The Colorado River has been flowing through the Grand Canyon for over 5 million years, but the two were not united to begin with. For millions of years before it found its current course, the river followed a path that did not run through the Grand Canyon. Scientists now think they have an explanation for how the river found its way in.
A study published in Science suggests that the Colorado River was joined to the Grand Canyon when several ancient lakes linked to the river gradually filled and spilled over into the canyon. The age of zircon crystals in the Bidahochi basin in northern Arizona supports this spillover idea and helps explain the river's change of course.
A precursor to the modern Colorado River existed in western Colorado at least 11 million years ago. Researchers have proposed many pathways for that early river. It may have flowed almost parallel to the present river, or northwest or southeast of the Grand Canyon. The river did not leave the Grand Canyon as it does now until 5.6 million years ago, leaving a long gap in its history. To explain how it was steered into the canyon, scientists have suggested several theories, from headwater erosion to karst piracy, in which large volumes of water move through cave networks.
The study, however, reports evidence that spillover from a paleolake system, a chain of ancient lakes, was the main process that channeled the Colorado River into the Grand Canyon.
The authors reached this conclusion after using uranium-lead dating on zircon crystals in volcanic ash beds and sandstones of the Bidahochi formation. The area holds a large basin that once contained ephemeral lakes, temporary bodies of water that may fill or dry out and are common in dry regions.
These lakes, which the National Park Service says once covered much of northeastern Arizona, laid down silt, clay and sand that now form the lower part of the Bidahochi formation. Ash and lava from nearby volcanoes also reached the basin, and the ash formed fine-grained layers in the lake sediments.
Uranium-lead dating of zircon is often used to date rocks. The method is especially useful because zircon grains act like fingerprints in the geologic record, letting researchers trace where sediment came from and how rivers were connected in the distant past.
The authors found that the makeup and age range of zircon grains in the upper Bidahochi formation, from about 7 to 6 million years ago in the late Miocene, match evidence from early Colorado River deposits. This suggests that the formation shares a sediment source with the river and that it was likely connected to the river by 6.6 million years ago.
Other clues point the same way. Sediment piled up faster, and fossil fish in the Bidahochi formation resemble those in surrounding rivers. Together, they indicate that the Colorado River was flowing into the basin and filling it for more than a million years before the water continued downstream into the Grand Canyon.
The authors believe that lakes spilling into the Grand Canyon likely set the river's present course, though groundwater flow or erosion may have played smaller roles.
In the nearly 5 million years since that change, the Colorado River has kept carving the canyon wider and deeper. One way it does this is through downcutting: the river carries large rocks and boulders that act like chisels, chipping away at the riverbed, according to the National Park Service. That lasting work has shaped the Grand Canyon into the geologic wonder seen today.