Figure 14. At river mile 246, the top of the Spencer flow exhibits abundant evidence that the free-flowing Colorado River was once established on top of this distal Black Ledge flow. Large, 5-m-diameter potholes and channel forms are preserved at this location. The river level here is now controlled by Lake Mead. Predam river levels were 30 m lower (Karlstrom et al., 2007). (Photo by Laura Crossey)