Prairie gully erosion in the Redwood Creek Basin, California

Walter, Tom

In this study, gully formation and enlargement in open grassland areas(prairies) were mapped and measured by reviewing aerial photographs takenin 1954 and 1978. Field measurements defined the relationship of gully widthto cross-sectional area. Roads traversing prairies were classified accordingto construction standard, and their lengths were measured. In addition, itwas determined whether or not each gully was associated with a road.Approximately 80 percent of the gully erosion (volumetrically) occurringduring the study period was either road-related or probably road-related.Logging roads, the most common type in the basin, accounted for roughly 60percent of the volume eroded from gullies. Higher standard roads (roadsdesigned for heavier traffic flow) required larger cut and fill slopes andgreater drainage alteration, and consequently produced more gully erosionper road kilometer than logging or ranch roads.There was an apparent relationship between road density (km of road perkm 2 of prairie) and gully erosion yield (volume of erosion per km 2 ofprairie) for the upper, middle, and lower sections of the Redwood Creekbasin. The few largest gullies contributed the vast majority of totalsediment. Cullies tended to be relatively inactive after initial formation.When gully enlargement did occur, they widened rather than lengthenedupslope. The gully erosion rate on prairies was roughly comparable to thatof Garrett Creek, a moderately erosive logged tributary basin and an orderof magnitude less than that of Copper Creek, a severely eroded loggedtributary basin within the Redwood Creek watershed.