Runoff and groundwater sapping processes are important, to varying degrees, in the initiation and evolution of terrestrial and Martian valleys. The resulting channel morphology and valley morphometry appear to be distinctive, depending upon which of the two processes are dominant. Our field reconnaissance and morphometric studies of runoff and sapping valleys on the slopes of Hawaii and Molokai in Hawaii indicate that valley types can be distinguished using remotely sensed data similar to the Viking orbital imagery of Mars. Principal components analysis of morphometric data clearly separate runoff valleys from sapping valleys. Hawaiian sapping valleys, like their possible Martian analogs, are characterized by: (1) steep valley walls and flat floors, (2) amphitheater heads, (3) low drainage density, (4) paucity of downstream tributaries, (5) low frequency of up-dip tributaries, and (6) strong evidence of structural and stratigraphic control on valley patterns.

The large Hawaiian sapping valleys are fed by groundwater from high-level dike-impounded aquifers breached by channel incision. Flume experiments in weakly cemented sand have produced similar channels by groundwater sapping. In addition, we have mimicked the sequence of valley evolution observed in Hawaii and have shown the importance of groundwater piracy as a major process for widening valley heads. These experimens are useful in testing fundamental ideas concerning how sapping valleys evolve in response to variable structure and stratigraphy. They also provide a model for developing geomorphic criteria useful in the recognition of landforms generated by groundwater sapping.