Casualty of historic arroyo incision in the southwestern United States

Arroyos in the American southwest are defined as steep-walled, flat-floored trenches, typically cut into alluvium. Intensive investigation of arroyos for more than a century has shown that arroyo cutting events have been temporally concentrated during the Holocene, the most recent (historic) interval occurring between 1850 and 1915; the 1880s and early 1900s were particularly important periods of formation. The restricted period of historic cutting suggests that their formation is related to semi-synchronous changes in environmental conditions throughout the southwestern U.S. However, determining causality is plagued by multiple, temporally overlapping drivers, that vary in magnitude and intensity over the region and that produce non-linear and divergent geomorphic responses that often lag well-behind the disturbance. The lag in response times is a result of sequential adjustments in vegetation, runoff, sediment yield, groundwater-surface water interactions, and/or the propagation of geomorphic adjustments through the system. In light of these difficulties, it may be more productive from a management perspective to determine the controls on valley/arroyo system response to disturbance and the potential impacts of these geomorphic responses on ecosystem conditions than to concentrate on the causality of arroyo formation.