CARBONDALE, Colo.

MOST visitors to the Hoover Dam and the Grand Canyon probably don’t realize that the mighty Colorado River, America’s most legendary white-water river, rarely reaches the sea.

Until 1998 the Colorado regularly flowed south along the Arizona-California border into a Mexican delta, irrigating farmlands and enriching a wealth of wildlife and flora before emptying into the Gulf of California.

But decades of population growth, climate change and damming in the American Southwest have now desiccated the river in its lowest reaches, turning a once-lush Mexican delta into a desert. The river’s demise began with the 1922 Colorado River Compact, a deal by seven western states to divide up its water. Eventually, Mexico was allotted just 10 percent of the flow.

Officials from Mexico and the United States are now talking about ways to increase the flow into the delta. With luck, someday it may reach the sea again.

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.

If the final reaches of this six-million-year-old delta were in the United States, they would have been declared a national park, with a protected free-flowing river. But because the river terminates in a foreign country, beyond the reach of the Endangered Species Act and most tourists’ cameras, it is suffering a slow death.

Yet even in its last gasp of fecundity, the delta is larger than the human imagination. Spring tides sweep, like heartbeats, from the upper Gulf of California and the Colorado River Delta Biosphere Reserve two dozen miles up the salt-crusted and rock-hard riverbed. From Arizona a canal runs farm wastewater about 50 miles south into the Mexican delta, creating an accidental, but now critical, bird sanctuary. Groundwater infuses verdant marshlands; newly planted trees line restored riverbanks; and an earthquake last spring destroyed farm irrigation canals, allowing the river to flow seaward again, but all too briefly.

The problems have been neglected amid attention on illegal immigration, the drug war and the debated border fence. But by the time this winter’s fogs burn off the delta, American and Mexican members of the International Boundary and Water Commission aim to complete negotiations on conserving water, responding to climate change and dedicating more water to the delta and its riverside forests instead of only to farms and distant cities.

These talks have gone on for years, but before Mexico’s election this summer, there is a rare ecological opportunity, if only political forces seize it. I hope the commissioners can transcend their differences and recall the wisdom of ancient empires, when civilizations flourished only as long as the Nile and the Euphrates and the Yangtze continued to flow. By strengthening the treaty between the United States and Mexico that governs the Colorado River, we have the opportunity to revive the river and show the world, as it is suggested in Ecclesiastes, that all rivers shall run to the sea.