Birth of a River
The extensive system of dams, reservoirs and aqueducts routinely diverts the Colorado’s entire flow for agricultural irrigation and water to 40 million people. Environmentalists have fought further development and controversy continues to swirl as demands on the river rise.
For Becky Dorsey, the story of the Colorado begins about 5 million years ago, with its birth. That’s a story she is just starting to tell.
The earth sciences professor is trying to determine when and how the Colorado started and how it evolved through time. Answers to those questions won’t just resolve debates about the river’s origins— they’ll help scientists understand how whole continents can split apart.
Dorsey and her team had scarcely begun a three-year project funded by the National Science Foundation when they notched their first breakthrough late last year, advancing a debate that has confounded geologists for decades: Did the Colorado River originally flow into a lake or the sea?
It’s long been thought that a key stretch of the river near the California-Mexico border first formed as a series of lakes, with water that flowed down from the Colorado plateau. Dorsey’s team argues instead that the Gulf of California once extended into this area, about 150 miles north of its northern shore today.
Under the conventional wisdom, scientists have explained the presence of marine fossils in the rocks of this area as evidence that birds once carried clams, barnacles and other catches there from the sea.
That idea is based on the composition of the rock. But Dorsey’s team—led by master’s student Brennan O’Connell—looked instead at its features: The rock shows orderly layers of varying thickness, suggesting instead that the sea once deposited sediment in the area in steady, high- and low-tide cycles.
“We took a new approach,” Dorsey said. “This is just what we do. We march up some lonely wash, we get right up there and we look really, really close at the rocks. It’s not rocket science and it’s not always easy to do this work, when the rain’s pouring down and it’s 30 degrees. But we love it.”
That sediment also holds clues to conditions that could cause entire continents to split in two.
The Colorado once dumped huge amounts of sediment into the gulf, a body of water that sits atop the continental plate boundary where Southern California and the Baja California peninsula are slowly drifting away from mainland Mexico.
Rifting between two continental plates may be intensified by the presence of sediment, leading to “localized strain,” Dorsey said—that’s one of the conditions necessary for continents to crack in two.
“We understand that continental rifting is driven by forces that try to pull two regions apart,” Dorsey said. “But a fundamental question is, what are the conditions that lead to the breakup of one continent to make two new continental plates?”
For her, the Colorado is about much more than just man-made changes of the last century.
“When I look at the river, it inspires me to contemplate the connections between deep time and the present day,” Dorsey said. “There’s a lot of mystery and beauty in that.”