For six million years

the Colorado River and its dozens of spectacular tributaries carved out the Grand Canyon and scoured the Rockies, flushing mineral-rich sediment to the sea.

These rivers weren't always in flood, but they never ran dry. Today, however, the second largest tributary, the Gila, is mostly bone dry in its lower reaches through Arizona; the Salt River—supplying Phoenix—no longer makes it to a confluence with the Gila; the Santa Cruz is seen beneath Tucson bridges only during rare floods; and the Colorado River itself, almost unbelievably, stopped running to the sea in most years after the completion of Glen Canyon Dam in 1966. The river hasn't flowed to the Sea of Cortez in Mexico since 1998.

What happened? In a word: agriculture. A steady march of population growth and climate change has exacerbated conditions, but it all began with farming, much of it supported by taxpayers in a feverish desire to "settle the West." After irrigation ditches and canals were hewed into the arid ground, farmers planted wheat, then hay. Those crops, along with cotton, citrus fruits, 90 percent of the nation's winter vegetables, and almost any other plant imaginable, now consume about 70 percent of the Colorado River Basin's water.

Our green lawns, golf courses, swimming pools, and generally thirsty cities also take a share of the dwindling water, as do reservoirs, where 15 percent of the basin's water evaporates into thin air each year.

Although people tend to think of the Colorado as a single channel that slices through the Grand Canyon, the river system is really made up of scores of major tributaries that are collectively known as the Colorado River Basin. Running as red and warm as fish blood, or as emerald and cold as a glacial lake, its 242,000-square-mile (626,777-square-kilometer) drainage area alternates between the primeval and the altered. The basin's drainage covers a percentage of the U.S. that's almost identical to the percentage of Africa drained by the Nile.

The Colorado River is both servant and scenic wonder to the 36 million Americans it supplies with water. It is also one of the siltiest, most litigated over, and frequently paddled rivers in the world.

It is now widely accepted that population growth and climate change will cause ever more intense water shortages in the West, much like the travails of Egypt over the long-disputed Nile.