Geomorphic response to minor cyclic climate changes, San Diego County, California

Abstract

Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region. Using aerial photographs and field investigations we found little response to the 1978–1983 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.