It is paradoxical that the Colorado stopped running consistently through the delta at the end of the 20th century, which — according to tree-ring records — was one of the basin’s wettest centuries in 1,200 years. Now dozens of animal species are endangered; the culture of the native Cocopah (the People of the River) has been devastated; the fishing industry, once sustained by shrimp and other creatures that depend on a mixture of seawater and freshwater, has withered. In place of delta tourism, the economy of the upper Gulf of California hinges on drug smuggling operations that run opposite to the dying river.

In 2008 I tried to float the length of the 1,450-mile river to the sea but had to walk the last week of the trip. Pools stagnated in the cracked riverbed. Like the 30 million other Americans who depend on the river, I worry about drinking water — but I also worry about the sorry inheritance we are leaving future generations.

Demand for water isn’t the only problem. Climate change also threatens to reduce runoff by 10 to 30 percent by 2050, depending on how much the planet warms, according to a 2009 paper in Proceedings of the National Academy of Sciences.

Although the river delta can’t yet be pronounced dead, its pulse is feeble and its once-vital estuaries and riverside forests are shrinking.

But a delicate beauty hangs on. Coyotes still bawl across the briny tang where a mirage-laden sky appears to pull the distant Sierra el Mayor down to sea level. The organic matter of this delta once sprawled 3,000 square miles to join Mexico and the United States in a miraculous mixture of fertility and desert; these sands have been washed out of the Rockies, carved from the Grand Canyon and tumbled through more than three million acres of river-dependent farms.