Why does the Colorado River no longer reach the sea?

The Colorado River no longer reaches the sea because people use almost all of its water before it gets there. About 40 million people depend on the river's water for agricultural, industrial and domestic needs (in 2010, that was the number living in areas it supplies), and it irrigates 5.5 million acres of farmland. The river is so intensively managed that each drop of its water is used an average of 17 times a year. Intensive water consumption has dried up the lower 100 miles of the river, which has rarely reached the sea since the 1960s.

The river's last miles tell the story. At Imperial Dam, over 90 percent of the river's flow is moved into canals that irrigate California's Imperial Valley. At Morelos Dam in Mexico, the entire remaining flow is diverted to irrigate the Mexicali Valley. Almost nothing is left for the final stretch.

It was not always so. Before the early 20th century, 10 to 20 million acre-feet of water per year reached the Colorado River Delta. Large-scale management began in the early 1900s, with guidelines set by treaties known as the Law of the River, and the US federal government built most of the major dams between 1910 and 1970, including Hoover Dam in 1935.

Now there is less water to share. Since 2000, extended drought has conflicted with growing demands, and flows have fallen further because of evaporation from reservoirs and reduced snowpack in a warming Rockies.

Not everyone has given up on the delta. In 2014, a planned pulse flow reached the sea on May 16, the first time in 16 years that any Colorado water flowed into the ocean.