Geomorphic response to seasonal variations in rainfall in the Southwest United States

The interaction of the North American Monsoon with watershed hydrology and landscape response is evaluated by observing geomorphic characteristics of hillslopes, hydrology, and stream channels in two mountain ranges with contrasting intensity of precipitation. The study compares watersheds in the Hualapai and Santa Catalina Mountains in Arizona, which are similar in lithology, elevation, tectonic setting, vegetation, and annual precipitation, but differ in the proportion of precipitation received in summer thunderstorms. The Hualapai Mountains receive most of their precipitation in winter, whereas rainfall in the Santa Catalina Mountains occurs mostly in summer. Drainages in the Santa Catalinas are more variable in local relief and exhibit much more exposed bedrock and higher drainage density. The trunk channel of major drainage in the Santa Catalina Mountains has measured discharges several orders of magnitude greater than a channel draining a comparable area in the Hualapai Mountains. The alluvial segment of the channel in the Santa Catalina piedmont additionally displays greater concavity, smaller width-to-depth ratios, and a larger caliber of bed material. In contrast to conventional interpretation, summer monsoonal precipitation is not primarily responsible for generating most of the discharges that modify channels in large-scale drainages. In the monsoonal climate regime of the Santa Catalinas, small basins flood most often in summer, whereas larger drainages exhibit peak discharges in response to low-intensity winter precipitation. We attribute the paradoxical discharge response of larger drainages to the small spatial scale of summer thunderstorms, which fail to deliver enough precipitation to generate floods in these larger basins, but do prime hillslopes by stripping colluvium and lowering hillslope-infiltration rates, making the large drainages more responsive to areally extensive winter storms.