GRAND FALLS, ARIZONA, AS AN ANALOG FOR FLUVIAL EROSION ON MARS (Invited Presentation)

Grand Falls is an ephemeral waterfall along the Little Colorado River (LCR) in northern Arizona located approximately 50 km northeast of Flagstaff, Arizona. While inactive most of the year, Grand Falls typically experiences two seasonal flow pulses: one during the late winter months (fed by upstream snow melt in Arizona’s White Mountains) and the other in the mid to late summer months (fed by monsoonal rains upstream of the falls). Discharges can exceed 1,000 cubic meters per second during floods.

The falls formed approximately 20,000 years ago when a lava flow dammed the LCR’s original course, diverting the river outside of its incised course and resulting in a 55-meter plunge back into the channel. Since then, lateral erosion through the regional sedimentary rock has occurred at an average rate of approximately 2 centimeters per year. The LCR primarily erodes through the resistant Kaibab Limestone, with the weaker Moenkopi Formation and Coconino Sandstone present above and below the Kaibab, respectively.

The LCR at Grand Falls is a valuable terrestrial analog for fluvial erosion on Mars for several reasons. First, the LCR erodes through a variety of sedimentary lithologies of varying chemical and physical properties, providing a unique opportunity to understand the different styles and extents of erosion through sedimentary rock while holding other variables constant. Second, the dearth of stabilizing plants at Grand Falls ensures that the observed processes are minimally influenced by vegetation. Lastly, the ephemeral nature of the falls allows for frequent revisiting of the landscape and detailed topographic and morphological characterization when flow is not occurring.

We have initiated a long-duration study at Grand Falls to detail the dominant modes and rates of physical erosion during fluvial activity. High-resolution imagery and topographic data are regularly collected, while rock hardness measurements and bedload sampling is performed to understand the susceptibility to and mechanisms of physical erosion.

Fieldwork on the Navajo Nation was conducted under a permit from the Navajo Nation Minerals Department. Any persons wishing to conduct geologic investigations on the Navajo Nation must apply for and receive a permit from the Navajo Nation Minerals Department, PO Box 1910, Window Rock, AZ 86515, (928) 871-6587.