By Robert Marcos
Geologists who dated ancient Colorado River sediment believe that the river began 11 million years ago as a network of streams that flowed across western Colorado. But when they dated the ancient river sediment at the base of the Grand Canyon, they ran into a very big problem. The ancient sediment where the Colorado River exits from the Grand Canyon is only 5.6 million years old. So where had the Colorado River been flowing during the “missing” 5.4 million years? 1
Note – this five-million-year gap in the river’s history is unrelated to the Grand Canyon’s famous “Great Unconformity,” which represents a much older missing chunk of Earth’s solid rock layers.
During the decades when the five-million-year gap continued to be a mystery, geologists debated conflicting theories to locate the missing river. Some hypothesized that the ancestral river might have bypassed the Grand Canyon entirely by flowing south into Mexico or routing toward the Mississippi River basin. Others actively hunted for evidence of alternative underground pathways – proposing that the river may have dissolved into subterranean limestone networks before emerging downstream, in a process known as Karst piracy.2
The Answer: Bidahochi Lake
The mystery was finally resolved by a scientific study that was published in Science, in April 2026.
The paper was written by Dr. John He (a geologist from UCLA), Dr. John Douglass (from Paradise Valley Community College), and Emma Heitmann (a doctoral student from the University of Washington), who had met during expeditions that studied the sedimentary deposits of Bidahochi Lake – an ancient lake that once existed on Navajo land in Northeastern Arizona.3
To help solve this multi-million-year cold case, the geologists used detrital zircon geochronology—a high-tech forensic technique that allows scientists to track the exact birthplaces of ancient river sediments. Zircon crystals are nearly indestructible, microscopic minerals that form inside cooling magma. As they crystallize, they trap trace amounts of uranium but completely reject lead. Over time, that uranium radioactively decays into lead at an incredibly steady, known rate. By measuring the ratio of uranium to lead inside a single zircon grain using a laser mass spectrometer, scientists can determine exactly when that crystal formed, providing a geological timestamp. Their research provded that the Colorado River had flowed into Bidahochi Lake for several million years, before the river changed its course and began to flow through the Grand Canyon. 4
Dr. Angela Hessler, an Earth Scientist from Stanford University said, “The Colorado River has not always flowed through the Grand Canyon area, and it is not known when and how that relationship began. He et al. collected uranium-lead ages for thousands of zircon crystals in formations located upstream and downstream of the canyon. Age patterns revealed a “fingerprint” of Colorado River sediment suggesting its arrival into an upstream lake by 6.6 million years ago and its subsequent integration with the Grand Canyon and downstream catchment. The direct connection may have happened through a combination of processes, but the zircon data give support to a long-debated “lake spillover” hypothesis.” 5