Overview
- The Science study, published Thursday, reports that the ancestral Colorado River pooled in the Bidahochi basin around 6.6 million years ago before spilling west about 5.6 million years ago to begin carving the Grand Canyon.
- Researchers traced sand sources using detrital zircon dating, a method that reads uranium–lead ages in tiny crystals to match sediments to their upstream origins like a fingerprint.
- Supporting clues include strontium isotope ratios, thicker sediment layers, river‑style ripple marks in lake beds, and fossil fish built for fast current, all pointing to sustained river inflow before spillover.
- The team argues spillover over the Kaibab uplift best explains the data, while experts such as Karl Karlstrom question lake size, notch elevations, and whether an older paleocanyon could have routed flow without pooling.
- The work helps close a five‑million‑year gap in the river’s history and tracks downstream integration to the Gulf of California roughly 4.8 to 5 million years ago.