Colorado River

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

It is easy to miss, in a short geographic description, that the Colorado River lower course now runs dry before reaching the sea most of the year. That single fact is the endpoint of a century of engineering. Seven US states and Mexico all draw irrigation and drinking water from the same river under a set of agreements, chiefly the 1922 Colorado River Compact, negotiated when the river's average flow was measured during an unusually wet stretch of years and has been over-allocated relative to its actual long-term flow ever since. Hoover Dam, completed in 1936, and Glen Canyon Dam, completed in 1966, turned the river into a chain of reservoirs, Lake Mead and Lake Powell foremost among them, that store water for cities as far away as Los Angeles and Phoenix, neither of which sits on the river at all. By the time what remains reaches the Mexican border, so little water is left that the Colorado River's historic delta on the Gulf of California, once a rich wetland, is now dry for most of the year, a river that carved the Grand Canyon over millions of years now unable, most years, to finish its own final few miles.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

The Grand Canyon is often described by its width and its exposed rock layers, but the Colorado River that cut it is the more interesting half of the story, because the canyon is really a record of a river winning a very slow argument with rising land. The Colorado Plateau, the broad tableland the river crosses through northern Arizona, has been slowly uplifted over millions of years by tectonic forces unrelated to the river itself; as the plateau rose, the Colorado, already established on its course, cut downward to keep pace rather than being pushed aside, carving through nearly two billion years of rock in the process, some of the oldest exposed rock anywhere on the continent. That kind of canyon, called an entrenched or superimposed river, only forms when the uplift is slow enough for the river to keep eroding faster than it is displaced, and the Grand Canyon's own nearly one mile of depth is the visible record of the Colorado managing exactly that balance for roughly five to six million years, a modest span in geological time for a mile of solid rock.