The difficult task of estimating recurrence intervals for large floods has long plagued hydrologists because statistical measures fail when return intervals of floods exceed the length of historical data sets. Sediments deposited in the backwaters of large floods may accumulate thick sequences in tributary mouths. Stratigraphic and sedimentologic studies of these sequences combined with radiocarbon dating have established a 10,000-year paleoflood record for the lower Pecos and Devils rivers in southwestern Texas. This technique is rapid and relatively inexpensive and can be used where historical records are short or entirely absent.

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 experiments 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.

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Detailed geomorphic mapping from Viking imagery of selected portions of Kasei Vallis, Maja Vallis, and vicinity reveals numerous similarities of channel morphology to erosional and depositional features of the Channeled Scabland. Characteristic scabland landforms which occur in Kasei and Maja Valles include erosional grooves, streamlined uplands and hills, scour zones around flow obstacles, inner channels with erosional head cuts, breached ridges and basin (crater) rims, pendant forms (bars?), erosional terracing of streamlined hills and channel margins, and possible midchannel bars. These features constitute an assemblage of landforms which on earth is most characteristic of catastrophic flood channeling in jointed bedrock. Prominent mass wastage and sapping features are associated with the high-wall relief in lower Kasei Vallis. Many cliffs along the channel margins exhibit steep upper slopes and gentler lower talus slopes which form the spur-and-gully topography that has also been described along chasma walls in the Valles Marineris. Landslides, debris fans, and debris cones can also be recognized. Much less wall modification occurs in the shallower Maja Vallis. Probably, the steep escarpments of Kasei were created by tectonic processes and subsequent channel incision. These escarpments later receded by mass wasting and sapping. bed forms that often mark the entire valley width. Our previous morphological mapping from Viking imagery of Kasei, Ares, and Tiu valles revealed a distinctive assemblage of landforms [Baker and Kochel, 1978a]. The macroforms are related to the overall channel pattern and include the following: (1) a regional anastomosis of channels, (2) discrete source areas (chaotic terrain) displaying a disparity between apparent released fluid volume and downstream trough volume showing fluid-erosional forms, (3) indistinct fluid sink relationships, (4) erosion of diverse rock types thousands of kilometers from probable fluid source areas, (5) residual uplands separating phologies. Some systems approach the degree of development channels, many of which are streamlined by fluid flow, (6) of terrestrial dendritic patterns with an elaborate hierarchy of pronounced flow constrictions and expansions, (7) high widthtributaries. The dry valley systems are widely distributed but depth ratio, (8) low sinuosity, and (9) differential erosion controlled by lithology and structure, including exhumation of show a general association with the relatively old, heavily cratered terrain surfaces [Pieri, 1976, 1979; Carr, 1979; Pieri and prefiow topography. The bed forms within the channels in-Sagan, 1979]. clude the following: (1) longitudinal grooves, (2) inner chan-The outflow channels of Mars are much larger than the dry valley systems. Channel widths range from 10 to 100 km, and some channel complexes (Maja, Ares, and Kasei valles) can be traced 2000 km or more. These channels appear full born at localized sources, usually collapse zones marked by chaotic terrain. They transect terrains of varying age, generally ...

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