Answer: The Colorado River

There’s little that can stop a mighty roaring river from eventually reaching the sea, little that is, except for the needs of humankind. Historically, the Colorado River moved a huge amount of water throughout the southwestern United States and northern Mexico, fed mainly by snowmelt water from the Rocky Mountains. In fact, huge might be an understatement. Before human development along the river, the average discharge from the Colorado River into the Gulf of California was around 20 cubic kilometers of water per year with an average peak discharge of roughly 2,800 cubic meters per second during the summer.

Today, shockingly, the Colorado River runs dry. While there’s plenty of water to see closer to the mountains, if you follow the river’s winding path through five U.S. states and northern Mexico, by the time you get to the end, you won’t see a powerful river emptying into the ocean, just a dry river bed with the occasional slow trickle of water that is more reminiscent of a country stream than the powerful river that cut the Grand Canyon into the earth.

Where did all the water go? Starting in the late 1880s, a series of irrigation and damming projects—from the Grand Ditch (completed in 1890) to high profile projects like the Hoover Dam (completed in 1935)—began altering the flow of the river to capture the water for farmland irrigation, energy generation, and to supply water to numerous cities along the way. The river is the sole supply of water for Imperial Valley in Southern California, seen in the photo here, which is among the most productive agricultural land in the United States.

While that made the arid badlands of the Southwest livable for millions of people, it also permanently changed the river. Starting in the 1960s, the river began running dry before it reached its mouth and since then, save for a few exceptions and several consecutive years in the 1980s when record-breaking precipitation and snowmelt sent water surging down the Colorado River watershed, it has never approached its previous flow rate.