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Piru Creek is a southerly drainage of the Transverse Ranges in southern California. Geologically recent tectonic activity has created a terrain in which many different kinds of rocks are exposed and in which landscapes of different maturity are adjacent. Estimates of the sediment yield of the watershed, obtained by a variety of techniques, are consequently variable and range from 165 to 303 acre-feet per year. The Piru Creek watershed has undergone a period of extensive aggradation that may be ending, possibly in response to changes in land use. Santa Felicia Reservoir, which impounds Piru Creek near its mouth, was surveyed to determine the sediment yield of the watershed. The 10 years of reservoir record (1955-65) was a period of subnormal precipitation. Consequently, streamflow data were used to extend the sediment record through a major wetdry fluctuation and to obtain an estimate of the probable long-term sediment yield. The probable sediment yield of the basin above Pyramid Rock, site of a proposed reservoir on Piru Creek upstream from Santa Felicia Reservoir, is 225 acre-feet per year, or 0.79 acre-foot of sediment per square mile per year. The yield was determined by applying a basin-size correction to the sediment yield of the entire basin. Measurements of suspended sediment indicate that 570,000 tons, a volume of 504 acre-feet, were transported past the Pyramid damsite during calendar year 1965. November and December 1965 constituted one of the most intense storm periods in southern California history, and the quantity of sediment transported during 1965 can be considered to represent the expectable maximum. The yield for 1965 confirms the long-term estimate of sediment yield as being substantially and unexpectedly less than that of surrounding watersheds. Comparable figures for sediment yield were obtained from other analyses of sedimentation in basins of the Transverse Ranges and by correlation of sediment yields in other basins. Multiple-regression equations were calculated using sedimentation records of seven nearby reservoirs. Regional climatic parameters, effective precipitation and precipitation intensity, were used in the analysis because of the general paucity of climatic data for the Piru Creek basin. Soil credibility was assessed by determining dispersion and surface-aggregation ratios for each of the seven major lithologic types in the watersheds. El E2 SEDIMENTATION IN SMALL BASINS Sediment yields based on correlations using drainage-area and physiographic factors were substantially higher than those based on water-discharge, climatic, land-use, and soil-erodibility factors. The differences in computed yields are due to marked heterogeneity in climate, lithology, and landscape maturity throughout the Transverse Ranges, factors which combine to cause an abnormally low sediment yield in the Piru Creek basin. The sediment yield of Piru Creek has the capacity to increase by approximately 50 percent if subjected to the wildfire prevalence typical of nearby watersheds. However, the premise of interpreting future conditions in light of those of the past probably is the best approach in forecasting sediment yields, unless trends are clearly evident. INTRODUCTION AND ACKNOWLEDGMENTS A primary concern in the design of a reservoir is the rate of depletion of reservoir capacity by the accumulation of sediment. This factor assumes added significance in a region of high, yet variable, sediment yield. Such a region is the Transverse Range province of southern California, in which the Piru Creek basin is located. Sediment yield here is generally greater than in most other parts of California where sedimentation in reservoirs has been measured, and the region is one of marked variability in factors affecting erosion and transport of sediment. A reasonably correct prediction of the long-term sediment yield for the drainage basin above a reservoir is essential. Overly large estimates of sediment yield will result in needless expense of design tolerance for large quantities of sediment. Low estimates may decrease the useful life of a reservoir through reduction of its capacity by the unexpectedly high rate of sediment accumulation and may cause economic loss as well as disruption of water-use planning. This report applies and compares a variety of methods of estimating sediment yield in the Piru Creek basin above the site of a planned terminal-storage reservoir that is part of the California Water Plan. The methods are applicable to most parts of the Transverse Ranges and to similar regions which are tectonically active and physiographically diverse. The report discusses the techniques and problems of estimating sediment yields where direct measurements have not been made. The primary method of determining sediment yield is measurement of suspended sediment and streamflow and, thus, the suspendedsediment discharge. The low-frequency and flashy character of runoff in the Piru Creek basin has made the establishment of a sedimentsampling program impractical to date (1966). However, installation of automatic sediment-sampling devices to collect samples at a series of predetermined stages is possible. Such devices were installed at three streamflow gaging stations at the following locations near the damsite: Piru Creek below Buck Creek, Canada de Los Alamos below Apple Canyon, and Piru Creek above Frenchman's Flat (fig. 1). PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E3 The sediment collected by the automatic sampler does not include bedload sediment which rolls and bounces along the stream bottom. Reservoir surveys, which consist of measurements of accumulated sediment along preestablished cross sections, are an additional source of direct sediment-volume determination. Such measurements include bedload but exclude suspended sediment passed through the reservoir, the quantity of which depends on the trap efficiency of the catchment. Sediment trapped by Santa Felicia Reservoir, near the mouth of Pirn Creek, was measured to establish the overall sediment yield for the drainage basin and was used as a basis for comparison of other methods of computing sediment yield. Another technique which frequently has been used to estimate sediment yields in the Transverse Range province is the correlation of direct sediment measurements for different basins according to the degree of similarity in the factors that affect sediment yield. The primary factors controlling erosion and sediment yields are watershed area, climate, land use, and soil erodibility. Soil erodibility is of particular importance in areas of young soils where parent material is a significant soil-forming factor generally unrelated to the other factors. Equations incorporating the primary factors were derived to estimate the sediment yieH. Sediment yield from the drainage areas above Matilija, Pacoima, Big Tujunga, Big Santa Anita, Sawpit, Big Dalton, and San Dimas Reservoirs, all near the Piru Creek watershed, (fig. 1) had already been measured. By correlation of the yields from the areas above the reservoirs with different parameters of basin characteristics, regression lines were drawn, and the yield for Piru Creek above Pyramid Rock was calculated. The writers wish to express their gratitude to the following individuals: H. W. Anderson and J. R. Wallis, for consultation and the supply of unpublished data collected by the Pacific Southwest Forest and Range Experiment Station; M. F. Burke, of the Los Angeles County Flood Control District, who provided long-term rainfall records for Los Angeles County; G. M. Kennedy, of the Soil Conservation Service, for aid in obtaining soil information; W. P. Price and Frank Beckwith, of the United Water Conservation District, who provided data and collected sediment samples at Lake Piru; and W. H. Hansen and W. T. Dresser supervisors of Los Padres and the Angeles National Forests, respectively, who provided fire histories for the Piru and adjacent watersheds. The project was conducted cooperatively with the State of California and was completed under the general supervision of W. W. Dean and George Porterfield, of the Water Resources Division, U.S. Geological Survey. The manuscript benefited substantially from the criticism of H. W. Anderson, D. M. Culbertson, and C. H. Hembree. 309-538 O 68 2 11 9° 30 ' 11 9° 00 ' 11 8* 00 ' M 34 -3 0' E X P L A N A T IO N _ _ _ _ _ _ _ K E R N C O U N T Y L O S A N G E L E S C O U N T Y D R A IN A G E A R E A S D ra in ag e di vi de

Sedimentation in the Piru Creek watershed, southern California