Sedimentologists are geologists that study rocks on the earth’s surface today in order to interpret ancient depositional environments. By studying deposits of river gravels, Andre Potochnik has been able to determine major changes in the drainages on the southern part of the Colorado Plateau. We talk with Andre about his work in Arizona on the Apache Reservation and how he has come up with the timing of significant drainage reversals for rivers in that area and how this impacts the interpretations of the Colorado River and the Grand Canyon.