Major-storm and long-term erosion rates in mountain watersheds of the western Transverse Ranges of Ventura County are estimated to range from low values that will not require the construction of catchments or channel-stabilization structures to values as high as those recorded anywhere for comparable bedrock erodibilities.
A major reason for this extreme variability is the high degree of tectonic activity in the area--watersheds are locally being uplifted by at least as much as 25 feet per 1,000 years, yet the maximum extrapolated rate of denudation measured over the longest available period of record is 7.5 feet per 1,000 years adjusted to a drainage area of 0.5 square mile. Evidence of large amounts of uplift continuing into historic time includes structurally overturned strata of Pleistocene age, active thrust faulting, demonstrable stream antecedence, uplifted and deformed terraces, and other results of base-level change seen in stream channels. Such evidence is widespread in the Transverse Ranges, and aspects of the landscape, such as drainage-net characteristics and hillslope morphology, are locally more a function of tectonic activity than of denudational process. Many of the 72 study watersheds are located on frontal escarpments of mountain blocks cut by recently active thrust faults, along which the upper part of the drainage basin has overthrusted either the lower part of the basin or the adjacent valley area.
To define erosion rates in 35 small watersheds in the western Transverse Ranges, a group of 37 similar watersheds with measured sediment yields in debris basins was selected from the eastern Transverse Ranges in Los Angeles County. Sediment yields from this group of watersheds during the record-breaking 1969 storms ranged from relatively low rates to values equivalent to reduction of the entire land surface of a watershed by more than 2 inches.
Correlation of erosion rates from the watersheds with measured rates to the group with unknown rates required definition of the chief factors that control the erosion rates. Numerous types and combinations of variables measuring physiography, soil erodibility, slope stability, hydrologic factors, wildfire effects, vegetation, and land use were analyzed by regression. A slope-stability variable retained in regressions at significant levels was the proportion of watershed drainage area underlain by slope failures, a logical measure of increased erodibility caused by uplift.
The importance in the area of debris flows, mudflows, and mass movements--forms of sediment transport not involving normal aqueous entrainment--is also a reflection of the active tectonic setting of the Transverse Ranges. Implicit in the detailed study of selected physiographic and slope-failure variables was the logical assumption that correlation with the probability of transport by these exotic but quantitatively important sedimentation processes would be achieved.
So prominent and widespread was evidence of debris flows in the small study watersheds after the 1969 storms, that it was possible to formulate a model for the dispersal of sediment in such watersheds: Lateral supply of sediment to stream channels is a relatively continuous process, accomplished in significant part during the dry season by dry-sliding, in addition to wet-season contributions from overland flow and mass movements. During periods without major storms, stream channels undergo more-or-less time-continuous fill. Then, during a storm of high recurrence interval, channel-bed material is mobilized and dispersed in large part by debris flows--coarse granular slurries, some of which are induced by mass movements triggered by the storm. Channels undergo substantial net scour, accomplished by removal of bed material in debris flows and by scour during recession flow. Valley-side slopes are undercut by bank erosion, and a new cycle of channel infilling by hillslope processes is initiated.
|Title||Erosion and sediment yields in mountain watersheds of the Transverse Ranges Ventura and Los Angeles Counties, California; analysis of rates and processes|
|Authors||Kevin M. Scott, Rhea P. Williams|
|Publication Subtype||USGS Numbered Series|
|Series Title||Water-Resources Investigations Report|
|Record Source||USGS Publications Warehouse|