Sedimentation in the 

Piru Creek Watershed 

Southern California

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1798-E

Prepared in cooperation with the 

California Department of Plater 

Resources

Sedimentation in the 

Piru Creek Watershed 

Southern California

By KEVIN M. SCOTT, JOHN R. RITTER, and JAMES M. KNOTT

SEDIMENTATION IN SMALL BASINS

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1798-E

Prepared in cooperation with the 

California Department of W^ater 

Resources

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 35 cents (paper cover)
CONTENTS

Page

Abstract_______________________________________________________ El

Introduction and acknowledgments._________________________________ 2

Location and physical features._____________________________________ 5

Climate._________________________________________________________ 6

Lithology and structure.___________________________________________ 8

Geomorphology _ __________________________________________________ 10

Soils and vegetation.___________________________-__--_-_--_---_-_-- 13

Land use and fire history___________________________________________ 13

Direct measurement of sediment deposited in Lake Piru________________ 14

Reservoir use and operation___________________________________ 14

Method of survey__ ___________________________________________ 16

Sediment sampling___________________________________________ 16

Sediment properties__________________________________________ 17

Computation of sediment volume________________________________ 21

Measurement of suspended sediment transported by floods______________ 22

Formulas developed for estimating sediment yield in southern California. 24 

Estimates of sediment yield by correlation with and regression analysis

of data in adjacent basins._______________________________________ 26

Comparison of sedimentation rates_____________________________ 26

Detection of changes in watershed conditions _________________ 26

Length of reservoir-sedimentation records____________________ 27

Correction of sediment yields for basin size.__________________ 28

Correction for trap efficiency..______________________________ 29

Correlation of sediment yield with basin size._____________________ 30

Correlation of sediment yield with physiographic parameters. _______ 31

Sediment-area factor.______________________________________ 31

Sediment-movement factor.________________________________ 31

Total stream length______________________________________ 31

Transport-efficiency factors.________________________________ 34

Additional factors affecting sediment yield._______________________ 37

Climatic factors.___________________________-_---_-__-----_ 37

Soil-erodibility factors________________________-__--__-_--- 39

Other factors affecting sediment yield________________________ 42

Probable accuracy of sediment-yield predictions in the Piru Creek watershed ___________________________________________________________ 44

Conclusions and probable long-term sediment yield at the Pyramid damsite. ___________________________________________________________ 46

References_ _ ____________________________________________________ 47

m
IV CONTENTS

ILLUSTRATIONS

Page 

FIGURE 1. Index map of the Piru Creek watershed.______________ E4

2. View of Piru Creek gorge at the Pyramid damsite looking

north_ __________________________________________ 5

3-5. Graphs:

3. Mean monthly temperatures at the Sandberg

weather station ____________________________ 6

4. Mean monthly precipitation at the Sandberg

weather station--___-__-_--_-_-_---..---_--_ 7

5. Annual precipitation at the Sandberg weather

station from 1933 to 1964___________________ 8

6. Map showing major rock types in the drainage area of

Santa Felicia Reservoir_________ ___________________ 9

7-9. View of 

7. Piru Creek looking upstream from a point 4 miles

upstream from Santa Felicia Dam___________ 11

8. Lockwood Valley, a broad alluviated area in the

western part of the Piru Creek basin.________ 12

9. Lake Piru looking from the east shore upstream

from Santa Felicia Dam____________________ 14

10. Map of locations of sediment ranges on Lake Piru_____- 15

11-24. Graphs:

11. Specific weight and particle size of sediment

deposited in main stem of Lake Piru________ 18

12. Mean particle size, sorting, and skewness of

sediment of Lake Piru_____________________ 20

13. Double-mass plot of peak discharge against 

precipitation effectiveness for the Matilija 

Creek drainage basin______________________ 27

14. Double-mass plot of peak discharge against 

precipitation effectiveness for the Santa Anita 

Creek drainage basin______________________ 28

15. Double-mass plot of peak discharge against 

precipitation effectiveness for the Piru Creek 

drainage basin____________________________ 29

16. Relation of sediment yield to basin area for

basins listed in text-______________-_-_--_- 30

17. Relation of sediment yield to the sediment-area

factor for basins listed in text_____________ 34

18. Relation of sediment yield to the sedimentmovement factor for basins listed in text_____ 35

19. Relation of sediment yield to the total stream

length for basins listed in text-_____________ 36

20. Relation of sediment yield to a transportefficiency factor (Ti) for basins listed in text__ 37

21. Relation of sediment yield to a transportefficiency factor (TV) for basins listed in text__ 38

22. Relation of sediment yield to a transportefficiency factor (Ts) for basins listed in text__ 39
CONTENTS V

FIGURES 11-24. Graphs Continued Page

23. Relation of basin size to total number of streams

for basins listed in text.___________________ E40

24. Relation of basin size to total stream length for

basins listed in text-______________________ 40

TABLES

TABLE 1. Proportions of lithologic types exposed in the Piru Creek

watershed-________________-________--___--__---- E10

2. Relation of lithology to first-order stream gradient in the

Piru Creek watershed.____________________________ 11

3. Specific gravity and specific weight of sediment samples

from Lake Piru_________________________________ 17

4. Particle-size distribution of sediment samples from Lake

Piru____-_________________________--___--_------ 19

5. Comparison of the quantity of suspended sediment 

transported past the damsite with that supplied to the 

head of Lake Piru for a storm in November 1965__--_ 23

6. Calculations of yearly sedimentation in Lake Piru_____- 25

7. Calculated annual sedimentation rates at Lake Piru___ 25

8. Data on reservoirs and sediment yield of watersheds in

southern California.______________________________ 28

9. Morphometric data and physiographic parameters of the 

Piru Creek drainage basin and watersheds near Los 

Angeles_ ________________________________________ 32

10. Sediment yield for Piru Creek basin upstream from 

Pyramid Rock calculated from physiographic parameters ____________________________________________ 37

11. Dispersion and surface-aggregation ratios as related to

lithologic type______________________________----_- 41

12. Summary of watershed parameters._________________ 43

13. Sediment yields calculated for Piru Creek basin upstream

from Pyramid Rock______________________________ 45
VI SYMBOLS

SYMBOLS

A Drainage area.

A' Area of reservoir quadrilateral.

A" Area of lake segment.

Ac h Area of main channel of a watershed.

Av Planimetric area of a watershed.

aP Precipitation during 21 days antecedent to day of maximum pre- 

	cipitation.

C Cover density.

D Diameter.

DR Dispersion ratio.

E Cross-sectional area of sediment at a range.

EP Effective precipitation.

Fire Percentage of watershed burned by fire.

7 Probable maximum 24-hour precipitation.

L Stream length.

M Mean diameter.

MAQ Mean annual runoff per unit area.

MAP Mean annual precipitation.

Ni,N2 Number of streams of order designated by subscript.

P Maximum daily precipitation of storm.

Pi,P2 Physiographic parameters.

q Maximum yearly peak discharge.

r Linear correlation coefficient.

Rb Stream bifurcation ratio.

Rc Ratio of stream-channel slopes.

RD Relative discharge.

RE Relative erodibility.

RRA Relative rain area modified for California latitudes.

SA Sediment-area factor.

SA Surface-aggregation ratio.

S/C Suspended silt and clay divided by colloids.

SM Sediment-movement factor.

Sy Sediment yield.

Sy',Sv" Sediment yield per unit area. 

TI, T2 , T3 Transport-efficiency factors.

TMAQ Total mean annual runoff for a basin.

V Volume.

W Width of sediment range.

a Skewness.

& g Ground-slope angle.

o> Sorting.

4> Iog2 D.
SEDIMENTATION IN SMALL BASINS

SEDIMENTATION IN THE PIRU GREEK WATERSHED 

SOUTHERN CALIFORNIA

By KEVIN M. SCOTT, JOHN R. RITTER, and JAMES M. KNOTT

ABSTRACT

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
119°30' 119°00' 

34-30'

_______ KERN COUNTY

LOS ANGELES COUNTY 

Area of

FIGURE 1. Index map of the Piru Creek watershed and the drainage areas of all 
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E5 

LOCATION AND PHYSICAL FEATURES

Piru Creek drains south from the center of the Transverse Ranges, 

a group of mountains which trend east-west and include the San 

Gabriel Mountains; the Transverse Ranges are athwart the general 

northwest trend of the Coast Ranges. Piru Creek joins the Santa 

Clara River near Piru, about 30 miles northwest of Los Angeles 

(fig. 1). The drainage basin is mainly in Ventura County but is partly 

in Los Angeles and Kern Counties. The headwaters of Piru Creek 

are in the southwest corner of the upper basin, from which the stream 

descends rapidly and meanders across an area of gentle relief; in 

general the flow is parallel to structural trends. The upper basin is a 

broad expanse of terrain ranging in altitude from about 3,000 to 6,000 

feet above mean sea level and is bounded on the south by part of 

the Topatopa Mountains and on the north by Mount Pinos (alt 8,831 

ft), the highest point in the basin. About the same proportion of 

slopes face north as face south in the area above the prdposed damsite (fig. 2), near Pyramid Rock (alt 2,250 ft).

FIGURE 2. View of Piru Creek gorge at the Pyramid damsite looking north. The highway will be rerouted 

around the reservoir site. The phyllitic shale cropping out at this locality is fairly resistant.
E6 SEDIMENTATION IN SMALL BASINS

Santa Felicia Dam, about 5 miles upstream from the confluence of 

Piru Creek and the Santa Clara River, impounds runoff from 425 

square miles of the basin. The proposed reservoir at Pyramid Rock will 

act as catchment area for 284 square miles of the basin. The physiography of the area above Pyramid Rock is distinctly different from that 

below. The general flow of streams in the upstream area is to the east, 

as opposed to the southward flowing streams in the downstream area, 

and the topography is more subdued. Recent tectonic activity has 

created the present southward drainage of the upper basin, which 

formerly drained northeastward into the Mojave Desert.

The part of the basin between Santa Felicia Reservoir and the 

Pyramid damsite is distinctly more mountainous and contains fewer 

valley flats than the area upstream. The area in the immediate vicinity 

of Santa Felicia Dam is marked by steep some precipitous slopes 

formed on rather easily credible rocks. As a result, the area near the 

dam probably contributes substantially more sediment per unit area 

than more remote areas. There is no way, unfortunately, to measure 

quantitatively the relative contributions of areas according to their 

position within a watershed.

CLIMATE

Piru Creek basin has a Mediterranean-type climate. Summers are 

dry and warm, and the winters are wet and cool. The mean annual 

temperature at Sandberg (fig. 1) is 55.5°F, and the mean monthly 

temperature ranges from 39.9°F in January to 74.4°F in July (fig. 3). 

The Sandberg weather station is in the northeastern part of the basin

< 70

Mean annual temperature

Apr. Juiy

FIGUEE 3. Mean monthly temperatures at the Sandberg weather station.
PIRU CREEK WATERSHED, SOUTHE.RN CALIFORNIA E7

at an altitude of 4,517 feet, or about 400 feet higher than the mean 

altitude of the basin.

Mean annual precipitation ranges from about 12 inches near 

Sandberg to about 24 inches in the southwestern part of the basin. 

Mean annual rainfall for the entire basin is 17.1 inches and is 15.9 

inches for the basin upstream from Pyramid Rock. Mean monthly 

precipitation at Sandberg ranges from about 0.02 inch in July to 

2.58 inches in February (fig. 4).

Mar. Apr. May June July Sept.

FIGURE 4. Mean monthly precipitation at the Sandberg weather station.

The only long-term precipitation record in the Piru watershed is 

that for Sandberg. However, 11 precipitation stations are distributed 

around the drainage area within 10 miles of the drainage divide. 

Consequently, the area is included in long-term, extended-record 

compilations and isohyetal maps. The record at Sandberg emphasizes 

the climatic variation over a long period of time (fig. 5). The mean 

annual precipitation at Sandberg is 12.1 inches, but during the 10 

years since Santa Felicia Reservoir was constructed and sediment 

began accumulating, precipitation has averaged 11.0 inches. During 

the first 10 years of record (1933-42), precipitation averaged 14.7 

inches, nearly double the low 10-year average (1947-56) of 7.5 inches.
E8 SEDIMENTATION IN SMALL BASINS

FIGURE 5. Annual precipitation at the Sandberg weather station from 1933 to 1964. 

LITHOLOGY AND STRUCTURE

The Transverse flanges are a complex structural and stratigraphic 

province. Piru Creek basin includes part of the eastern end of the 

Ventura stratigraphic basin, the site of dominantly marine deposition 

throughout most of the Tertiary Period. The part of the Piru watershed 

between the Pyramid damsite and Santa Felicia Dam is formed in this 

thick sedimentary sequence (fig. 6) composed of moderately well 

consolidated sandstone and conglomerate with interbedded shale. 

Basement rock for the sedimentary series is an assemblage of preCretaceous igneous and metamorphic rocks which underlies much 

of the high country in the upper part of the basin.

The area is bisected by the northwest-trending San Gabriel fault, 

a major rift with a large well-defined lateral separation. The northeastern part of the basin above Pyramid Rock, between the San 

Gabriel fault and the San Andreas rift (the major structural lineament of California), is in the Ridge stratigraphic basin. The Ridge 

stratigraphic basin is the site of one of the thickest recorded nonmarine sedimentary sections of Miocene and Pliocene age. The 

generally soft, readily credible sequence is exposed in the northeastern 

part of Piru Creek basin which receives the quantity of precipitation 

near the optimum for erosion (Langbein and Schumm, 1958, p. 1077), 

and consequently, badland topography has resulted locally. A comparison of relative sedimentation rates in California and the extent 

to which they reflect rock type suggests that a much higher than 

average sediment yield per unit area is attributable to this terrain.
EXPLANATION

Alluvium

Continental sedimentary rocks

Continental sedimentary rocks 

Includes some rocks of Oligocene age

Marine sedimentary rocks

M 

Marine sedimentary rocks

gm Granitic plutonic rocks 

Metamorphic complex

SANTA FELICIA DAM

FIGURE 6. Major rock types in the drainage area of Santa Felicia Reservoir. .. indicates drainage divide. Geology from Crowell (1952), Kundert (1955),

and Carman (1964).

CO
E10 SEDIMENTATION IN SMALL BASINS

The diverse types of rocks in the Piru Creek basin (fig. 6, table 1) 

must be considered in determining sediment supply at any particular 

point. Determinations are further complicated by the intensity of 

tectonic activity, which must have radically influenced landform 

evolution within the basin in comparatively recent geologic time. 

The Pleistocene orogeny of the Coast Ranges is the latest in several 

mountain-building episodes that are sufficiently recent to be reflected 

in the present topography.

Piru Creek basin may approximate the ultimate in tectonic effects 

on sedimentation rates. The drainage basin is immediately south of 

the juncture of the San Andreas, San Gabriel, Big Pine, and Frazier 

Mountain faults, each of which transects the area and has large 

demonstrable displacement. The recently active San Cayetano thrust 

fault underlies part of the lower Piru Creek basin. A dip-slip component of displacement of 3,500 feet (Shepard, 1960) on the Bayley 

fault, a major branch of the San Gabriel fault, has occurred within 

the basin since middle Pleistocene time. About 3.5 miles of horizontal 

displacement is apparent along the same shear zone of the Bayley 

fault, which forms the channel of Piru Creek for 3 miles upstream 

from Pyramid Rock.

TABLE 1. Proportions of lithologic types exposed in the Piru Creek watershed

Percentage of area

Lithologic types Upstream from Upstream from

Santa Felicia Pyramid

Reservoir damsite

Continental sedimentary rocks of Miocene age (includes some rocks of

7.3

20.1

13.5

9.9

13.4

20.5

15.3

9.9

22.8

.5

13.3

12.1

19.9

21.5

Tectonism probably will not change the sediment yield throughout 

the lifetime of any reservoir as much as will climatic and land-use 

factors. Tectonic events have brought about the present distribution 

of rock types, and differential erosion has produced the present 

topography. However, differential movement in the recent past 

between drainage basins and parts of basins may influence sediment 

yields. Differences in maturity of landscapes between and within 

basins introduces uncertainty in correlations of rates of sediment 

yield; this interaction of the effects of lithology and of erosion stage 

thus provides additional uncertainty in the assessment of sediment 

yields. The relation between lithology and a slope parameter, the
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA Ell

first-order stream gradient, calculated from U.S. Geological Survey 

topographic maps at a scale of 1:24,000 illustrates the dependence of 

physiography on rock type (table 2).

TABLE 2. Relation of lithology to first-order stream gradient in the Piru Creek

watershed

First-order 

stream gradient

Lithology 

Alluvium of Quaternary age___________________________________

Continental sedimentary rocks of Pliocene age____________________

Continental sedimentary rocks of Miocene age (includes some rocks 

of Oligocene age)____________________________________________

Marine sedimentary rocks of Miocene age________________________

Marine sedimentary rocks of Eocene age_________________________

Granitic plutonic rocks_______________________________________

Metamorphic complex__ _ _____________________________________

(ftperff) 

0.083 

. 130

.118

.246

.213

.305

.263

One unusual feature of the area is the alternation of highly credible 

and highly resistant rocks, particularly along the course of Piru 

Creek. This variation in rock type results in broad alluviated subbasins 

alternating with gorges incised in bedrock. In addition to the primary control of stream gradients by regional tilting of bedrock and 

by contrasts in erodibility of different rock types, many resistant 

layers in the sedimentary sequences form local base levels and slope 

inflections and create local segments of alluviation numerous valley 

flats (fig. 7) alternate with bedrock channels.

FKHJKE 7. View of Piru Creek looking upstream from a point 4 miles upstream from Santa Felicia Dam. 

A typical valley flat is visible in the foreground. Peak at left is Blue Point.

309-588 O 68 3,
E12

The headwaters of Piru Creek are in readily credible sediments. 

The stream passes abruptly into highly resistant gneiss, then into 

extremely credible rocks of Pliocene age in the eastern part of the upper 

basin, and then again into the gneiss for a short distance. The anomalous course of Piru Creek, which flows southeast across many structural trends and against the predominant northwest dip of the rocks, 

was first noticed by Hershey (1902).

Alluviation of much of the upper basin seems to be abnormal (fig. 

8) compared with alluviation of surrounding areas and thus suggests 

a previous period of aggradation. Active channel erosion and local 

gullying suggest that the period of alluviation may be ending, possibly 

in response to grazing. A large potential source of sediment exists upstream from Pyramid damsite, and a substantial increase in sediment 

yield could be triggered by only moderate changes in climate or land 

use.

FIGURE 8. Lockwood Valley, a broad alluviated area in the western part of the Piru Creek basin. Frazier 

Mountain forms the skyline. Snow remaining more than a few days is rare except on the highest elevations.

Several erosion surfaces, some representing stripped nonconformities at the base of thick sediment accumulations of Pleistocene age, 

are conspicuous in the upper basin at altitudes of approximately 

4,000 and 7,000 feet above mean sea level and are locally affected by 

faulting (Crowell, 1952). At least two terraces, one 30 to 50 feet and 

one 15 to 20 feet above the bottom of the stream, occur along lower 

reaches of Piru Creek. The lower course of the stream has aggraded
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E13

appreciably, 90 feet of sediment having been reported near Blue 

Point (Shepard, 1960, fig. 7) and 80 feet at the Santa Eelicia damsite 

(Cordova, 1956).

Landslides are a significant mechanism of sediment movement in 

the area. Ten unpublished theses on the geology of approximately 

one-third of the total basin record many mappable landslides and 

additional landslide deposits. The slides are most common on slopes 

formed in sedimentary rocks adjacent to fault lines and are sedimentsupply mechanisms with probable strong periodicity related to climatic 

variations. Additional fault zone or "rift" topography in the area 

includes offset alluvial remnants and terraces, sagponds, and faceted 

spurs.

SOILS AND VEGETATION

Most of the area is covered by shallow, irregular stony soils formed 

in place by residual weathering. Typical of such soils is the Altamont 

Loam in the southernmost part of the basin, the only part covered 

by a soil survey (Nelson and others, 1920). The Altamont Loam is 

8 to 15 inches deep, contains shale fragments and caliche concentrations, and occurs locally on upland areas. Soils along stream 

channels are young, sporadically located alluvial soils, such as the 

Yolo Sandy Loam, 12 to 30 inches in thickness, and vary to lighter 

textured varieties.

Chaparral is the commonest form of vegetative cover, particularly 

on slopes underlain by sandstone and conglomerate. Seasonal grasses 

dominate in soil formed on finer grained sedimentary rocks and 

alluvium. Groves of oak, cottonwood, and willow occur in stream 

channels and near springs. Oak, transitional to pine, and some fir 

and juniper occur at altitudes above 5,000 feet. The pattern is modified 

by topography; higher altitude forms of vegetation occur on northern 

slopes and in canyon bottoms as a result of shading and the downhill 

flow of cold air.

LAND USE AND FIRE HISTORY

Most of the area is culturally undeveloped. The main human factor 

affecting sediment yield is road construction. U.S. Highway 99, the 

new ridge route, was built in the late 1930's but has been widened and 

straightened subsequently. The necessary rerouting of the highway 

around the new reservoir will be the most immediate cultural factor 

increasing sediment yield at the proposed damsite.

Some cattle ranching is practiced in the upper basin along with 

some dry farming on lowland flats.

In southern California, brush-covered watersheds with seasonal rain 

are susceptible to fires, some of which cause high but temporary sediment yields during the following years. This is particularly true of the
E14 SEDIMENTATION IN SMALL BASINS

basins used for sediment-yield correlation with the Piru Creek basin; 

however, less than 10 percent of the Piru Creek watershed has been 

burned since 1940 (less than 0.4-percent burn per yr).

DIRECT MEASUREMENT OF SEDIMENT DEPOSITED IN

LAKE PIRU

A reservoir survey of Lake Piru (Santa Felicia Reservoir), which 

stores runoff from nearly the entire Piru Creek basin, was made in 

October and November 1965 to measure the sediment stored in the 

reservoir. Lake Piru was formed in 1955 by the construction of Santa 

Felicia Dam by the United Water Conservation District and is 4.4 

miles northeast of Piru in Ventura County (fig. 1). The dam is an 

earthfill structure 85 feet high and 1,275 feet long and forms a reservoir 

in a narrow gorge that extends about 4.3 miles upstream (figs. 9, 10).

FIGURE 9. View of Lake Piru looking from the east shore upstream from Santa Felicia Dam. 

RESERVOIR USE AND OPERATION

Water released from Lake Piru is used for ground-water recharge 

and irrigation on the Oxnard Plain. Water-discharge records for Piru 

Creek downstream from Santa Felicia Dam indicate an average 

annual outflow of 24,900 acre-feet for the period October 1955- 

October 1965. Complete records of streamflow into and out of the 

reservoir are published by the U.S. Geological Survey in the watersupply paper series, "Surface Water Supply of the United States 

Part 11, Pacific Slope Basins in California," and the annual series,
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E15

4000 FEET

SANTA FELICIA DAM

FIGUKE 10. Locations of sediment ranges on Lake Piru.

"Water Resources Data for California Part 1, Surface Water Records of California, Volume 1, Colorado River basin, Southern Great 

Basin, and Pacific Slope basins excluding Central Valley."
E16 SEDIMENTATION IN SMALL BASINS

Operations at Lake Piru, like those at many reservoirs in southern 

California, are oriented toward water conservation. Water that is not 

used during wet years is held for the inevitable dry years. The storage 

of water in Lake Piru has ranged from zero for a few weeks in 1961 

to 78,500 acre-feet in 1958. The reservoir has never filled to maximum 

capacity, 101,225 acre-feet.

Water releases for ground-water recharge are generally uniform and 

average about 10 to 20 acre-feet per day throughout much of the year. 

Water required for irrigation is generally released during March and 

April and sometimes exceeds 200 acre-feet per day. The quantity of 

water released for irrigation is variable, depending on the availability 

and the demand, and is less than 10 percent of the total outflow. 

This mode of operation of the reservoir minimizes seasonal fluctuations of reservoir levels and redistribution of sediment within the 

reservoir.

METHOD OF SURVEY

A series of sediment ranges (fig. 10) was established by the United 

Water Conservation District before completion of Santa Felicia Dam, 

and a resurvey of these established ranges was selected as the best 

method of measuring sedimentation in Lake Piru. The amount of 

sediment was calculated by the prismoidal method (Eakin and Brown, 

1939, p. 158-159) from the difference between the 1955 and 1965 cross 

sections. The elevation of the sediment surface was determined by 

transit-stadia traverse where the lakebed was exposed and by leadline sounding where the lakebed was submerged.

A reconnaissance of the reservoir when the lake was low indicated 

that much sediment was deposited as a delta between ranges 4 and 

5 (fig. 10). Range 4 is established along a bedrock ridge extending 

about a third of the way across the reservoir from the right bank. 

All sediment originally deposited along the ridge has been removed 

be subaerial erosion. The resistant bedrock layer forming the constriction impounds a moderate amount of sediment that is unmeasured 

along range 4. Therefore, a new cross section, 4A, was established 

upstream from the constriction formed by the ridge. The original 

bottom of the reservoir along range 4A was determined by core holes, 

and the measured depth of sediment was used in calculating the 

volume of the deposited sediment.

SEDIMENT SAMPLING

Samples of lakebed sediment were collected at several points along 

each survey range. Veihmeyer tubes were used at ranges where the 

lakebed was exposed to obtain samples which represent an accurate 

vertical distribution of the sediment layer. A split-core sampler
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E17

suspended by standard streamflow-measuring equipment was used at 

ranges where the lakebed was submerged. The split-core sampler 

penetrated the sediment layer by the force of its own weight, the 

depth of penetration being restricted by various properties of the 

deposited sediment such as cohesiveness, grain size, and compaction.

SEDIMENT PROPERTIES

Specific weight was determined for most of the sediment samples 

(table 3). Samples were reconstructed in the laboratory to obtain 

values of specific weight that would represent conditions of initial 

deposition. Water was added to the samples, which were stirred and 

allowed to settle. The reconstructed specific weight was computed by 

dividing the dry weight by the volume of the settled sediment.

Specific weight of the sediment deposited in Lake Piru ranges from 

33 to 96 pounds per cubic foot (fig. 11) and generally increases with 

distance upstream from the dam. Pronounced variation from range to 

range may be due to such factors as compaction, grain size of sediment, 

turbidity currents, erratic inflow characteristics, and reservoir shape. 

Mean specific weight of lake-bottom sediment is 52 pounds per cubic 

foot. The value was computed by adjusting the average specific 

weight of each reservoir segment to the volume of sediment deposited 

therein. Specific gravity of the sediment samples, determined by the 

flask method (U.S. Bureau of .Reclamation, 1960, p. 443), ranged 

from 2.63 to 2.70 (table 3).

TABLE 3. Specific gravity and specific weight of sediment samples from Lake Piru

Range

I........ .........

2..................

3..................

4A_. ..............

Specific 

gravity

2.66

2.70

2.70

2.68

Specific 

weight (Ib per 

cuft)

43.0

36.4

33.0

74.6

Range

5. ............ ....

6. ............ ....

11. 

12-. 

Specific 

gravity

2.66

2.63

2.65

2.66

Specific 

weight (Ib per 

cuft)

96.5

88.9

66.0

96.0

Particle-size distribution was determined for all samples of lake 

sediment (table 4). Samples in the sand and gravel range (>0.062 

mm, Wentworth scale) were analyzed by sieving, and samples in the 

silt and clay range «0.062 mm) were analyzed by the pipet method. 

Particle size of lake sediment from range 3 downstream to the dam 

is quite uniform, and the sand content is small (fig. 11). Lack of sand 

in this area is due to the settling of coarse-grained material in the 

upper reaches of the lake. Most of the sediment in the lower reaches 

of the reservoir is transported to the site of deposition by turbidity 

flows, a mechanism only rarely capable of transporting sand. The 

median grain size in this area ranges from 0.0024 to 0.0091 mm.
E18 SEDIMENTATION IN SMALL BASINS

'Sand (6.0625-2.00 mm)-'; : '

0246 8 10 12 14 16 18 

DISTANCE UPSTREAM FROM SANTA FELICIA DAM, IN THOUSANDS OF FEET

FIGTJEE 11, Specific weight and particle size of sediment deposited in main stem of Lake Piru.

Sediment deposited in the reservoir above range 3 increases in coarse-

-r\ f\c*c* firl4-\\ /-\lc*4-c\~r\f*f\ -i-t-rm4-nf\c\'m -pT»y\TV* 4~\-\ £1 f\ aYYl \/l oH 1Q Tl O"T£I 1TI Ql 7.P. 1T1 

L1XOI1 U V^IOL/V^OJ-UOVA 111 UilW JL t/Ot/JL V \JM-L <JUW V *J JL U/J.J.g^l/ l-/ J.AJ.V/J. v/M/kJv»-- J.AJ- ^v^v*.

ness with distance upstream from the dam. Median grain size mcreases from 0.0024 mm at range 3 to 0.88 mm at range 6. Most of the
TABLE 4. Particle-size distribution of sediment samples from 

[All samples analyzed by sieve, by pipet, in distilled water, and by chemic

Rang*

1 . __ . .

2

3 -

4A. ....... .....................

5

6 .. .. .

11 

12 .... ..- . --------

0.002

... . - 26

26

 . ... .. 12

1

... ....... ... ... 9

Percent finer than indicated siz

0.004

31 

33

59 

14

1 

10

0.008

47 

48 

68 

17

1 

12

0.016

64 

64 

87 

26 

1 

2 

18 

1

0.031

88 

82 

98 

32 

2 

3 

27 

1

0.062

96 

98 

100 

60 

4 

4 

38 

2

0.125

100 

100

88 

11 

6 

58 

8

0
E20 SEDIMENTATION IN SMALL BASINS

sediment is in the main stem of the reservoir; that deposited in tributary arms is only about 2.2 percent of the total volume.

The mean particle size, sorting, and skewness of the samples at 

each range are shown in figure 12. Parameters were calculated by the 

methods of Inman (1952, p. 130) using phi units where <£= logzD, 

D being the particle diameter in millimeters (Krumbein, 1936, p. 37). 

In general, the sediment becomes finer grained, more poorly sorted, 

and more positively skewed from the head of the reservoir toward 

the dam. Most of the sand entering the reservoir is deposited in the

5 10 15 

DISTANCE UPSTREAM FROM SANTA FELICIA DAM, IN THOUSANDS OF FEET

FIGURE 12. Mean particle size, sorting, and skewness of sediment in Lake Piru.
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E21

delta and is bypassed by the silt and clay fractions, which are transported and deposited farther down the reservoir.

COMPUTATION OF SEDIMENT VOLUME

The 1965 reservoir survey indicated that 2,470 acre-feet of sediment 

(an average sedimentation rate of 247 acre-ft per yr or 0.58 acre-ft 

per sq mi per yr) had accumulated in Lake Piru since 1955. The 

sediment volume was computed from the prismoidal formula (Eakin 

and Brown, 1939, p. 158-159)

+ft)

where V=Volume of sediment, in acre-feet,

A'=Area, in acres, of the quadrilateral formed by connecting 

the points of range intersection with maximum lake level, 

A"=Area of lake segment, in acres, 

E Cross-sectional area of sediment at the range, in square

feet, 

and W= Width of range at maximum lake level, in feet.

The volume of sediment deposited in reservoirs is usually representative of the total sediment discharge to the reservoir. A small percentage of sediment may pass through the reservoir in suspension. The 

percentage of the total sediment inflow retained in the reservoir is 

known as the trap efficiency and, in general, is affected by such factors 

as slope of the reservoir, ratio of inflow to capacity, sediment properties, water salinity, turbidity currents, and reservoir operation. 

The principal factors affecting trap efficiency of Lake Piru have been 

the mode of operation and the inflow to the reservoir. The water 

level has never reached the spillway elevation and, consequently, the 

only way sediment could leave the lake is via gate valves in the dam. 

Normal procedure has been to release only minimum flows during and 

immediately after storm periods. The trap efficiency of Lake Piru 

probably is nearly 100 percent, and the sediment volume measured 

by the 1965 reservoir survey represents nearly the total sediment 

inflow.

Any reduction in trap efficiency that may result from sediment 

passed through Santa Felicia Dam is probably compensated for by the 

addition of sediment derived from within the reservoir. Throughout 

the history of Lake Piru, lateral slopes and tributary stream channels 

in the upper part of the reservoir have actively been degraded. The 

sediment removed from these areas is deposited with sediment eroded 

from the watershed and is included in the total measured sediment
E22 SEGMENTATION IN SMALL BASINS

volume. However, the quantity of sediment added in this manner is 

probably small in relation to the total inflow.

MEASUREMENT OF SUSPENDED SEDIMENT 

TRANSPORTED BY FLOODS

The quantity of suspended sediment transported by Piru Creek 

was measured at two stream-gaging stations one immediately 

downstream from the proposed Pyramid damsite and the other 

upstream from Lake Pirn (fig. 1). Samples of the water-sediment 

mixture were obtained at low flow with hand-operated depth-integrating samplers. Higher flows were sampled at the gaging station on 

Pirn Creek belowT the Pyramid damsite by an automatic sampler 

which obtained samples at a series of fixed elevations on the rising 

stage of each runoff event. Data were obtained from automatic 

samplers installed at two additional gages Piru Creek below Buck 

Creek and Canada de Los Alamos below Apple Canyon (fig. 1) 

but lack of water-discharge data for high flows prevented calculation 

of the quantity of suspended sediment transported at those localities.

The concentration of suspended sediment in each of the flow samples 

was determined so that the relation between water discharge and 

sediment concentration could be established. Sediment concentration ranged from 2 to 268,000 parts per million. Such extreme variability is typical of semiarid and arid regions with flash floods and 

ephemeral runoff. Particle size, specific gravity, and specific weight 

were determined for selected samples.

The installation of the automatic samplers in January 1965 preceded one of the most intense storm periods in the history of southern 

California, November and December 1965. The Sandberg weather 

station received 9.80 inches of precipitation in November, a month 

in which the mean precipitation is 0.92 inch. An additional 3.90 inches 

fell in December, which previously had averaged 2.49 inches. The rain 

gage at Lake Piru collected 15.76 inches in November and 6.08 inches 

in December months in which an average of only 1.67 and 1.87 inches 

of precipitation had fallen previously. The November rainfall was the 

second greatest monthly precipitation in the 33-year record at Sandberg and the greatest recorded monthly total in 14 years of data 

collection at the Lake Piru station. The runoff during calendar year 

1965 can be considered as being in the uppermost range of reasonably 

expected flows to pass the damsite and to have transported a nearmaximum sediment load under present watershed conditions. The 

short period of flow measurement at the Pyramid damsite prevents 

a more specific discussion of the frequency of the 1965 floods.

The total amount of suspended sediment to pass the damsite during 

this wet year was 570,000 tons. This is the sum of all daily sediment
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E23

discharges during calendar year 1965, but virtually the entire amount 

was transported by the flows of November and December. Sediment 

loads for periods during which suspended-sediment concentrations or 

water discharge changed rapidly were obtained by subdividing each 

day and computing a mean concentration and load for each day. The 

quantity of suspended sediment that was transported during unsampled periods was estimated by comparing recorded water discharges with similar flows which were sampled. Conversion of the 

amount of 570,000 tons to volume, using a specific weight of 52 pounds 

per cubic foot based on sediment deposited in Lake Piru, gives a sediment yield at the damsite of 504 acre-feet, or 1.77 acre-feet per square 

mile of drainage area. These values represent only suspended sediment 

to which, for strict comparison with the other estimates of total 

sediment yield presented here, must be added values for bedload and 

an inestimable, but relatively small, quantity of unmeasured load. 

Because the sediment-yield rate determined from the 1965 reservoir 

survey of Santa Felicia Reservoir is used in the following sections as a 

basis for comparing the validity of yields derived by other techniques, 

a comparison of the rate of suspended-sediment movement past the 

damsite with that into the reservoir is significant. Collection of samples 

upstream from the head of the reservoir allows us to make such a 

comparison for one of the November 1965 flood peaks. The results 

(table 5) indicate a considerably greater sediment yield per unit area 

at Piru Creek upstream from Lake Piru (494 tons per sq mi) than at 

the Pyramid damsite (374 tons per sq mi). The higher yield at the 

downstream station is attributable to a much higher rainfall in November at Lake Piru (15.76 in.) than at Sandberg (9.80 in) and to a 

greater proportion of erodible rocks in the drainage area downstream 

from the damsite.

TABLE 5. Comparison of the quantity of suspended sediment transported past 

the damsite with that supplied to the head of Lake Piru for a storm in November 1965 

[Results are estimated, except where value is preceded by *]

Piru Creek above Frenchmans Flat 

(Pyramid damsite)

Piru Creek above Lake Piru 

(Santa Felicia Reservoir)

Date

Nov. 15......

16

17..-...--.-.

18 -.. .

19 .. -.

20....- .....

21.. . .

Total..-..-.

Mean 

discharge 

(cfs)

44

798

1,120

325

250

114

56

2,707 .

Suspended sediment

Concentration 

(ppm)

300 

14,100 

11,000 

840 

580 

210 

160

Tons Tons d 

per per 

day sq mi

170 .. _

*57,600 - ....

47,000 .... ..

7on

430 ... ..

*65 ....- 

24 ...

106, 079 374

Mean 

.ischarge 

(cfs)

64 

862 

2,170 

837 

406 

188 

109

4,636 .

Suspended sediment

Concentration 

(ppm)

1,680 

20,100 

14,000 

4,000 

2,000 

1,100 

760

Tons Tons 

per per 

day sq mi

*969 -., .

*73,700 _-.---_

*96,400 ....... ...

9,700 ..........

2,300 

560 -.._.-.--.

*221 .-_.-..--.

183,850 M94

1 Based on a drainage area of 372 sq mi.
E24 SEDIMENTATION IN SMALL BASINS

FORMULAS DEVELOPED FOR ESTIMATING SEDIMENT 

YIELD IN SOUTHERN CALIFORNIA

Two formulas that have been derived to estimate sediment yields 

in southern California were applied to the Piru Creek watershed 

above Santa Felicia Dam. The first of these, based on surveys of 

sediment accumulation in Gibraltar Reservoir (U.S. Department of 

Agriculture, 1953, app. 2, table 3) is

log $,,=log +log ^j+0.689

+ 0.866 log q- 1.236 log (7+0.370 log Ach , (2) 

where

RE= Relative erodibility, antilog (1.678+0.0183 S/C), with

Gibraltar being taken as 100,

S/C= Suspended silt and clay divided by colloids, in percent, 

RD= Relative discharge, 0.370 power of ratio of annual flow to 

peak flow of year to 0.866 power (Gibraltar being taken 

as 100), 

q= Maximum yearly peak discharge, in cubic feet per second

per square mile,

(7= Cover density on the watershed, in percent, 

.4Cft=Area of main channel of the watershed, in acres per square

mile.

The U.S. Department of Agriculture (1953, app. 2, table 3) determined the relative erodibility, RE, of Piru Creek basin as 399; 

the average relative discharge, ED, as 105; and the area of main 

channel, Ach , as 4.9 acres per square mile. A value of 77.4 percent 

was used for the cover density, C. Substitution of these constants 

reduced equation 2 to

log $,,=0.866 log g- 0.769. (3)

Maximum yearly peak discharges from 1956 to 1965 were substituted for g, and the yearly sedimentation in Lake Piru was computed 

(table 6). The total sediment yield for the 10 years, 4,255 acre-feet, 

is considerably higher than the 2,470 acre-feet measured in the 1965 

survey of Lake Piru.

The second formula was derived by Anderson (1949, p. 579) from 

sedimentation records of many reservoirs in the San Gabriel and the 

San Bernardino Mountains. This formula, with the same symbols as 

equation 2, is

log $,, = 1.041 + 0.866 log g+0.370 log Ach -1.236 log C (4)
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E25

The values for Ach and C are the same as those used in equation 1, 

and the equation is reduced to

log £,, = 0.866 log 2-1.041. (5)

Table 6 shows that for the years 1956-65 the total sedimentation, 

calculated by use of equation 5, was 2,285 acre-feet.

TABLE 6. Calculations of yearly sedimentation in Lake Piru

Water year (Oct. 1-Sept. 30)

1956....--. ............................. ..

1957... ....... .......................... ..

1968......................................

1959...... .................................

I960... .................................

1961.......................................

1962.......................... .... .......

1963...-..-------.---....---..-....-...

1964.. .................................. ...

1965. ....... .........................

Total....... ........................

Peak 

discharge 

(cfs)

1,230

3,600

8,600

3,820

292

528

12,200

512

318

484

Equation (3)

B,> Sy

(acre-ft/per (acre-ft) 

sqmi)

0.480 

1.216 

2.682 

1.279 

.137 

.231 

3.499 

.225 

.148 

.214

204 

517 

1,097 

544 

58 

98 

1,487 

96 

63 

91

4 255

Equation (5)

Sy 

(acre-ft/per 

sqmi)

0.258 

.653 

1.387 

.687 

.074 

.124 

1.879 

.121 

.079 

.115

Sy 

(acre-ft)

109 

278 

589 

292 

31 

53 

799 

51 

34 

49

2,285

Andersen's formula (eq. 5) for annual sediment yeild in southern 

California provides a remarkably close approximation to the measured 

sedimentation rate in the resevoir survey. The formula was used, 

therefore, to calculate annual sediment yields from the area above 

Lake Piru from 1932 to 1965 (table 7). The measured sediment yield 

was 1.08 times that calculated by Anderson's formula, and all yields 

computed from the formula were adjusted by this factor to correspond 

with measured rates. Cover density (CT) and channel area (Acjl) were 

assumed to be constant throughout this period. The expansion of the

TABLE 7. Calculated annual sedimentation rates at Lake Piru

Year

1932

1933

1934

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

Peak 

discharge 

(cfs)

15,800

1,200

12,000

3,660

1,040

5,000

35,600

2,600

1,470

7,500

948

16,000

8,500

3,350

3,000

2,500

420

Ss, sedimentation rate 

(acre-ft per yr)

934

101

736

264

89

346

1,890

196

120

491

82

1,148

547

245

222

190

40

Cumulative 

acre-ft

934

1,035

1,771

2,035

2,124

2,470

4,360

4,556

4,676

5,167

5,249

6,397

6,944

7,189

7,411

7,601

7,641

Year

1949

1950

1951

1952

1953

1954

1955

1956

1957

1958

1959

1960

1961

1962

1963

1964

1965

Peak 

discharge 

(cfs)

165

472

30

7,010

618

1,150

330

1,230

3,600

8,600

3,820

292

528

12,200

512

318

484

S9 , sedimentation rate 

(acre-ft per 

yr)

18

45

1

464

56

96

32

118

301

637

316

34

57

864

55

37

53

Cumulative 

acre-ft

7,659

7,704

7,705

8,169

8,225

8,321

8,353

8,471

8,772

9,409

9,725

9,759

9,816

10,680

10, 735

10,772

10,825
E26 SEDIMENTATION IN SMALL BASINS

data to 1932, the time of first flow measurement, gave an approximation of sediment yield during the wet years of the 1930's and early 

1940's. The overall average annual sediment yield covers at least one 

wet period and one dry period. The average sediment yield for the 

area above Lake Piru for the 34-year period was 318 acre-feet per year. 

This yield will be compared with sediment yields of neighboring basins 

which have longer records of reservoir sedimentation.

ESTIMATES OF SEDIMENT YIELD BY CORRELATION

WITH AND REGRESSION ANALYSIS OF DATA IN

ADJACENT BASINS

COMPARISON OF SEDIMENTATION BATES 

DETECTION OF CHANGES IN WATERSHED CONDITIONS

In making long-term comparisons of sediment yields, the assumption is that all environmental factors within the different basins have 

remained approximately constant. Because of fires, road construction, 

or other changes in the watershed environment, the sediment yield 

may change. For example, Anderson (1955, p. 121) has shown that 

sediment yield in Gibraltar watershed (fig. 1) increased markedly 

following forest and brush fires and decreased as the watershed recovered from the fire damage. Thus, the history of the basins should 

be evaluated wherever possible. Basins in the same general area do 

not necessarily have similar sedimentation histories and may not be 

correlative in terms of sediment yield.

Anderson (1955, p. 125) suggests double-mass plotting of peak 

discharges against precipitation effectiveness as a method of detecting 

the effects of changes in watershed characteristics. Precipitation 

effectiveness is defined as

where P is the maximum daily precipitation of the storm producing 

the peak discharge, and aP is the precipitation occurring 21 days 

antecedent to the maximum day. Any major deflection of the doublemass curve indicates a change in characteristics.

Such analysis by double-mass plots can be made for only three of 

the studied basins for which sediment yields are compared Piru, 

Matilija, and Big Santa Anita (fig. 1) because there are nu records 

of peak discharges available for other basins (figs. 13, 14, 15). The 

Piru Creek watershed has been virtually untouched by fire, relative 

to some surrounding areas. The only fire of consequence occurred in 

1960 and burned less than 5 percent of the drainage area. Matilija 

and Big Santa Anita watersheds have been completely burned, 

Matilija in 1948 and Big Santa Anita in 1953. Figures 13 and 14 show
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E27

100

co 80

3=0

60

40

« 20

Gag! moved 2 miles 

upstream

1939,

1955

1963

100 200 300 400 

CUMULATIVE PRECIPITATION EFFECTIVENESS

500 600

FIGURE 13. Double-mass plot of peak discharge against precipitation effectiveness for the Matilija Creek

drainage basin.

that these burns are not detectable in the double-mass plots. In 

general, trends for the three watersheds are similar from 1947 on.

LENGTH OF RESERVOIR-SEDIMENTATION RECORDS

A summary of the reservoir-sedimentation data (table 8) indicates 

that sediment records for the six reservoirs in the San Gabriel Mountains (fig. 1) are of comparable length, 31 to 40 years. There have been 

marked fluctuations of precipitation in the region since 1932, and, 

consequently, the relatively short records at Santa Felicia and Matilija 

Reservoirs, 10 to 15 years, must be extended to a comparable length 

before comparisons of sediment yields can be made. The record at 

Matilija Reservoir has been extended back to 1927 by use of flow data 

(Boyle Engineering, written commun., 1964). The formula developed 

by Anderson (1949, p. 579) from watersheds in the San Gabriel 

Mountains also utilizes flow data and was used to extend the Piru 

Creek record to 1932. These extended sedimentation rates are used 

in the following correlations:
E28 SEDIMENTATION IN SMALL BASINS

TABLE 8. Data on reservoirs and sediment yield of watersheds in southern

California

Year of Year of Total Sediment Sediment

Reservoir dam com- last number of yield yield

pletion survey surveys (acre-ft (acre-ft

per yr) per sq mi)

Sawpit..... .-.--__---_-.__.____-_.

Big Dalton ____ __________

Big Santa Anita.. _ _ ._ _. .

SanDimas ____ . ___ ... _ . __ .

Pacoima ___ ...... _________ .

BigTujunga _ . _____ ______ . Matillja.. ........................... .

Santa Felicia (Lake Pim).: .-._-_..

......... 1927

 ..... 1929

...-.--.. 1927

.-..-... 1922

......... 1929

------ 1931

.... .. 1948

.. ... 1955

1962

...do.......

...do.... 

-.do.......

...do.. .

...do.......

1963

1965

9

6

16

9

8

11

2

1

8.4

5.7

43.2

21.7

52.6

113.2

35.6

152.0 

246.9

a 318. 0

2.52

1.27

4.00

1.34

1.87

1.38

.70

»1.03

.58

a.75

1 Calculated average from 1927 to 1963.

2 Calculated average from 1932 to 1965.

200 400 600 

CUMULATIVE PRECIPITATION EFFECTIVENESS

800 1000

FIGURE 14. Double-mass plot of peak discharge against precipitation effectiveness for the Santa Anita

Creek drainage basin.

CORRECTION OF SEDIMENT YIELDS FOR BASIN SIZE

Comparison of sediment yields from basins of different size is 

complicated by the fact that yield per unit area depends on the size 

of the basin. Several factors such as longer flow paths and consequent greater opportunity for intrabasin deposition, lower stream 

gradients, and the lesser probability of intense storms covering all 

the drainage hi larger areas combine to create this effect.
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E29

50 75 100 

CUMULATIVE PRECIPITATION EFFECTIVENESS

FIGTJBE 15. Double-mass plot of peak discharge against precipitation effectiveness for the Piru Creek

drainage basin.

Brune (1948) has shown that sediment yield is inversely proportional 

to the 0.15 power of the basin area. Calculation of the exponent 

for all the basins in this study results in an exponent of 0.22. In 

spite of the fact that the Brune exponent was developed in the midwestern United States, it is probably the more valid of the two 

figures because of the relatively small number of reservoirs used in 

this study. The Brune coefficient is used to adjust the sediment yield 

per square mile for each watershed to that for a watershed of 78.0 

square miles, which is the mean area of all the basins. Two of the 

watersheds have areas of less than 5 square miles, a size for which 

Brown (1950, p. 381-383) has shown variation in sediment yield to 

be extremely dependent on land-use and terrain factors.

CORRECTION FOR TRAP EFFICIENCY

Prior to correlation of the reservoir sediment yields the trap efficiency of each impoundment was also considered. A method of 

approximating trap efficiency from the relation of reservoir capacity 

to inflow (Brune, 1953, p. 414) was applied to each reservoir. Trap 

efficiencies estimated on this basis ranged from. 94 to nearly 100 

percent. Correction of the sediment yields for this small range of
E30 SEDIMENTATION IN SMALL BASINS

trap efficiencies was not justified because of the unmeasurable effects 

of sluicing, reservoir operation, and limitations of survey accuracy.

CORRELATION OF SEDIMENT YIELD WITH BASIN SIZE

The fundamental factor in correlating sediment yields within a 

region is the size of the contributing basin. Sediment yields of 

eight of the basins shown in figure 1 can be plotted against basin 

area to establish the yield-area relation (fig. 16). The linear correlation 

coefficient, r, is 0.949. On this basis, by substituting the area of the 

basin above Pyramid Rock in the regression equation, a value of 

260 acre-feet per year is obtained. However, by assuming a constant 

yield per unit area for the entire Piru Creek basin, a value of 212 

acre-feet per year is obtained for the drainage area above Pyramid 

Rock.

1000

100

10

i i i i i i i i i i i i i i

10 100 

AREA, IN SQUARE MILES

i i i i i i i

1000

FIGURE 16. Relation of sediment yield to basin area for basins listed in text. The vertical line represents 

the area of the Piru Creek drainage basin upstream from Pyramid Rock. The corresponding sediment 

yield Is 260 acre-feet per year.
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E31

CORRELATION OF SEDIMENT YIELD WITH PHYSIOGRAPHIC

PARAMETERS

Many physiographic variables have been proposed in an attempt 

to improve on the use of basin size as a geometric factor influencing 

sediment yield. Several such physiographic parameters have been 

applied by Lustig (1965) to the Castaic watershed, which is adjacent 

to and east of the Piru Creek basin, and the same factors are applicable to Piru Creek. Lustig used data from six reservoirs included in 

this study Pacoima, Big Tujunga, Big Santa Anita, Sawpit, Big 

Dalton, and San Dimas (fig. 1).

The physiographic parameters defined by Lustig are as follows: A 

sediment-area factor (SA), a sediment-movement factor (SM), total 

stream length (SL), and three transport-efficiency factors (Ti, T2 , 

and Tz) (table 9). Linear regression lines (figs. 17-22) relating each 

of these parameters to sediment yield were drawn using the leastsquares method.

SEDIMENT-AREA FACTOR

The sediment-area factor is defined as

SA=AP/ cos e~g, (7)

where Ap is the planimetric area of a watershed, in square miles, 

and Og is the mean ground-slope angle for values obtained at 100 

random locations as described by Lustig (1965). The intercept value, 

using this sediment-area factor for the Piru Creek watershed upstream 

from Pyramid Rock, is 251 acre-feet per year (fig. 17). The linear 

correlation coefficient, r, is 0.957.

SEDIMENT-MOVEMENT FACTOR

The sediment-movement factor is defined as

SM =SA X smTg, (8)

where SA is the previously defined sediment-area factor, and sin Qg 

is the mean of the sines of the ground-slope angles at 100 random 

locations. An intercept value of 200 acre-feet per year is obtained 

for the sediment yield upstream from Pyramid Rock (fig. 18) by 

using this correlation factor (r= 0.964).

TOTAL STREAM LENGTH

Total stream length (SL) is the sum of the lengths of all streams 

within a watershed. Figure 19 indicates that by using this correlation 

factor (r= 0.946) a sediment yield of 303 acre-feet per year is 

obtained for the basin upstream from Pyramid Rock.
TABLE 9. Morphometric data and physiographic parameters of the Piru Creek drainage 

[Data for Sawpit, Big Dalton, Big Santa Anita, San Dimas, Paeoima, and Big Tujunga Res

Watershed

Sawpit.. .....

Big Dalton ..

Big Santa

Anita

San Dimas ...

Paeoima

Big Tujunga.

Basin

order

3

3

3

4

3

5

Area

of

basin

(sqmi)

3.34

4.50

10.79

16.17

28.10

82.00

Number of

streams of

order u

Ni= 13

AT2 = 5

AT3= 1

ATi= 6

AT3= 2

AT3 = 1

ATi= 17

AT2= 3

AT3= 1

ATi = 12

AT2= 4

AT3= 2

AT4 = 1

Ni- 49

AT2= 7

AT3 = 1

Ni= 136

AT2= 31

AT3 = 8

Nt= 2

JV«= 1

Total length

of streams of

order u

(miles)

Sii= 8.13

S£2= 9.89

Sis= 12.17

Sii= 6.31

2£2 = 8.60

S£3 = 8.80

2Li= 15.50

Si2= 21.47

Sis= 23.72

S£l = 19.02

Si2= 23.50

Sis= 27.21

Si4= 30.27

S£l = 46.26

S£2= 55.66

Si3 = 61.24

S£i= 116.01

SZ/2= 156.98

S£3 = 176.32

Si4= 185.11

21/5= 189.07

Mean length

of streams

of order u

(miles)

Zi= 0.63

Z2= 1.98

Z3= 12.17

Zi= 1.05

Z2 = 4.30

is= 8.80

Zi= .91

Z2= 7.16

i3= 23.72

Li= 1.59

Z2= 5.88

1/3= 13.61

14= 30.27

Zi= .94

Z2= 7.95

Z3 = 61.24

Li= .85

ia= 5.06

Z3 = 22.04

Z4= 92.56

Zs= 189.07

Mean

channel

slope of

streams of

order u

(ft per ft)

9i=0.292

92 = .201

93= .083

9i= .207

92 = .060

93 = .038

9i= .279

92= .142

83= .055

9i= .155

92= .109

83= .059

§4= .020

0i= .228

92= .109

93 = .073

9i= .216

§2= .091

83= .056

94= .046

95 = .018

Mean

basin

altitude

(ft

above

sea

level)

3,010

2,620

3,485

3,305

4,050

4,540

Mean

groundslope

angle

(degrees)

33.5

30.5

34.0

33.0

33.5

30.5

Bifurcation

ratio

ATi/AT2=2.60

AT2/AT3 =5.00

Rb =3.80

AT1 /^V2 =3.00

Ar2/AT3 =2.00

Rt =2.50

ATi/AT2 =3.67

AT2/.ZV3=3.00

Rk =3.34

ATi/JV2 =3.00

AT2/JV3 =2.00

AT3/AT4=2.00

1?6 =2.33

Ni/N2=7.QO

N2/Ns=7.00

Rb =7.00

ATi/AT2 =4.39

AT2/AT3=3.88

AT3/AT4=4.00

^V4/AT5 =2.00

#& =3.57

s

0

02

T

0

0

R

0

02

R

0

0

0

R

0

0

R

0

0

0

0

R
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA 

S B

E33

r ,£|0!

S8

T-l CO C» "3 M

CO 1-1 G O O

50 US ifS 0

M W 1^ N M

T-I O» OS CO US

T-l T(< * . O O

us' <N to us oi

O CO ^ US US

! i !L 1 '«

W W W M W

S r- fr- IN i-i

SS? S3

rH rH

S

t~ 00 O t- us

iH 00 CO. CO US_

 co !> ' gi uj to

US 00 00 Ol O us

OO CO *-J US T-I US

us t^ OO 00 Oi Oi

W M M M W W

S S£ S8

US T-I -^ t- 00 US

i I C<l t^ CO i I T-I

(N r-t O O O O

II II II II II II

co' t~ <o o

IS N W h5 IS H

OS US -^ 1A US

o co * us us

T-I' 1-^ CO O (N

OS 00 t~ Tfl TH I-I
E34

1000

SEDIMENTATION IN SMALL BASINS

100

10

i i I

10 100 

SEDIMENT-AREA FACTOR (SA =Ap/cos

1000

FIGUKE 17. Relation of sediment yield to the sediment-area factor (Sx) for basins listed in text. The 

vertical line represents the sediment-area factor of the Piru Creek drainage basin upstream from Pyramid 

Rock. The corresponding sediment yield is 251 acre-feet per year.

TRANSPORT-EFFICIENCY FACTORS

The first of the three transport-efficiency factors is defined as

(9)

where Rj, is the mean bifurcation ratio, and 2L is the total stream 

length. Sediment yield upstream from Pyramid Rock, determined by 

this correlation factor (r= 0.957), is 280 acre-feet per year (fig. 20).

The second transport-efficiency factor is defined as

(10)

where 2N is the total number of streams of all orders, and Rc is the 

mean channel-slope ratio for a watershed. The intercept value, determined by use of this correlation factor (r=0.902), is 296 acre-feet 

per year (fig. 21).
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E35

1000

100

10

10 100 1000

SEDIMENT-MOVEMENT FACTOR (SM=S1 X sin Qg )

FIGURE 18. Relation of sediment yield to the sediment-movement factor (Sat) for basins listed in text. 

The vertical line represents the sediment-movement factor of the Piru Creek drainage basin upstream 

from Pyramid Kock. The corresponding sediment yield is 200 acre-feet per year.

The third transport-efficiency factor is defined as

(11)

where N is the number of streams of the order designated by the 

subscript, and Rc is the ratio of the stream-channel slope of each 

stream order to that of the next highest stream order. The value of 

the intercept of the regression line (r=0.931) is 281 acre-feet per 

year (fig. 22).

Sediment yield was also calculated by the ratio method

£=318 (12)
E36

1000

100

z

Q

_i

LJ

> 10

SEDIMENTATION IN SMALL BASINS

= 8

'mo:

» E

0) Ho|

10 100 1000 

TOTAL STREAM LENGTH (SL), IN MILES

10,000

FIGURE 19. Relation of sediment yield to the total stream length (Si) for basins listed in text. The vertical 

line represents the total stream length of the Piru Creek drainage basin upstream from Pyramid Rock. 

The corresponding sediment yield is 303 acre-feet per year.

where Sa is the sediment yield upstream from Pyramid Rock, in 

acre-feet per year, 318 is the sediment yield upstream from Santa

r>

Felicia Dam, in acre-feet per year, and is the ratio of the geo-

-T2

morphic parameters of the two areas. Table 10 shows that sediment 

yield determined by this method is considerably lower than the 

yield obtained by using the regression method.

The average of the values obtained from the correlations with 

geomorphic parameters, 269 acre-feet, is about the same as that 

obtained from the relation with basin size, 260 acre-feet. The geomorphic parameters are highly area dependent when applied to this 

set of data. The parameters do not correlate significantly with sediment yield in acre-feet per square mile, a correlation which would be 

more valid than those employing total sediment yield. The sedimentarea and sediment-movement factors are based on drainage area; 

the total stream length and the total number of streams are also 

directly related to drainage area (figs. 23, 24). Thus, use of most of 

the geomorphic parameters does not improve on the use of basin 

size to compute sediment yield. However, use of the sedimentmovement factor (SM) does produce a value for sediment yield that
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E37

cc

UJ

cc

UJ 

Q.

t 100

UJ

L.

UJ 

CC 

0

z

Q 

UJ

>- 10

2 

UJ

O 

UJ

1 1 1 1 II 1 1

-

-

-

-

-

"

-

X

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1

<s s0

/*

i i i 1 1 1 1 1

t i i i 1 1 1 1

>X *^* .^^^

p ^&**

$^X

1 1 1 1 1 1 1 1

1 1

^1

WE

KJ jo

'(DO.

J£ O 

 " t

"1

E £

Q-lfl

D

1 1 1 1 L

/*

~

:

-

-

:

1 1 1 1 1 1 1

10 100 JOOO 

TRANSPORT-EFFICIENCY FACTOR (T1 =RbX2L)

10,000

FIGUEE 20. Relation of sediment yield to a transport-efficiency factor (Ti) for basins listed in text. The 

vertical line represents the transport-efficiency factor of the Piru Creek drainage basin upstream from 

Pyramid Rock. The corresponding sediment yield is 280 acre-feet per year.

approximates those obtained from the reservoir-sediment survey and 

by use of the Anderson method (1949, p. 579). Validation or invalidation of the physiographic-factor approach must await further 

evaluation.

TABLE 10. /Sediment yield for Piru Creek basin upstream from Pyramid Rock 

calculated from physiographic parameters

Physiographic parameter

Sediment yield above Pyramid Rock 

(acre-ft per yr)

Transport-efficiency factors: 

Ti.. ..................................

Ti

T3

Regression method

.............. 251

.............. 200

.............. 303

.............. 280

.............. 296

.............. 281

Ratio method

208

184

232

223

228

246

ADDITIONAL FACTORS AFFECTING SEDIMENT YIELD 

CLIMATIC FACTORS

Second in probable importance to basin size in influence on sediment 

yield are climatic factors. Annual precipitation and rainfall intensity
E38

1000

Q: 

<

LlJ >-

o:

LLJ 

0.

KlOO

LLJ 

L_

LLJ Q: 

o 

<

z

Q

LLJ

> 10

1-

z

LLJ

2

O

LLJ 

(f)

1

SE-DIMENTATION IN SMALL BASINS

:

:

: s

1 1 1 I I i i I

1 1 IIM

* *$

;/

.s 

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1

9 /

& -$ /

1 1 1 1 1 1 1 1

'v> o:

00 (Q 

(0 ^_

1 E

* o

0) >fc

O (o

D ^_

_

-

-

1 1 1 1 1 1 1 1

10 100 1000 

TRANSPORT-EFFICIENCY FACTOR (T-,=2N X Rc)

10,000

FIGURE 21. Relation of sediment yield to a transport-efficiency factor (Ti) for basins listed in text. The 

vertical line represents the transport-efficiency factor of the Piru Creek drainage basin upstream from 

Pyramid Rock. The corresponding sediment yield is 296 acre-feet per year.

are the climatic parameters most commonly used. In addition, Leopold 

(1951, p. 351-352) has shown that variations in storm frequency can 

produce changes in erosion rates even without any changes in annual 

precipitation.

Langbein and Schumm (1958) have considered the effects of low 

runoff in dry areas and dense vegetative cover in wet areas and have 

shown that maximum sediment yield occurs when effective precipitation is about 10 to 14 inches. Effective precipitation was plotted 

against sediment yield by Langbein and Schumm and is calculated 

by use of the relation of runoff to precipitation and temperature 

(Langbein and others, 1949, fig. 2). Data for each basin discussed here 

are adjusted to a mean annual temperature of 50° F.

A positive correlation between sediment yield per square mile and 

effective precipitation, an r of 0.61, was determined for the basins 

studied. This positive correlation was in range of effective precipitation for which Langbein and Schumm fourd a negative relation. 

A rank-correlation coefficient of 0.5 is significant at the 80-percent 

level. Lack of agreement with the Langbein and Schumm relation can 

be attributed to masking of the precipitation effect on sediment yield 

by other factors, to the general variability of small watersheds, or to
PIRU CREEK WATERSHED, SOUTHERN CAUFORNIA E39

1000

100

> 10

I I I I I I I

~1 I I I II LI

10 100 1000 

TRANSPORT-EFFICIENCY FACTOR 

(«e )+.

10,000

FIGURE 22. Relation of sediment yield to a transport-efficiency factor (Ts) for basins listed in text. The 

vertical line represents the transport-efficiency factor of the Piru Creek drainage basin upstream from 

Pyramid Rock. The corresponding sediment yield is 281 acre-feet per year.

the fact the sediment-yield data for areas such as the Transverse 

Ranges were unavailable to Langbein and Schumm.

SOIL-ERODIBILITY FACTORS

Anderson (1954, p. 278) has shown that sediment yield is significantly related to differences in the erodibility of soils within a watershed. In a study of variation of soil erodibility as related to soilforming factors, Andr6 and Anderson (1961, p. 3354) have shown 

that parent material is more important than topography or vegetation. 

The mountain soils of the Pirn Creek basin are more youthful than 

those studied in northern California by Andre and Anderson and 

should be even more closely related to rock type.

Soil samples in the Piru Creek watershed were collected on westerly 

slopes of 20°-30° to conform with previous studies of soil erodibility. 

The samples were cored from a 0- to 6-inch depth by several penetrations with a Viehmeyer sampler.

In previous studies a variety of erodibility indexes has been used, 

and most of them are measures of the degree of aggregation of the 

finer particle sizes into larger, more stable aggregates. Two indices 

were computed for soils of the Piru Creek basin the dispersion ratio
E40

1000

w

y 100

i

SQUARE

z

£ 10 

<

i

]

SEDIMENTATION IN SMALL BASINS

i i i i i M i

~-

-

i i i i i i i i

1 1 1 1 1 I! 1

 /x

 /

I/

1 1 1 1 1 1 1 1

1 1 1 1 1 1 II

/".

£/

 /&

/

\ \ 1 1 1 1 1 1

1 1 1 1 1 II 4- / ;

=

-

1 1 1 1 1 1 1 1

10 100 1000 10,OC

TOTAL NUMBER OF STREAMS

1000

\ 100 

J

D>1)

r

 10 

t

1

FIGURE 23. Eelation of basin size to total number of streams for basins listed in text.

1 1 I I 1 1 1 1

-

I i I I i i H

1 1 1 1 1 1 1 1

/

i I I i I I I I

1 1 1 1 I I I I

/*

^

i i i i i i i i

1 1 1 | 1 1 1 L

-

1 1 1 1 1 1 1 1

10 100 1000 10,0 

TOTAL STREAM LENGTH, IN MILES

FIGURE 24. Belation of basin size to total stream length (Si) for basins listed in text.
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E41

of Middleton (1930, p. 3), which Anderson (1951, p. 131) indicates 

quantitatively relates to sediment yield, and the surface-aggregation 

ratio (Anderson, 1954, p. 272). The indices are defined as follows: 

Dispersion ratio (DE): Total percentage of measured silt- and claysize particles in an undispersed soil divided by the percentage of 

these sizes when the soil is dispersed.

Surface-aggregation ratio (SA): Surface area of particles coarser than 

silt divided by the aggregated silt plus clay. Surface area, in 

square centimeters per gram, is obtained by treating the particles 

as spheres with a specific gravity of 2.65 and by assigning mean 

diameters to the sand, granule, and pebble subdivisions. Aggregated silt plus clay is the percentage of dispersed silt and clay 

minus the percentage measured before dispersion. 

Size distribution of the silt and clay was determined by hydrometer (Bouyoucos, 1936) before and after dispersion with sodium 

hexametaphosphate. Samples were then wet sieved to determine the 

distribution within the sand, granule, and pebble ranges.

Three samples were taken from soils developed on each of the major 

rock types in the basin (table 11). The dispersion ratios correlate well 

with determinations on soil lithosequences in other areas, but surfaceaggregation ratios tend toward higher yet relatively similar values 

(Anderson and Wallis, 1965, p. 27). An average soil erodibility for the 

basin was calculated from the average erodibility values and the 

proportional area of exposure of each rock type and was used in the 

correlations and analyses discussed below.

The seven correlative basins, excluding the Santa Felicia drainage 

area, are each dominated by a single lithologic assemblage; Matilija 

basin, by marine sediments of early Tertiary age; and those basins in 

the San Gabriel Mountains, by plutonic and metamorphic assemblages. Soil erodibilities corresponding to rock types in the Piru Creek 

area were applied to the other basins.

TABLE 11. Dispersion and surface-aggregation ratios as related to lithologic type

Dispersion ratios

Lithologic type

Alluvium of Quaternary age. . __ .

Continental sedimentary rocks of

Continental sedimentary rocks of 

Miocene age (includes some OligoMarine sedimentary rocks of MioMarine sedimentary rocks of Eocene

Metamornhic comDlex. .............

Ratios

50, 59, 67 

52, 55, 59

35, 37, 37 

43, 45, 45

33 

40, 43, 46 

46. 55. 45

Mean

59 

55

36 

44

43

49

Confidence 

limits 

at 0.95 

probability

±22 

9

3 

3

8 

13

Surface-aggregation ratios

Ratios

146, 152, 216 

95, 103, 110

64,68.68 

61, 68, 68

71 

118, 120, 143 

130. 178. 190

Mean

171 

103

67 

66

127 

166

Confidence 

limits 

at 0.95 

probability

±90 

19

9

9

32

77
E42 SEDIMENTATION IN SMALL BASINS

OTHER FACTORS AFFECTING SEDIMENT YIELD

The previous discussions establish a good correlation between 

sediment yield and basin area. However, the values of sediment yield 

upstream from Pyramid Rock that were obtained by using area and 

physiographic parameters are consistently higher than the sediment 

yield determined by the reservoir survey and the formula of Anderson 

(1949). These values of sediment yield suggest that climatic, land-use, 

and soil-erodibility factors cause the sediment yield in the Piru 

Creek basin to be substantially less than that in the other basins. 

The small quantity of data available, however, preclude any elaborate 

statistical analysis to assess these factors.

Obtaining a significant result from multiple-regression analysis 

with more than three independent variables is virtually impossible 

with only eight watersheds for comparison. The number of variables 

used (table 12) was therefore reduced; this was accomplished by a 

logical rather than an empirical approach because of the small sample 

size. A preliminary step in assessing the data was to correlate each of 

the factors with sediment yield adjusted to remove the effect of area 

by expressing the yields in acre-feet per square mile and by basing 

all data on a mean basin size using the power adjustment of Brune 

(1948). However, multiple correlation based only on variables with 

the highest gross or net correlation is generally too high. Removal of 

the dominant factor, area, does not appreciably reduce this possibility. 

This procedure may result in discarding factors which would show a 

truly important relation to sediment yield through a more sophisticated analysis impossible to use in this study.

Instead, parameters for multiple-regression analysis were selected 

on the basis of having been shown to significantly affect sediment 

yield in previous studies and on the basis of the logical effect of 

each variable on sediment yield. Runoff, effective precipitation, mean 

annual precipitation, precipitation intensity, and surface-aggregation 

ratio were chosen as factors for correlation. Inclusion of two factors, 

such as area and runoff, for the same effect results in multicollinearity 

and erroneous correlations.

A regression of sediment yield ($ ") corrected with the power 

adjustment for basin size against several factors is

logSy»= 0.64+2.25 log MAP 1.80 log 7+0.11 log SA, (13)

where

MAP=mean annual precipitation, in inches,

/= probable maximum 24-hour precipitation, in inches (U.S.

Weather Bureau, 1960), 

and SA=surf ace-aggregation ratio.
TABLE 12. Summary of watershed -parameter

Oj/ Oj/ Oy

Basin (acre-ft (acre-ft (acre-ft 

per yr) per sq per sq 

miperyr) miperyr)

Piru (at Santa Felicia Dam)..-.

52.0 

318.0

52.62 

113. 23 

43.18 

8.43 

5.71 

21.70

0.956

.748

1.87 

1.38 

4.00 

2.52 

1.27 

1.34

0.902 

.959

1.61 

1.39 

2.96 

1.58 

.830 

1.06

A MAP EP RRA I

(sqmi) (inches) (inches) per- (inches) 

cent

54.40 

425. 00 

284.00 

28.10 

82.00 

10.79 

3.34 

4.50 

16.17

28.7 

17.1 

15.9 ..

25.6 

25.5 

35.1 

34.0 

27.0 

28.0

25.8 

16.0

24.5 

22.5 

30.2 

22.3 

21.4 

19.5

95 

91 

83 

92 

88 

97 

98 

100 

98

38.0 

24.5 

21.5 

25.0 

27.5 

37.5 

37.5 

33.0 

33.0

Fi

(p

ce

Estimated.
E44 SEDiMEJsnwnoN IN SMALL BASDSTS

The adjusted standard error of estimate is ±0.165 log unit. 

A regression with sediment yield per square mile (Sy>) gives

log S>= 6.48+1.70 log EP+0.70 log 7+1.61 log SA, (14)

where EP= effective precipitation, in inches (Langbein and others, 

1949, fig. 2). The adjusted standard error of estimate is +0.111 log 

unit.

The following relation is obtained, regressing with sediment yield

log Sy= -4. 13+0.95 log MAQ -0.22 log 7+1.25 log SA, (15)

where M4Q=mean annual runoff, in acre-feet per square mile. The 

adjusted standard error of estimate is ±0.156 log unit. The negative 

coefficient for log 7 in equations 13 and 15 is contrary to experience; 

sediment yield should increase as precipitation intensity increases. 

Therefore, equation 14 with a positive correlation with precipitation 

intensity is probably the most valid.

Only observed coefficients of multiple correlation greater than 

0.885 will shown any true correlation based on eight samples and 

three independent variables at the 0.95-probability level (Fisher, 

1928; Ezekiel and Fox, 1959) . Although the most significant regression 

correlation is that which includes the effect of basin area in the form 

of mean annual discharge, the other factors included in the regressions 

probably explain some of the variation not due to area. Coefficients 

of multiple correlation of the three equations are 0.730, 0.930, and 

0.979. Equations 14 and 15 show true minimum correlations of 0.53 

and 0.87 at the 0.95-probability level. The multiple-correlation 

coefficient deteriorates as area is progressively removed from the 

analysis, and this emphasizes the strong dependence of sediment 

yields on that factor.

The three above equations produce estimates of sediment yield at 

Santa Felicia Reservoir of 238, 257, and 201 acre-feet per year, all of 

which are below the value of 318 acre-ft per yr used in the regression. 

Applying the equations for Sy» and Sy> to the data for that part of 

the Piru Creek basin upstream from Pyramid Rock provides values 

of 184 and 175 acre-feet per year.

PROBABLE ACCURACY OF SEDIMENT-YIELD 

PREDICTIONS IN THE PIRU CREEK WATERSHED

The difficulty of precisely predicting sediment yield in watersheds 

as diverse as that of Piru Creek is evident from the results of this 

multivariate approach. The range of sediment yield given by the 

methods used (table 13) plainly indicates that only a reasonable
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E45

estimate can be obtained. This estimate must be further qualified 

because the assumption is made that future conditions will be similar 

to past conditions. Changes in either dynamic or geometric factors 

will affect sediment yield, particularly in an area as unstable and 

varied from the standpoint of land use and intensity of geomorphic 

processes as the Piru Creek watershed has been and is likely to be. 

Study of the area indicates that marked periods of aggradation and 

degradation have taken place during development of the present 

landscape and that pronounced changes in factors controlling sediment movement and yield have occurred.

TABLE 13. Sediment yields calculated for Piru Creek basin upstream from Pyramid

Rock

Method Sediment yield

(acre-ft per yr)

1. Rate based on yield per unit area obtained from 10-year accumulation in Santa Felicia Reservoir (ratio method, using areas) _____ 167

2. Method 1 extended back to 1932, using the Anderson (1949)

formula________________________________________________ 212

3. Method 2 modified for effect of basin area._____________________ 225

4. Regression, using area_______________________________________ 260

5. Regressions, using physiographic parameters (table 10)________ 200

251

280

281

296

303

6. Method 2 subdivided by ratio method, using physiographic parameters (table 10)______________________________________ 184

208 

223 

228 

232 

246

7. Multiple regressions, using climatic, soil-erodibility, and flow

parameters ______________________________________________ 175

_________________________________________________184

A period of high sediment yield could result from changes in either 

land use or tectonic activity. Burned areas in the San Gabriel Mountains have shown 10 times the preburn rate of erosion, much of which 

occurred as dry creep during the dry season (Krammes, 1960, p. 4). 

The U.S. Forest Service in 1950 estimated an average expectable burn 

percentage in the Piru Creek watershed of 1.4 percent of the area each 

year. The actual burn rate for the 26-year period 1940-66 has been 

only 0.4 percent. The change in average cover density associated with 

a burn rate of 1.4 percent as opposed to 0.4 percent would increase 

sediment yield by a factor of 1.46 (H. W. Anderson, oral commun, 

1966). Greater fire protection and increasing effectiveness of firefighting techniques probably reduce the future burn probability in the
E46 SEDIMENTATION IN SMALL BASINS

watershed and may have accounted in part for the less-than-expected 

burn since 1940.

Slope rejuvenation resulting from recent uplift in the San Gabriel 

Mountains increased local sediment yield as much as five times 

(Anderson and others, 1959, p. 6). The same effects could occur in the 

Piru Creek area. In fact, the area above Pyramid Rock may be more 

sensitive than most owing to the recent period of pronounced alluviation. The potential exists for a sediment yield substantially higher 

than estimated values. If future conditions are relatively similar to 

those of the past, however, the estimate will be confirmed.

CONCLUSIONS AND PROBABLE LONG-TERM SEDIMENT 

TIELD AT THE PYRAMID DAM SITE

The largest values for projected sediment yield above Pyramid 

Rock were obtained from the regression analysis involving area, 260 

acre-feet per year, and the physiographic variables, 200 to 303 acrefeet per year with a mean value of 269 (table 13). The value obtained 

by dividing the sediment yield measured at Santa Felicia Reservoir 

by area is 212 acre-feet per year.

The use of physiographic parameters does not give a more accurate 

result than the use of area. Wallis (1965, p. 11) notes that physiographic variables other than area are generally of secondary importance compared to area, climate, and land use as factors affecting 

sediment yield. Such an apparent impasse, reinforced by the results 

of this study, is by no means an indictment of the use of physiographic 

factors. The attempts to segregate meaningful physiographic variables 

from the large number proposed in the literature should be continued. 

The sediment-movement factor, for example, gives the value (200 

acre-feet per year) most in line with the measured rate of 212 acrefeet per year upstream from Pyramid Rock.

Consideration of climatic, land-use, and soil-erodibility factors 

apparently improves somewhat on correlation with area, and results 

in calculated sediment yields of 175 and 184 acre-feet per year. These 

moderately low yields, compared with the yield of 212 acre-feet per 

year, are probably due to the effects of the basin variability 011 the 

correlations. The Piru Creek watershed, in particular the area upstream from Pyramid Rock, is markedly more arid than the surrounding basins for which data are available. Were there no direct measurement of the sediment yield at any point in the basin, however, the 

correlations including climatic, land-use, and soil-erodibility factors 

would have to be favored on the basis of the results of previous 

sediment-yield studies and are seen, in this study, to provide a result 

closer to the measured yield. Generalized and regional trends in 

climatic, land-use, and soil factors may be a useful addition to areal
PIRU CREEK WATERSHED, SOUTHERN CALIFORNIA E47

sediment-yield correlations in diverse areas, such as the Transverse 

Ranges.

In light of the extreme variability and the small number of basins 

available for correlation of reservoir sediment accumulations, the 

most acceptable estimate of the sediment yield upstream from 

Pyramid Rock is that which is based on the amount of sediment 

measured in Santa Felicia Reservoir. The yield obtained from the 

reservoir survey, extended back over a range of climatic conditions 

by the Anderson technique (1949) and increased to correspond to a 

smaller area by use of the Brune power adjustment (1948), is 225 

acre-feet per year. An estimated range in rates is ±50 acre-feet per 

year. Confidence limits applied to the mean of the values in table 13 

(231 acre-feet per year) are ± 19 at the 0.95 probability level.

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US. GOVERNMENT PRINTING OFFICE: 1968 O-309-538