Repeated excursions in the Mohave Desert region of southeastern California have led to this comprehensive summary of the work of sheetfloods and streamfloods in arid physiographic cycles. Comparisons are also made with accounts of a few other arid regions, especially those in which better examples of external drainage or locally degrading base levels have been observed.
In regard to the controversial question of the importance of lateral erosion in deserts, it is concluded, at least for the region under observation, that lateral erosion is of practically no importance, except in the mountains and in the few other places where there are well-developed washes or water courses. Sheetfloods, not streamfloods, normally grade piedmont slopes and the fan bays where desert mountain valleys open out between diminishing spurs. The areas subject to sheetfloods there are enlarged mainly by back-weathering subaerial degradation of the mountain and spur slopes, essentially in the manner analyzed by Lawson (1915). Where gradual uplift, degrading base levels, or other moderate changes allow streamfloods to take mild possession of these graded areas, their work too is almost entirely down-wearing rather than lateral cutting.