HILLSLOPE GULLYING AND RELATED CHANGES, SANTA CRUZ ISLAND, CALIFORNIA

This study examined the multiple processes involved in initiating and shaping the hillslope gully under varied steepland conditions on Santa Cruz Island. Gullies cut into bedrock of steep watersheds are the result of varying influences of rock and soil type, vegetation reduction, and intense rainfall. The vegetation reduction coincides with commencement of sheep grazing in the mid 19th century. Ensuing rapid erosion of the Island's steep slopes in the late 19th century and 20th century is documented by accelerated aggradation of adjacent canyon floors. Intense rainfall influence was documented during the 1977-78 and 1978-80 storm seasons. The 1977-78 storm season is one of the most severe of the century.; Hillslope gullies were surveyed on five different watershed types ranging from ungrazed to heavily sheep-grazed sites on three rock types. Propensity for gullying varied with rock type. The most severe gullying occurred where there had been almost complete loss of previously substantial woody cover. Gullying especially was facilitated by mass movement on the disturbed watersheds and locally on fine-grained regoliths prone to soil piping.; Gully morphology varied with respect to different rock types. Gully depths were constrained by unweathered bedrock, while width of the gully channel was more dependent on gully depth and gully wall stability. However, on some sites there were no trends or interrelationships among gully dimensions while there were strong interrelationships among the same dimensions in other populations. The type of gully head varied considerably among gully populations demonstrating the process-selective influence of rock type.; Three gullies were monitored for 2.5 years, by erosion pins and placement of painted stones on the gully floor between storms. Varying scales of mass movement were found to shape the hillslope gullies rather than fluvial processes. Amelioration of gully erosion occurred in an area recently excluded from sheep-grazing.