GROUNDWATER SAPPING: A PLANETARY-SCALE EROSIONAL PROCESS?

The surfaces of Earth, Mars and Titan host canyons and valleys cut by rivers. Establishing the source of river flow, whether from rainfall or groundwater, has major implications for planetary atmospheres, hydrology and habitability. It has long been argued that valleys with stubby amphitheater-shaped headwalls were formed by groundwater sapping, the process of undermining and erosion by seepage flow, and the abundance of these canyons on Mars therefore may indicate widespread valley formation by groundwater seepage in the absence of rainfall. This interpretation has been a subject of significant debate in the last decade, however, and new techniques, such as cosmogenic exposure age dating and sediment transport constraints, have revealed that some amphitheater-headed canyons on Earth originally attributed to groundwater sapping may instead owe their origin to overland-flow erosion. Other Earth-analogs remain ambiguous, and the link between erosion processes and canyon form appears to be strongly dependent on rock type and stratigraphy. For twenty-five years Alan Howard has pioneered investigations into amphitheater-headed canyons on Earth and Mars, including field measurements, experiments, and numerical modeling. In this presentation I will review the history and rationale of the groundwater-sapping hypothesis for valley formation, discuss recent insights from canyons in Idaho, and evaluate opportunities and challenges in linking process and form in putative groundwater-sapping landscapes and in landscape-evolution models.