Limited Response of Ephemeral Stream Dynamics to Extreme Vegetation Change Related to Historic Overgrazing, Catalina Island, California, U.S.

The role of climatic transition and related changes in vegetation in arroyo development remains a controversial topic. Most studies in the semiarid southwest US suggest that either latest Holocene climate change or overgrazing triggered widespread historic aggradation and incision in ephemeral stream systems, yet a clear consensus has not been reached. The fluvial response question is important when trying to anticipate landscape changes in response to global climate change. A history of extensive vegetation disturbance caused by 150 years of intense grazing on Catalina Island and a record of fluvial erosion and deposition provides clues about ephemeral fluvial system response to extreme vegetation change. Deposits exposed along axial channels contain multiple buried soils that have strong A horizons and weak Bw horizons. Radiocarbon dates indicate that these sediments were deposited between ca. 6-2 ka. New radiocarbon dates and soil stratigraphy of several tributaries along the main valley washes also indicate that widespread deposition occurred episodically up to about 2 ka. The presence of multiple buried soils indicates that several pronounced cycles of deposition, stability, and erosion occurred long before overgrazing and extensive reduction in vegetation cover. Deep (>75 cm), moderately developed soils examined along steep hillslopes also indicate relative slope stability over the past several centuries to several thousand years, although, scattered but localized areas of historic erosion are observed. Holocene deposition and erosion is well represented in the valley bottoms, but little stratigraphic evidence exists for widespread historic hillslope erosion and fluvial deposition in tributaries or trunk streams. The relation implies that an extensive decrease in vegetation alone appears insufficient for triggering a cycle of historic hillslope erosion and valley aggradation. Limited geomorphic response to overgrazing suggests that other response processes such as complex geomorphic response, extensive fires, or an increase in extreme storms may be required to mobilize sediment and trigger extensive arroyo filling and incision.