LIMITED RESPONSE OF EPHEMERAL STREAM DYNAMICS TO EXTREME VEGETATION CHANGE- LATE HOLOCENE ALLUVIAL HISTORY OF EPHEMERAL STREAMS, CATALINA ISLAND, CALIFORNIA

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 arroyo incision question is important when trying to anticipate geomorphic changes in the landscape in response to global climate change. A well-documented history of extensive vegetation disturbance due to 150 years of intense grazing on Catalina Island and a record of late Quaternary fluvial erosion and deposition provides an opportunity to address how ephemeral fluvial systems responded to extreme vegetation change. Alluvial and colluvial deposits and soils were characterized in the Middle, Cape, Skull, and Bulrush canyons of Catalina Island. These drainages contain ephemeral streams in narrow alluvial valleys that are incised locally more than 3 m and preserve up to 6 discontinuous terraces. Fluvial deposits contain multiple buried Holocene soils that contain strong A horizons and weak to moderate Bw horizons. These soils record cycles of deposition, stability, and erosion. Most hillslopes contain soils indicative of relative stability over the past several centuries to several thousand years, although, scattered but localized areas of historic erosion are observed. Whereas the Holocene depositional and erosional record is well represented in the valley bottoms, there is little stratigraphic evidence for widespread historic hillslope erosion and fluvial deposition in tributaries or trunk streams. The relation implies that considerable and extensive decrease in vegetation by itself appears to be insufficient for triggering a cycle of historic hillslope erosion and valley aggradation. This means that historic incision and sedimentation is considerably less extensive than what previously occurred in the Holocene. Limited geomorphic response to grazing 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.