...