Erosion and Sediment Yields 

in the Transverse Ranges, 

Southern California

By KEVIN M. SCOTT and RHEA P. WILLIAMS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1030

Prepared in cooperation with the

Ventura County Department of Public Works

and the Ojai Resource Conservation District

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Scott, Kevin M 1935-

Erosion and sediment yields in the Transverse Ranges, southern California. 

(Professional paper—U.S. Geological Survey ; 1030) 

Bibliography: p.

1. Erosion—California—Transverse Ranges. 2. Sediments (Geology)—California—Transverse Ranges. 3. Sedimentation and deposition. I. Williams, Rhea P., joint author. II. Title. III. Series: United States. 

Geological Survey. Professional paper ; 1030.

QE571.S4 551.3'02'0979492 77-608034

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

Washington, D.C. 20402 

Stock Number 024-001-03034-9
CONTENTS

Abstract ________________________________

Introduction _______________________________

Previous work __________________________

Purpose, scope, and methods ___________________

Acknowledgments ___________________________

The environment _________________________________

Location and physical features _________________

Bedrock geology ___________________________

Climate and vegetation _______________________

Flood history—the 1969 storms ____________ 

General geomorphic history ___________________

Watershed form and process ________________________

Relation of geomorphic history to tectonics ______

Mass movements and sedimentation processes ____

Rockfalls and slides _____________________

Rock-fragment flows _______________________

Debris flows __________________________

Mudflows ______________________________

Relative importance of sediment-transport

processes __________________________________

Slope failures as related to debris flows and 

mudflows ___________________________________

How 1969 sedimentation processes relate to 

determinations of erosion rates _______________

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Methods of data analysis ____-----__---_---- — _____

Physiographic characteristics _ — ___ —— ___ — - 

Soil erodibility _____________-_-_--_--_--------

Slope failure _______--_---- ———— _- ——— ---- —

Hydrologic factors _________-_-_--__-----------

Precipitation _____________-____----__----_

Peak discharge ___________-____---_----_—

Total time of concentration ________________

Fire effects _________________-_---__-----------

Cover and land use __________________--------_-

Results of analysis -------------------------------

Predictive equations ______________________ — ___

Areal variation in erosion rates _______________

Erosion rates for planning and design __________

Accuracy of erosion-rate estimates _____________

Long-term erosion rates ____________-____--------_-

Estimation of rates ________________--____--- —

Implications of long-term erosion rates _________

Present stage in cycle of alluviation and channel 

entrenchment __________________-_----------_

Time variation of erosion rates _________________

References cited ______________-__________-__------

ILLUSTRATIONS

FIGURE 1.

2.

3. 

4-6.

7.

8.

9.

10.

11.

12.

Index map showing location of the Transverse Ranges and the principal areas of study _____-_____---

Map of location of drainage basins in the eastern Transverse Ranges, Los Angeles County ___ — ______

Map of location of drainage basins in the western Transverse Ranges, Ventura County ___________ — __

Photographs of:

4. Fire-denuded hillslope in Hook Canyon watershed __________________ — __________ — __________

5. Mud Creek watershed viewed from the drainage divide _______________ — ________-_-----____

6. Eroded dry-sliding deposits in Englewild Canyon ___________________ — _________________ — _

Diagrammatic cross sections of typical bedrock-channel deposits in an area of high erosion rates in the

Transverse Ranges __________________________________________________ — __________ — — _

Photograph of typical Transverse Range slump showing features used to identify slope failures ______

Graph of sediment yields for watersheds near Ojai (area 1) compared with debris-production curve based

on estimated actual yield in Stewart Canyon _______________________ — __________________________

Graph of long-term sediment yields at selected sites in Los Angeles County _______ — _______________

Photograph of typical silt-rich valley fill in tributary of Tapo Canyon _______ — ___ — _____ — ____ — — _

Photograph of undercut debris-retention structure in tributary of Tapo Canyon ______________________

TABLES

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TABLE 1. Summary of physiographic variables, discharges, and sediment data for selected watersheds in eastern 

Transverse Ranges of Los Angeles County _______._________________________-_---_---_--__ — _--

Page

18

in
IV CONTENTS

Page

TABLE 2. Dispersion and surface-aggregation ratios as related to all geologic units in debris-producing areas of

Ventura County and in selected drainage basins in Los Angeles County ___ — ___ — __ — — __ — — 21

3. Depth-duration comparison of 50-year precipitation and nearest actual precipitation at stations in the

Transverse Ranges ___________________________________________ — _ — __ — __ — —— — ___ 23

4. Fifty-year peak discharges at selected stream-gaging stations in Los Angeles and Ventura Counties —_ 24

5. Comparison of 1938 and 1969 peak flow to the 50-year peak flow in Ventura County ———— ————— _ 24

6. Data for selected drainage basins with January 1969 sediment yields in Los Angeles and eastern Ventura

County ________________._______.___________________________———————— 26

7. Data for drainage basins above proposed and selected existing debris-basin sites in Ventura County _ — __ 28

8. Data and comments on estimated January 1969 sediment yields per unit area (erosion rates) in selected

existing debris basins in Ventura County ______.____________________--__----_-_-———————— 31

SYMBOLS

A Area of a drainage basin.

a Area above a certain reference altitude within a drainage basin.

C Runoff coefficient.

Dd Drainage density.

DR Dispersion ratio; a measure of soil erodibility.

ER Elongation ratio.

FF Fire factor.

H Total relief of a drainage basin; highest altitude minus lowest altitute.

h Altitude of a contour line above outlet of drainage basin.

I Precipitation intensity.

K Precipitation factor defined as: 10-dayX (24-hour precipitation) 2.

L Watershed length.

SL Total stream length.

L,!* Length of stream or stream channel of order p..

MAP Mean annual precipitation.

P Precipitation.

Qso Peak discharge with a recurrence interval of 50 years.

Qcfs Peak discharge in cubic feet per second.

Qeim Peak discharge in cubic feet per second per square mile.

Rb Bifurcation ratio.

Rr Relief ratio.

SA Sediment-area factor.

Sx Sediment-movement factor.

Sv Sediment yield.

Sv' Sediment yield per unit area.

SA Surface-aggregation ratio; a measure of soil erodibility.

SF Proportion of basin underlain by slope failures.

Ti Transport-efficiency factor.

TC Total time of concentration.

VI Vegetation index.

Z Land-use coefficient.

6g Ground-slope angle.

{M Stream order; unbranched tributaries are designated as first order; their con­ 

	fluence forms a stream of second order; and so forth.
CONTENTS

CONVERSION FACTORS

Factors for converting English units to metric units are given below to four significant figures. However, in the text the 

metric equivalents are shown only to the number of significant figures consistent with the values for the English units.

English Multiply by

°F (degrees Fahrenheit) 5/9(F-32)

ft (feet) 3.048 xlO-1

ft3/s (cubic feet per second) 2.832 xlO"2

in (inches) 2.540x10

mi (miles) 1.609

mi2 (square miles) 2,590

yd3 (cubic yards) 7.646xlO"1

ydVacre (cubic yards per acre) 1.889 

yd3/mi2 (cubic yards per square mile) 2.952XlO"1

Metric

0 C (degrees Celsius)

m (meters)

nf/s (cubic meters per second)

mm (millimeters)

km (kilometers)

km2 (square kilometers)

nr (cubic meters)

mVhm2 (cubic meters per square hectometer)

mVknf (cubic meters per square kilometer)

EROSION AND SEDIMENT YIELDS IN THE TRANSVERSE RANGES,

SOUTHERN CALIFORNIA

By KEVIN M. SCOTT and RHEA P. WILLIAMS

ABSTRACT

Major-storm and long-term erosion rates in mountain watersheds of the western Transverse Ranges of Ventura County 

are estimated to range from low values that will not require 

the construction of catchments or channel-stabilization structures to values as high as those recorded anywhere for comparable bedrock erodibilities.

A major reason for this extreme variability is the high 

degree of tectonic activity in the area—watersheds are locally 

being uplifted by at least as much as 25 feet (7.6 meters) 

per 1,000 years, yet the maximum extrapolated rate of denudation measured over the longest available period of record is 

7.5 feet (2.3 meters) per 1,000 years adjusted to a drainage 

area of 0.5 square mile (1.3 square kilometers). Evidence of 

large amounts of uplift continuing into historical time includes 

structurally overturned strata of Pleistocene age, active thrust 

faulting, demonstrable stream antecedence, uplifted and deformed terraces, and other results of base-level change seen in 

stream channels. Such evidence is widespread in the Transverse 

Ranges, and aspects of the landscape, such as drainage-net 

characteristics and hillslope morphology, are locally more a 

function of tectonic activity than of denudational process. Many 

of the 72 study watersheds are located on frontal escarpments 

of mountain blocks cut by recently active thrust faults, along 

which the upper part of the drainage basin has overthrust 

either the lower part of the basin or the adjacent valley area.

To define erosion rates in 35 small watersheds in the western 

Transverse Ranges, a group of 37 similar watersheds with 

sediment yields measured in debris basins was selected from 

the eastern Transverse Range in Los Angeles County. Sediment yields from this group of watersheds during the recordbreaking 1969 storms ranged from relatively low rates to 

values equivalent to reduction of the entire land surface of 

a watershed by more than 2 inches (51 millimeters).

Correlation of the measured erosion rates to the watersheds 

with unknown rates required definition of the chief factors 

that control the erosion rates. Numerous types and combinations of variables measuring physiography, soil erodibility, 

slope stability, hydrologic factors, wildfire effects, vegetation, 

and land use were analyzed by regression. A slope-stability 

variable retained in regression at significant levels was the 

proportion of watershed drainage area underlain by slope 

failures, a logical measure of increased erodibility caused 

by uplift.

The importance in the area of debris flows, mudflows, and 

mass movements—forms of sediment transport not involving 

normal aqueous entrainment—is also a reflection of the active 

tectonic setting of the Transverse Ranges. Implicit in the detailed study of selected physiographic and slope-failure variables was the logical assumption that correlation with the 

probability of transport by these exotic but quantitatively important sedimentation processes would be achieved.

So prominent and widespread was evidence of debris flows 

in the small study watersheds after the 1969 storms, that it 

was possible to formulate a model for the dispersal of sediment in such watersheds: Lateral supply of sediment to 

stream channels is a relatively continuous process, accomplished in significant part during the dry season by dry sliding, 

in addition to wet-season contributions from overland flow and 

mass movements. During periods without major storms, stream 

channels undergo more-or-less time-continuous fill. Then, during a storm of high recurrence interval, channel-bed material 

is mobilized and dispersed in large part by debris flows— 

coarse granular slurries, some of which are induced by mass 

movements triggered by the storm. Channels undergo substantial net scour, accomplished by removal of bed material in 

debris flows and by scour during recession flow. Valley-side 

slopes are undercut by bank erosion, and a new cycle of channel infilling by hillslope processes is initiated.

INTRODUCTION

The vast urban area of southern California has 

developed progressively outward from intermontane 

flatlands to alluvial fans formed around the bases 

of precipitous, fault-block mountain ranges. Continued population pressure has extended urbanization up the fans and, in recent years, almost into 

the mouths of the rugged mountain watersheds, 

which periodically disgorge their storm runoff and 

loads of coarse sedimentary detritus to the surfaces 

of the fans.

The results of flooding in areas of urban expansion may be catastrophic when neither proper zoning nor flood-control measures exist. However, the 

problems due simply to rising floodwater in existing channels have been of historically lesser importance relative to the problems caused by a group of 

complex geomorphic processes common to the region. Processes that have caused extensive damage 

on alluvial fans, for example, include lateral scour 

in existing channels, the formation of new channels 

by sudden redirection of flow at the fan apex (Scott,
TRANSVERSE RANGES, SOUTHERN CALIFORNIA

1973), and inundation by debris flows and mudflows 

(Scott, 1971). When unimpeded, channels in the 

noncohesive fan deposits that are themselves the 

products of former storms are free to migrate unpredictably throughout the roughly semicircular arc 

of the fan. Numerous other processes related to 

major storms are active on hillslopes and in the 

confined bedrock channels of the watersheds themselves.

The prevention of damage from these causes is 

one of the most important environmental-geomorphic problems in the area today. At present, the 

most economical solution often is the construction 

of debris basins at the mouths of watersheds with 

high flooding potential and erosion rates. These 

structures trap sedimentary detritus and divert 

storm discharges into lined channels. A major criterion for both justifying and designing the basins 

is the amount of detritus that will be eroded during a major storm.

The object of this study is to estimate majorstorm and long-term erosion rates for planning 

purposes in the western Transverse Ranges (fig. 1). 

The study basically is an analysis of the variables 

that affect the quantities of sediment eroded from 

small, steep drainages in an area that is as diverse

as any in the world with respect to the tectonic, 

geologic, and geomorphic controls on erosion rates. 

The term "erosion" is used here in the established 

general sense to include both weathering and the 

transportation of weathered products, which consist predominantly of detrital alluvium and colluvium in the study area. Sediment yields are the 

volumes of sediment retained in impoundment structures over a given period or during a certain magnitude of storm. Erosion rate is used here synonymously with sediment yield per unit watershed area 

in some contexts, with the qualification that absolute 

values of erosion rates can be derived from sediment yields only by measurement of the dissolved 

products of weathering, and with a correction for 

trap efficiency of the impoundments. Both corrections are minor throughout most of the study area.

PREVIOUS WORK

The sediment system of mountain watersheds 

like those in southern California is unique compared 

with the sediment system of watersheds for which 

sediment-measurement procedures have been developed. Direct measurements of sediment discharge 

in surface flow are not feasible for sediment-yield 

analysis in the study watersheds because, among

116°

\

115°

25 50 75 100 MILES

i I I I I I i

0 25 50 75 100 125 150 KILOMETERS

FIGURE 1.—Location of the Transverse Ranges and the principal areas of study.
INTRODUCTION

other reasons, of the scarcity and short duration of 

such flows. Use of standard sedimentation procedures in mountain watersheds may be objected to 

for many reasons, such as the subjectivity in calculation of bedload when bed material is in extremely coarse size ranges. Other problems unique 

to quantitative sedimentation study of these watersheds will be discussed throughout the report.

In addition to application of bedload functions 

and rating curve-flow duration extensions of direct 

measurements of sediment discharge, methods by 

which sediment yields are determined indirectly 

have been widely used. The most common of these 

involves the calculation of the amount of eroded 

soil, using one of several classical soil-loss equations, followed by sediment routing by means of 

sediment-delivery ratios. However, soil-loss equations are products of agricultural research, and this 

approach should be confined to areas with zonal 

soil development. It has not proved useful in southern California.

Sediment volumes that have accumulated in reservoirs and debris basins are the only reliable source 

of sediment-yield data in the mountain watersheds 

of southern California. Erosion rates determined 

from these accumulations must be transferable to 

other watersheds if any conclusion of more than 

local interest is desired. It is the transfer value of 

the erosion rates, the watershed variables by which 

the rates may be correlated, and the assessment of 

the limits beyond which the rates no longer apply 

that are the crux of any such attempt. Past studies 

in the Transverse Ranges have attempted the widespread application of a single set of criteria calculated from a limited group of data.

In a typical regression analysis using graphical 

techniques, Ferrell (1959) estimated erosion rates 

in Los Angeles County, mainly in the frontal San 

Gabriel Mountains, to derive criteria for debrisbasin design. The resulting equation was

Sv ' — -•

35,600

(5 + F/) 2 - 67

where Sv ' is sediment yield, in cubic yards per 

square mile, Qcm is peak discharge, in cubic feet 

per second per square mile, Rr is relief ratio, and 

VI is a vegetation index. The standard error of estimate is 0.386 log units ( + 143, -59 percent).

What is essentially a regression of sediment yield 

against a series of variables—slope, drainage density, hypsometric-analysis index, and 3-hour rainfall—was developed by Tatum (1965). Basic to the 

analysis was the concept of an ultimate erosion

rate, the idea that under conditions of a 100-percent burn followed by a major storm there is a 

maximum rate of erosion to which correction factors 

for the above variables can be applied. Data were 

obtained from watersheds in the eastern Transverse Ranges.

Although the inflow of sediment into an impoundment is a stochastic process, attempts to develop a 

stochastic model for prediction of sediment yields 

have thus far foundered in practicality on several 

critical but necessary assumptions. As will be clear 

from subsequent discussion, the extreme character 

and diversity of the subject watersheds would virtually preclude modeling, even were procedures 

standardized.

A reconnaissance of all existing impoundments 

and many stock pounds in September 1970 led inescapably to the conclusion that previous attempts 

at erosion-rate analysis would not apply to the 

western Transverse Ranges, primarily because of 

the limited nature of the sample populations from 

which the previous criteria were derived. At the 

time of the 1970 reconnaissance, the effects of the 

recordbreaking 1969 storms were still visible, and 

large differences in erosion rates between adjacent 

parts of the Transverse Ranges were distinct. The 

differences were in most cases related to complex 

differences in watershed characteristics, and not 

simply to variations in storm intensity.

PURPOSE, SCOPE, AND METHODS

The most logical approach to estimation of erosion rates in the Transverse Ranges is empirical 

correlation of actual sediment yields on the basis 

of watershed characteristics (see Anderson and 

Wallis, 1965). This study will use the same type of 

data as the studies by Ferrell (1959) and Tatum 

(1965)—actual sediment yields from debris basins 

in the eastern Transverse Ranges in Los Angeles 

County—but will attempt to modify the predictive 

results to a more variable range of conditions, 

especially to those that exist in the western Transverse Ranges of Ventura County. Objections to extension of the previous studies can be met in the 

following ways:

1. The sample of watersheds with known erosion 

rates will specifically include watersheds with 

characteristics similar to those of watersheds 

in the western Transverse Ranges. Sample size 

and selection will still be sufficient to assure 

diversity of parameters in most other respects.

2. All possible watersheds from areas of sedimentary rocks in the normally granitic-meta-
TRANSVERSE RANGES, SOUTHERN CALIFORNIA

morphic eastern part of the ranges will be included in the analysis. This broader range of 

bedrock and soil characteristics should include many of the conditions in the western 

part of the ranges. The chief difficulty in comparing watersheds in the two areas is this 

general difference in rock type. Anderson 

(1949a, p. 622) met a similar problem in correlating peak discharges but found that functions relating discharge to watershed variables were remarkably similar in areas of both 

sedimentary and granitic-metamorphic rock 

types. Anderson's result, though encouraging 

with respect to hydrologic behavior of the two 

types of terrain, points out the need for other 

variables to explain the large observed differences in erosion rates.

3. A larger number of variables than analyzed in 

previous studies will be considered, not only 

for reasons given in 2 but also because of the 

known importance of additional factors. Substantial new data became available as a result of the 1969 storms.

4. Variable selection will be based on logic and the 

results of other studies in southern California, 

as well as statistical inference. It is not possible to consider all the potential variables on 

a purely statistical basis, and it is at this point 

that sedimentation theory and the known response of watersheds elsewhere can be used 

for determination of variables.

The net end product of the study is the development of criteria by which the erosion potential of 

selected mountain watersheds in the western Transverse Ranges in Ventura County can be determined. 

Specifically, the foci of the study are as follows:

1. Estimates of erosion rates from a major storm 

(approximate 50-year recurrence interval) are 

prepared as an aid to the planning and design 

of debris basins or zoning regulations. Differences between rates under burned and unburned conditions are assessed to the maximum extent possible.

2. Estimates of long-term erosion rates are made 

to assist in planning cleanout costs and selecting debris-disposal areas. Long-term rates 

estimated under present conditions also establish a natural base against which the effects 

of future environmental changes can be compared.

3. Modes of coarse-sediment transport such as debris flow and mudflow, and their effects on 

erosion rates, are evaluated. Previous study

(Scott, 1971) has indicated that such processes 

may be the dominant means of sediment transport in some small watersheds in southern 

California.

ACKNOWLEDGMENTS

The writers are grateful to the many agencies 

and individuals who aided in the preparation of this 

report. Especially helpful were Gerald D. Bickel 

and Eugene Bluzmanis, Ventura County Department of Public Works; and William R. Barr and 

Robert Smith, Debris Reduction Section, Los 

Angeles County Flood Control District.

Technical advice on selected aspects of the study 

was provided by Henry W. Anderson and Raymond 

M. Rice, Pacific Southwest Forest and Range Experiment Station, U.S. Forest Service; William B. 

Bull, University of Arizona; and Saul E. Rantz and 

Mark W. Busby of the U.S. Geological Survey. The 

junior author supervised the data analysis and prepared the discussion of hydrologic factors. All or 

parts of the manuscript benefited substantially from 

critical review by H. W. Anderson and W. B. Bull 

and others, and any remaining shortcomings are the 

responsibility of the authors.

THE ENVIRONMENT

LOCATION AND PHYSICAL FEATURES

The 72 drainage basins discussed in this report 

are in the Transyerse Ranges in Ventura and Los 

Angeles Counties, Calif, (fig. 1). Most of the 37 

drainages with known erosion rates are in the eastern Transverse Ranges in Los Angeles County (fig. 

2) ; the 35 watersheds for which rates are determined by indirect methods are in the western Transverse Ranges in Ventura County (fig. 3).

The group of watersheds studied in Los Angeles 

County drains into both the Los Angeles and the 

San Gabriel River systems. Both steepness of terrain and erosion rates generally are high in watersheds along the front of the San Gabriel Mountains, 

and are by local standards relatively low to moderate in other parts of the area selected for correlation of erosion rates.

The watersheds in Ventura County are located 

in each of the three major drainages that flow to 

the ocean: The Ventura River, the Santa Clara 

River, and Calleguas Creek. Terrain and erosion 

rates decrease in severity from north to south within the county, so that the watersheds in the Ventura 

River drainage have the highest erosion rates, and 

those in the Calleguas Creek drainage have the
118°30'

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PAC/F/C OCEAN \N Santa Monica

1. Bell Canyon

2. Limekiln Canyon 

EXPLANATION 3 M ay Canyon (No.

4. May Canyon (No. 

T Existing debris basin 5 Row|ey Canyon

— — Drainage-basin divide 6. Bluegum Canyon

1 Irhaniyatirm hnnnrlarv 7. Blanchard Canyon wrudmzciiion uuunuciry ' 

8. Shields Canyon

9. Pickens Canyon

,

Base from U.S. Geological Survey

Los Angeles 1:250 000, 1959 and

San Bernardino 1:250 000, 1959

Hollywood • <fj

13?

("

Los Angeles

10. Snover Canyon 19.

11. Hall Beckley Canyon 20. 

1) 12. La Tuna Canyon 21.

2) 13. Birmingham Canyon 22. 

14. Sunset Canyon 23.

15. Childs Canyon 24.

16. Hillcrest Canyon 25. 

17. Deer Canyon 26.

18. Nichols Canyon 27.

i

0 5 10 

I ' '

/ i/g°/

cf/

'

Lincoln Canyon

West Ravine 

Kinneloa Canyon (West)

Kinneloa Canyon 

Sierra Madre Villa

Bailey Canyon

Auburn Canyon 

Little Santa Anita Canyon

Lannan Canyon

i

15 MILES

—————

""•4//V

Q/ j

i Santa 

Cont

28

29

30

31

32

33

34

35

36

37

0 5 10 15 20 KILOMETERS

FIGURE 2.—Location of drainage basins in the eastern Transverse Ranges, Los Angeles County.
119°00'

34° 

30'

1

EXPLANATION

v Site of proposed de

T Existing debris basin 

— — Drainage-basin divid

Stream channel 

Urbanization bounda

1. Cozy Dell Canyon

2. Stewart Canyon

3. Fox Canyon near Ojai

4. Dron Canyon

5. Gridley Canyon

6. Senior Canyon

7. Horn Canyon (Thacher Creek)

8. Wilsie Canyon (Reeves Creek)

9. Fresno Canyon

10. Sexton Canyon (Arundell Barranca)

11. Harmon Canyon

12. Peppertree Canyon (Wason Barranca)

13. Aliso Canyon (Ellsworth Barranca)

14. Adams Canyon

15. Fagan Canyon

16. Mud Creek Canyon 

M. Orcutt Canyon 

18. Keefe Ditch

19. Jepson Wash (Snow Canyon)

20. Pole Creek Canyon

21. Real Canyon

22. Warring Canyon

23. Fox Canyon near Somis

24. Gill Barranca

25. Coyote Canyon

26. Mahan Barranca

27. Long Canyon

28. Sou

29. Gab

30. Alam

31. Brea 

32. Dry 

33. Tap

34. Tap

35. Tap

Base from U.S. Geological Survey 

Southern California, Sheet No. 3 

1:250000, 1910

10 MILES

0 24 6 8 10 12 14 KILOMETERS 

FIGURE 3.—Location of drainage basins in the western Transverse Ranges, Ventura County.
THE ENVIRONMENT

lowest. The large, diverse Santa Clara River drainage contains watersheds with vastly differing rates 

of erosion; those studied, however, are generally 

intermediate in intensity.

No large, easily delineated mountain ranges exist 

within the Transverse Range system in Ventura 

County. Rather, the area is a complex of discontinuous ranges with local names (fig. 3).

The altitude range of watersheds studied is 310 to 

4,485 ft (94 to 1,367 m) above mean sea level in 

Ventura County and 500 to 5,440 ft (152 to 1,658 

m) in Los Angeles County.

All the watersheds discussed in this report debouch into areas that are inhabited or have development potential. Sites of existing and proposed debris basins are commonly near the point at which 

streams leave the mountain terrain and flow out 

upon the alluvial fans. The natural change in channel is from a confined, bedrock channel with little 

alluviation to an ephemeral wash formed in the deposits of the fans. No significant surface flow occurs in any of the study watersheds except during 

and after storms.

BEDROCK GEOLOGY

Watersheds in both Ventura and Los Angeles 

Counties are underlain by a great variety of bedrock lithologies. Generally, however, watersheds in 

Ventura County are underlain by sedimentary sequences that are from Cretaceous to Holocene in 

age, and those in Los Angeles County are dominated 

by igneous and metamorphic assemblages mainly of 

Mesozoic age. This distribution is not exclusive, and 

a number of basins were found in Los Angeles 

County that are at least partly underlain by sedimentary rock types. Otherwise, correlation of erosion rates from one area to another could possibly 

have been less significant.

Even if the lithologic differences were mutually 

exclusive in the two areas, correlation would still 

have been possible if variables reflecting surface 

erodibility were correctly assessed. The U.S. Forest 

Service (1953, app. 2, p. 12-13) applied a regression analysis developed with sediment yields in 

igneous-metamorphic terrain to yields from sedimentary terrain in Ventura County and found that 

little change in the discharge function or cover factor was necessary. Marked differences in the equation constant were found and were attributed to 

geologic and soil differences.

CLIMATE AND VEGETATION

The Transverse Ranges, like most of southern 

California, have a Mediterranean-type climate. At

the low and intermediate altitudes considered here, 

summers are dry and warm; winters are wet and 

cool. Summer high temperatures in the 90's F (32°- 

37° C) are common. Winter temperatures are only 

occasionally below freezing.

Approximately 90 percent of the annual precipitation occurs in the 6 months from November to 

April. The rest of the year is just one long dry spell. 

In any one year there may be almost no precipitation, or there may be five times the mean annual 

precipitation. Mean annual precipitation in the 

study watersheds ranges from a low of 15 in (381 

mm) at a site in the Simi Hills to 28 in (711 mm) 

in several watersheds along the front of Nordhoff 

Ridge north of Ojai.

Although light snowfalls occur above altitudes 

of 3,000 ft (914 m), snow is not a significant factor 

in hydrologic analysis of the watersheds. Less than 

0.1 percent of the study-watershed area in Los 

Angeles County is above an altitude of 5,000 ft 

(1,524 m), a common lower limit used to assess effects of snow cover on runoff in the area. No part 

of any watershed studied in Ventura County is 

above 5,000 ft (1,524m).

Hillslopes on steep mountain fronts are covered 

with a dwarf forest of chaparral, an association of 

xerophytic shrubs and stunted tree forms that reflects the semiarid climate. The dense, highly flammable chaparral gives way to a sage or sagechamise association on lower slopes, especially in 

the Santa Clara River basin. This brush association 

in turn grades to grassland, which is dominant in 

many parts of the low rolling hills in the Calleguas 

Creek drainage.

Individual adjacent watersheds vary greatly in 

cover density. To this natural areal variation is 

added the variation through time caused by wildfire (fig. 4) and changes in land use. Figure 4 shows 

the post-wildfire development of many lowest order 

stream channels, formed by the natural tendency to 

approach the hillslope configuration required for 

more efficient transport of the erosional products 

made available by removal of vegetation. These rills 

and first-order streams will heal in time as the restoration of vegetation increases the hydraulic 

roughness and decreases the surface erodibility.

Wildfire effects on vegetation, as they pertain to 

increased erosion potential, can be predicted and 

quantified; changes in cover related to land use can 

only be estimated with great subjectivity. Fortunately, this latter problem is largely academic 

when applied to areas with high erosion rates. 

Hillslope-stability problems attendant to construe-
TRANSVERSE RANGES, SOUTHERN CALIFORNIA

FIGURE 4.—Fire-denuded hillslope in Hook Canyon watershed. Date of fire: August 24, 1968; date of photograph: July 10, 1969.

Major storms occurred in both January and February 1969.

tion will prevent any significant degree of urbanization or manmade change in most watersheds with 

relatively moderate to high erosion rates. In addition, chaparral is neither penetrable nor palatable 

to grazing animals when it occurs in the density 

normal to watersheds with high erosional potential. 

Thus, except for wildfire destruction of cover, no 

other changes in cover can be predicted, or are important, in areas that have high yields, as do many 

of the watersheds in the Transverse Ranges.

FLOOD HISTORY—THE 1969 STORMS

Major storms, like those of March 1938 and January and February 1969, have followed a generally

similar pattern that can be expected to recur in 

future major storms. The storm of January 18-27, 

1969, was typical: The circulation pattern and 

stagnation of a low-pressure center in the Pacific 

permitted intense streaming of northeastward-moving, moisture-laden air as a succession of storm 

fronts. Precipitation was light until January 19 

when intensity increased sharply. Heavy precipitation occurred throughout most of January 19-26, 

was interrupted by a brief respite on January 22, 

and then climaxed on January 25.

The January 1969 storm generally produced peak 

discharges equal to or greater than those of the 

1938 storm in Ventura County and western Los
THE ENVIRONMENT

Angeles County. The 1938 storm had been the greatest storm of recent times, at least since the legendary floods of 1862, and its peak discharges and sediment yields have been widely used as standards of 

flood magnitude and as criteria for impoundment 

structures. A much larger quantity of sedimentyield data resulted from the 1969 storm. Only eight 

debris basins in Los Angeles County were available 

for sediment measurements in 1938; more than 70 

existed in 1969. The suitability of January 1969 

storm yields as a basis for future planning of sediment-retention structures is discussed later in this 

report.

In both the 1969 storm periods, surface saturation 

occurred well before the most intense precipitation. Similar antecedent conditions existed during 

the 1938 storm and can logically be assumed to 

recur during any future major storm.

GENERAL GEOMORPHIC HISTORY

Surface form of the study watersheds reflects a 

Quaternary history that is unusual in its combination of complexity and youthfulness.

Tectonic activity was intermittent throughout the 

late Pleistocene epoch, especially following an interval of earlier Pleistocene marine deposition. The 

folded Pleistocene sedimentary deposits were uplifted and now border the Santa Clara River valley 

on the north. This period of mountain building 

produced much of the bedrock structure seen in the 

region, even in terrain of Mesozoic age. Putnam 

(1942, p. 712) believed that local summit levels 

record a significant episode of erosion following the 

middle-Pleistocene orogeny. Although this interpretation is unlikely, late Pleistocene time was marked 

by alternating periods of alluviation and valley cutting, shown by paired terraces which are j uxtaposed 

and incised at a variety of levels. The relative importance of tectonic activity and climatic change in 

the formation of these terraces is not known. Uplift was extensive and variable in intensity throughout the area, however, and was doubtless an important component in the process.

The latest period of alluviation was profound and 

probably corresponds with similar episodes of alluviation noted by many workers throughout the 

southwestern United States. In parts of the Calleguas Creek drainage basin, alluvium, including 

colluvaal or weathered-mantle materials, extends 

from stream channel to within a few feet of hillslope crests, forming a broadly rounded terrain 

with few rock outcroppings even though the 

weathered mantle is thin and soil development is

poor. In much of the study area, erosion has been 

too active or hillslopes steepened too rapidly by 

tectonic activity for evidence of former alluvial 

cycles to be preserved.

Channel entrenchment indicates that the latest 

period of alluviation has ended. Renewed incision 

has been caused, or at least accelerated, by historical changes in climate and land use.

Vertical-walled valley trenches reveal thick alluvium composed predominantly of silt-size sediment (0.004-0.0625 mm). This material is finer 

than the present or past bed material and represents 

a substantial source of sediment that will add to 

future sediment yields if the erosion cycle is further 

accelerated. Although the material is fine grained, 

it is largely noncohesive, and this property, combined with its occurrence in thick deposits adjacent 

to active channels, indicates a considerable and continuing potential erosion hazard.

WATERSHED FORM AND PROCESS

RELATION OF GEOMORPHIC HISTORY TO 

TECTONICS

The Transverse Ranges, named for their anomalous east-west trend relative to the rest of the Coast 

Ranges in California, reflect earth structure at the 

largest scale. Although the actual dynamics are unknown, the range system was formed by the interaction of an east-west oceanic fracture zone with 

the San Andreas fault, a continental transform fault 

marking a major plate boundary- One effect of the 

interaction of the two fault systems has been extensive and rapid local uplift. The episodes of alluviation reflect periods of greater or lesser intensity in the tectonic processes, in concert with 

changes in climate. Historical movement on major 

faults in the area illustrates the activity of the 

processes today and their effects on the study watersheds.

The dominating influence on uplift of the Transverse Ranges is a recently recognized, 110-mi (177- 

km) system of thrust faulting that extends along 

the fronts of the ranges from Ventura eastward 

along the Santa Clara River valley, across San 

Fernando Valley, and along the abrupt southern 

front of the San Gabriel Mountains. The eastern 55- 

mi (89-km) segment of the system is known as the 

Sierra Madre fault zone (Proctor and Payne, 1972, 

p. 220). Movement on this zone at the front of the 

San Gabriel Mountains has resulted in watersheds 

so rugged and youthful that some drainages near
10 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Glendora are little more than avalanche chutes 

(Scott, 1971, p. 242).

These extremely rugged watersheds are produced 

by extreme rates of uplift. In the newly completed 

Glendora water-diversion tunnel, a minimum of 

700 ft (213 m) of throw is observed along the fault 

at the point where the basement complex has been 

thrust over probable Holocene deposits (oral commun., R. J. Proctor, 1972). This apparent separation is equivalent to a probable rate in excess of 70 

ft (21 m) per 1,000 years. The attitude of the thrust 

at this point is one of shallow dip, but elsewhere 

the same zone is steeply dipping, thus probably accentuating rates of uplift.

The central part of the thrust complex, the San 

Fernando fault system, was responsible for the San 

Fernando earthquake of February 9, 1971. A sudden vertical displacement of more than 3 ft (0.9 

m) occurred in valley alluvium along the base of 

the westernmost San Gabriel Mountains. Several of 

the watersheds in this study occur on the uplifted 

block. An older scarp of the same magnitude on the 

same zone has been identified as probably dating 

from 1769 (written commun., M. G. Bonilla, 1972). 

A scarp 10 ft (3m) in height occurs in young alluvium east of Glendora and has also been correlated 

with the 1769 quake (Proctor and Payne, 1972, p. 

221).

West of the San Fernando Valley, the thrust system splays into a series of faults, the relations of 

which are poorly known. The San Cayetano thrust, 

the best known of the group, either bisects or underlies several of the study watersheds in Ventura 

County, including Mud Creek Canyon, Orcutt Canyon, Jepson Wash, Keefe Ditch, Pole Creek Canyon, 

Real Canyon, and Warring Canyon. The fault dips 

north along the north side of the central Santa 

Clara River valley, parallel in strike but opposed in 

dip to the Oak Ridge thrust on the south side of the 

valley.-So recent is movement on this pair of faults 

that Sharp (1954, p. 23) believed that the Santa 

Clara River valley was "primarily a fault valley 

and not a secondary product of erosion." The San 

Cayetano thrust is noteworthy for its recent activity, shown by displacement of oil-well casings in 

holes penetrating the fault zone, and faulted lateQuaternary terraces that have been back-tilted toward their source.

Watersheds in the Transverse Ranges have also 

been uplifted by folding. The Ventura River is 

antecedent at least to the extent that it has preserved its course across terrain being uplifted in the 

form of a broad flexure. The antecedence is shown

by the convex-upward longitudinal profiles of river 

terraces described by Putnam (1942, fig. 8). Warping of the terraces is coincident with a major anticline that is normal to the river course and that 

involves a large thickness of Pliocene and Pleistocene sedimentary rocks. Folding probably is continuing at the present.

Deposits of late Pleistocene age are in a steeply 

inclined attitude at many localities in the area, and 

are actually overturned at several—to within 50° 

of complete overturn in Orcutt Canyon, a study 

watershed. In fact, the structural deformation of 

deposits of the youngest geological time periods in 

the Transverse Ranges is without known parallel in 

North America.

It is clear, from the above evidence, that tectonic 

processes are generally important and in local areas 

exceed erosional processes in their role in landscape 

formation. The significance of this conclusion to 

erosional analysis is that, in such a steep area, 

gravitional force acts directly on sediment particles 

in various forms of mass movement, plastic flow, 

and viscous fluid flow, rather than indirectly 

through entrainment in normal stream runoff. Correlation of erosion rates would be impossible without assessment of factors reflecting the relative 

amounts of uplift between watersheds. It is, of 

course, the differences in tectonic activity in the 

recent geologic past, and not the possible future 

tectonic effects, that will most influence erosion 

rates within the time span with which this study is 

concerned.

MASS MOVEMENTS AND SEDIMENTATION PROCESSES

ROCKFALLS AND SLIDES

The downslope movement of sediment as individual particles is locally an important erosional process in the Transverse Ranges. Talus deposits, 

formed by rapid downslope movement of individual 

particles or dry or unsaturated masses of particles 

(granular flow may occur), form laterally to stream 

channels at the base of steep hillslopes. Frontal 

watersheds in the San Gabriel Mountains and those 

north of Ojai in Ventura County contain many such 

deposits, which are finer grained than the talus 

characteristically associated with igneous or metamorphic rocks. The process supplies detritus to 

stream channels, where it must await a major flood 

before further transportation can occur.

In most cases, talus accumulations are associated 

with either rockslides or debris slides (mass-movement terminology after Highway Research Board; 

Varnes, 1958). Where the bedrock is poorly con-
WATERSHED FORM AND PROCESS 11

FIGURE 5.—Mud Creek watershed viewed from the drainage divide. Large slumps are visible at left and on opposite hillslope. 

Terrace in foreground is the top of a slump unit abuting a basal plane of failure. Note intense erosion along toes of 

the slope failures.

solidated fine-grained sedimentary rock of the Pico 

Formation, as in the Mud Creek watershed (fig. 5), 

mass movement may take the form of a slump. Although the view of Putnam (1942, p. 727-728), 

that "nearly every square foot of surface on hillslopes underlain by upper Pico clay shale is in motion downslope, or has moved in the very recent 

geologic past" is somewhat extreme, so active is 

this type of sliding in this appropriately named 

drainage, that Mud Creek itself usually contains 

high concentrations of silt- and clay-size material 

at low flow. This sediment reflects the nearly continuous activity at the snouts of large slumps. The 

sedimentary bedrock yields much silt and clay when 

fractured during slumping, concomitant with development of permeability to allow water to percolate through the masses and remove the material 

and transport it to the stream.

Large-scale slumps with little downslope movement affect the bedrock of the study watersheds 

north of Glendora. Although contributing little 

sediment directly to the stream channels, the slope 

failures are an index of the extreme hillslope instability of these watersheds.

Soil slips, small slides that involve only material 

above the bedrock surface (Bailey and Rice, 1969, 

p. 172), are common throughout the Transverse 

Ranges. They are the most visible aftereffects of 

major storms like those in 1969. The saturated soil 

mantle fails and moves downslope to the drainage 

network where it supplies sediment directly to 

existing runoff. In some cases in 1969, movement 

continued downstream as a debris flow or mudflow. 

U.S. Forest Service studies in the eastern Transverse Ranges (summarized in Bailey and Rice, 

1969, p. 176) found erosion rates due only to soil
12 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

slippage of 81 ydVacre (153 m3 /hm2 ) in converted 

grass areas and 11 ydVacre (20.8 m3 /hm) in brush 

areas, measured during a single storm in 1965. Renewal of slippage in 1969 produced 14 times as much 

erosion in brush areas and 5.4 as much in grass 

areas due to this cause as occurred in 1965 (Rice 

and Foggin, 1971, p. 1496).

ROCK-FRAGMENT FLOWS

Dry granular flowage on slopes during the dry 

season is the dominant form of sediment transport 

on hillslopes in many watersheds along the front 

of the San Gabriel Mountains, in the Ojai area, and 

in the drainage of the Santa Clara River. This type 

of movement is the end member in the continuum 

of movement with individual particles at the other

extreme, and is a more poorly sorted form of the 

sand runs or gravel runs observed in aggregate 

processing. It is largely confined to particles of 

pebble size (4-64 mm) and smaller. The process is 

known locally as dry sliding (fig. 6) and has been 

quantitatively evaluated by Anderson, Coleman, and 

Zinke (1959) and by Krammes (1960) in cases of 

brush-fire denudation.

DEBRIS FLOWS

Where bedrock weathers, to particles predominantly coarser than silt size (0.004-0.0625 mm), 

change from slumps, rockslides, and debris slides to 

channelized debris flows may occur during movement. More commonly, these and other types of mass 

movement do not change directly to flows, but inFIGURE 6.—Eroded dry-sliding deposits in Englewild Canyon. Vertical depth of erosion during 1969 storms is indicated by the 

height of the pedestaled fragments. Date of photograph: July 9, 1969. Length of ruler is 6 in (15.2 cm).
WATERSHED FORM AND PROCESS 13

stead provide source material and act as triggering 

mechanisms for, or undergo transformation to, 

debris flows at the time of major storms.

Evidence of debris-flow movement was widespread following the 1969 storms throughout the 

Transverse Ranges. Debris flows occurred in the 

following study watersheds: Cozy Dell Canyon, 

Stewart Canyon, in a tributary of Senior Canyon, 

Orcutt Canyon, Jepson Wash, May Canyon (No. 1), 

La Tuna Canyon, Sierra Madre Villa, in a tributary 

of Little Santa Anita Canyon, Hook Canyon (East), 

Harrow Canyon, Englewild Canyon, and in several 

tributaries of Little Dalton Canyon.

Diagnostic features of debris-flow activity in 

small watersheds included fronts of coarse detritus, 

which formed transverse to the direction of flow, 

lateral levees of coarse (>2 mm), poorly sorted 

material along channel sides, and the characteristic 

form and texture of the depositional phases of the 

flows. Actual flows were occasionally observed where 

they debouched from mountain fronts. In most 

cases, however, debris flows were not directly observed, because of downstream dilution to less viscous flow before leaving the mountain front, or because of the presence of a two-phase flow in which 

normal water flow occurred as an upper layer to a 

debris slurry in the manner documented by Scott 

(1971, p. 247).

A number of interesting historical reports support the evidence of debris flow concomitant with 

flood discharges in larger drainages of the Transverse Ranges during major floods. Reports include 

the appearance of boulders at the surface of what 

seemed to be normal water discharge, and standing 

trees seen moving downstream. To this can be added 

the plight of an individual, submerged and pinned 

upright against his car by floodwaters, who was 

buried to his shoulders by a wavelike mass of sediment and who survived to describe the experience. 

Clearly, however, the process of debris flow is of 

markedly less quantitative importance the larger 

the drainage area and the more subdued the relief.

Evidence of debris flows or viscous surges of sediment-water slurry associated with storm runoff was 

also present in some larger watersheds. In 1969 

coarse fill occurred in downstream reaches of Sespe 

Creek (fig. 3), a 251-mi2 (650 km2 ) watershed. Evidence from buried vegetation indicates that the fill 

was continuous, both across the channel and to a 

point 0.8 mi (1.3 km) away from the mountain 

front. Fill ceased at that point as if a pronounced 

front, 4-5 ft (1.2-1.5 m) in height, existed across 

the channel. The front was subsequently cut by recession flow. Though the presence of such fronts is 

not total proof of debris flow (see Scott and Gravlee, 

1968, p. 20-22), when combined with the character 

of the deposits—the size, sorting, and continuous 

nature of the fill—the evidence is strong. Deposits 

of the filled reach differed from most of those farther downstream in that they were coarser and 

more poorly sorted.

So prominent and widespread was evidence of 

debris flows in the small watersheds after the 1969 

storms that it is possible to formulate a model for 

the dispersal of sediment in such watersheds: Lateral supply of sediment to stream channels is a relatively continuous process, accomplished during the 

dry season by dry sliding and in the wet season by 

mass movements and overland flow. During periods 

between major storms, stream channels undergo 

more-or-less time-continuous fill. Then, during a 

major storm, channel bed material is mobilized and 

dispersed in large part by debris flows, some of 

which are induced by mass movements triggered by 

the storm. Channels undergo much net scour, accomplished by incorporation of bed material in the 

debris flows and by scour during recession flow. 

Valley-side slopes are undercut by bank erosion, 

paving the way for a new cycle of channel infilling.

This model is probably most applicable to watersheds less than 5 mi2 (13 km2 ) in size. It is empirically obvious, but not quantitatively demonstrable, 

that debris flows account for the bulk of coarsesediment (>2 mm) dispersal in such watersheds. 

It is, of course, with catchment of the coarse sediment that debris-basin planning is most concerned.

MUDFLOWS

The modal class in size distributions of the 1969 

Glendora debris flows was the 8-16 mm range 

(Scott, 1971, table 2). In the continuum of flow 

types, as grain size decreases and when mud content (<0.0625 mm) exceeds 10 percent dry weight 

of the contained sediment, the muddy, viscous appearance of the mixture will usually result in designation as a mudflow. Although no widely accepted 

distinction between mudflows and debris flows 

exists, the limit of 10-percent mud used here 

achieves an adequate working distinction in the 

Transverse Ranges.

Mudflows occurred in 1969 where thick deposits 

of fine-grained alluvium became saturated near the 

end of each storm period. Because of the sealing 

effect of the finer sediment, mudflows were able to 

continue in movement well beyond the mountain 

fronts. Two such 1969 flows in the Transverse
14 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Ranges were described by Scott (1971, p. 246), but 

in neither case does a parallel situation exist in any 

study watershed. In contrast, loss of fluidity by infiltration halted most of the more coarsely grained 

debris flows in the fan channels within a short distance of mountain fronts.

Mudflows commonly occur in watersheds which 

are underlain by fine-grained sedimentary rocks and 

which have been burned by recent wildfire. Mudflows occurred in December 1971 in small watersheds of an area burned in October 1971 20 mi (32 

km) northwest of Ventura. Witnesses described 

these flows of surges of what appeared to be mud 

covered with water backed up behind a moving 

boulder front. High water marks of the storm period 

were formed by the initial surge of mud in each 

channel. Succeeding surges were of an equally short 

duration but were accompanied by less pronounced 

boulder fronts.

Only small, localized mudflows occurred in 1969 

in the watersheds shown in figures 2 and 3, probably because of a lack of recent burns in the susceptible watersheds. A few shallow slumps continued as mudflows, but in no known case in the 

study watersheds did movement progress much beyond the foot of the failed slope. However, a definite 

mudflow potential exists in the Mud Creek watershed mentioned above. Many active slumps bordering Mud Creek could change to rapid earthflows or 

mudflows after long periods of saturation or a 

change in ground-water conditions. Such flows could 

continue down channels and could be destructive to 

development downstream. There is also potential 

for channel blockage by a slump, followed by release of a surge.

RELATIVE IMPORTANCE OF SEDIMENT-TRANSPORT 

PROCESSES

The preceding emphasis on mass movement and 

related 'processes should not give the impression 

that movement of sediment as bedload or suspended 

sediment in normal stream runoff is necessarily of 

subordinate importance. The significant point is 

that, compared with sediment-transport processes 

operating on a year-to-year basis in these unstable 

watersheds (and in stable watersheds at all times) 

the processes operative in small watersheds in the 

Transverse Ranges in a major storm are markedly 

different. Many kinds of mass movements can be 

triggered, sediment may sluff from precipitous hillslopes at record-setting rates, and debris flows and 

mudflows may pour down steep canyon bottoms,

mobilizing and incorporating most channel-bed material in their path.

Throughout this discussion, emphasis has been 

placed on transport of the coarse sediment fractions 

which constitute the volumetric bulk of the sediment produced in these watersheds. Even during a 

major storm most of the silt and clay (<0.0625 

mm) and part of the sand (0.0625-2 mm) would 

continue to be transported as suspended sediment 

in flood discharge.

SLOPE FAILURES AS RELATED TO DEBRIS FLOWS 

AND MUDFLOWS

An inspection of all watersheds after the 1969 

storms showed that the frequency and size of debris 

flows in a given watershed was directly related to 

the amount of slope failure in the watershed. If this 

was the case, were the flows triggered in part by 

the slope failures ? This possibility was investigated 

by Scott (1971, p. 243-244) in watersheds above 

Glendora. In one place the relation was definitely 

established by tracing debris-flow levees to their 

point of origin. In others the relation was very suggestive but not beyond doubt. It was not doubtful, 

however, that slope failures were an important 

source of sediment to the flows, indicated by abrupt 

increases in height of debris-flow levees at the sites 

of slides. Conversely, some slope failures were 

triggered or enlarged by bank erosion by the flows.

The slope failures that generated some of the 

debris flows and accounted for surges in others 

were small and generally involved only surficial or 

highly fractured material. Many were soil slips. In 

the Glendora watersheds, these small slope failures 

were not directly related to the large slumps and 

block glides that were present in all the watersheds. 

However, the number and extent of these larger 

slope failures did appear to be related to watershed 

stability and thus indirectly to sediment yields.

HOW 1969 SEDIMENTATION PROCESSES RELATE TO 

DETERMINATIONS OF EROSION RATES

Study of the results of the 1969 storms in the 

Transverse Ranges led to several conclusions important to the determination of erosion rates (Scott, 

1973).

First, sediment-yield rates for design storms must 

be based on yields from actual major storms of 

similar recurrence interval. The noteworthy differences in the way mountain watersheds respond to 

major versus minor storms could result in actual 

major-storm rates being greater than would have 

been expected from an analysis based on data from 

lesser storms.
WATERSHED FORM AND PROCESS 15

Secondly, peak discharges based on indirect determinations of discharge in watersheds like the 

study watersheds should not be used as a variable 

by which to correlate sediment yields. There are 

several reasons to doubt the reliability of some discharge measurements in small watersheds after 

major storms. The possible presence of debris flow 

or mudflow at the time high water marks were 

formed, and the unknown degree of change in channel cross section after passage of a peak are two 

such reasons. Processes of scour and fill are especially variable and unpredictable in the heterogeneous sediment in which stream channels of semiarid regions are formed.

These observations lead to another important 

consideration. Sediment yields and channel processes are clearly time-evolutionary. Many of the 

channel changes and unusual processes would not 

have occurred, or would have been less pronounced, 

had not abundant bed material been available for 

transport in channels. Channels that before the 

storms were choked with detritus were scoured to 

bedrock by storm runoff and removal of bed material in debris flows (fig. 7), and such was probably also true during the 1938 storms. For equivalent conditions to produce equivalent sediment

Channel fill of interstratified 

debris -flow deposits, 

stream alluvium, end colluvium derived directly 

from adjacent hillslopes.

BEFORE MAJOR STORM

Debris-flow deposits

Upper, finer, water-borne 

layer; preserved evidence 

of bedforms.

Lower, coarser, debris-flow 

deposits; poorly sorted 

and unstratified. Modal 

class commonly in 8-16 

mm range.

AFTER MAJOR STORM

15-25 ft 

4.6-7.6 m

FIGURE 7.—Diagrammatic cross sections of typical bedrockchannel deposits in an area of high erosion rates in the 

Transverse Ranges.

yields, a similar quantity of bed material would 

have to be available. Consequently, sediment-yield 

studies from a storm closely preceded by another 

major storm may be of limited practical value. For 

this reason, sediment yields from the second 1969 

storm (in February) were relatively low, and the 

storm could not be used for correlation purposes.

Most importantly, it is obvious that erosion rates 

in the area are directly dependent on the geological 

youthfulness—the interplay of relief and credibility—of the terrain in each watershed. The following analysis of watershed characteristics is in part 

a search for a combination of variables that will 

assess the effect of watershed instability on sediment yields. Possibilities include geomorphic variables, measures of soil erodibility, or a variable that 

defines the areal extent of slope failures. This is 

particularly important in analysis of highly variable sediment yields, because in the type of analysis 

commonly used, multiple regression, most of the 

standard error generally is due to neglect of significant factors (or to incorrect model formulation), 

not to errors in measurement.

METHODS OF DATA ANALYSIS

Because of its quick and effective applicability to 

the problem (Shen, 1972), multiple linear regression was selected as the technique by which erosion 

rates would be correlated from one set of watersheds 

to another (see criticism of factor analysis by 

Matalas and Reiher, 1967). Basically, multiple regression creates a linear mathematical equation of 

the relation between a dependent variable, in this 

case sediment yield, and a group of explanatory 

independent variables, the watershed characteristics 

that control the amount of sediment eroded. Results of previous studies have shown that hydrologic events are most nearly linearly related to 

watershed characteristics with logarithmic transformation of all variables.

Many logical series and combinations of watershed variables were analyzed by means of a program for stepwise regression (Dixon, 1968). In 

this procedure, the independent variables are progressively added, with calculation of the regression 

equation, standard error of estimate, and effectiveness of the most significant independent variables 

as each is added in order of its significance. A variable may be significant at an early stage of the 

computation but may be deleted after other variables are added.
16 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

This step-by-step analysis allowed for rigorous 

study of the effects of the variables, their interaction with variables already entered, and the progressive changes in accuracy of the relations. Variables 

were grouped and ordered according to their logical 

effects on sediment yield.

Where independent variables are themselves correlated, regression coefficients can be unstable and 

most statisticians believe that correlation might be 

spurious. To reduce this possibility, a simple partial 

correlation matrix was prepared and, in the case of 

correlation between two similar variables, only that 

variable giving the greatest reduction in standard 

error was included. There are, however, those who 

believe that if, as in this study, the purpose is to 

derive a regression formula that predicts the dependent variable accurately rather than to interpret 

individual regression coefficients, bias due to intercorrelation may actually be advantageous (Snedecor 

and Cochran, 1967, p. 395).

No absolute cause and effect is necessarily implied by correlation of the variables. The method 

simply provides an optimum fit of selected variables 

in the form of a predictive equation where success 

is reflected in a measure of the accuracy of the relation—the standard error of estimate. Using the 

regression equations for prediction at a number of 

sites, two-thirds of the estimates would be within 

the range of the standard error from the true 

value, and 95 percent would be within a range of 

twice the standard error. The assumption of normality of the residuals is necessary for tests of significance, but not for other standard properties of regression estimates.

PHYSIOGRAPHIC CHARACTERISTICS

Many physiographic variables have been developed in attempts to define the shape of watersheds. 

The number is probably well in excess of 100, and 

many are claimed to relate watershed shape to 

erosion rate more significantly than simple measures like basin size and channel slope.

An opportunity existed in data compiled by Ferrell (1959) to test a number of these variables in 

the Transverse Ranges. Using a selected group of 

seven watersheds, each with three sediment accumulations, physiographic variables were correlated 

with 21 separate sedimentation events (table 1), 

including some resulting from the major 1938 

storm. Variables were regressed against sediment 

yield as the dependent variable. Discharge and a 

vegetation factor were included as additional independent variables. The discharges in these analyses are actual measurements in large part and are 

not subject to the uncertainties described in previous sections.

Two stepwise multiple regressions were computed 

to compare the relative ability of each physiographic variable to explain the residual error after 

two important variables with known significance 

had already been included:

1. Total sediment yield (Sy ) as the dependent variable regressed against peak discharge (Qc/s), 

the vegetation index (VI) of Ferrell (1959), 

and the group of physiographic variables. The 

result, after insertion of QCfS and VI, is

Log Sy = 3.720+ 0.921 log Qcfs - 1.267 log VI.

The standard error of estimate is 0.387 log 

units ( + 143, — 59 percent).

2. Sediment yield per square mile (<S/) as the dependent variable regressed against peak discharge per unit area (Qcsm ), vegetation index, 

and the group of physiographic variables. The 

result, after insertion of Qcam and VI, is:

Log S/ = 3.655 + 1.070 log QC8m - 1.488 log VI.

The standard error of estimate is 0.388 log 

units (+145, — 59 percent).

Considerable care in data selection was necessary 

to insure that basins in the analysis were similar 

in respect to variables not included, such as soil 

erodibility, lithologic type, slope stability, and urbanization. Calculation of the vegetation index involves a subjective weighting procedure after both 

cover density and subareas dominated by each of 

eight vegetation types are determined from aerial 

photography. It includes the effects of wildfire.

Physiographic variables in the analysis were confined to those found significant in correlations with 

erosion rates in southern California and similar 

areas (Ferrell, 1959; Lustig, 1965; Anderson and 

Wallis, 1965; Tatum, 1965; and Scott, Ritter, and 

Knott, 1968). Several potentially useful variables, 

such as Anderson's variable (1949b, p. 571), area 

of main channel of the watershed, were excluded because of inapplicability to small, steep watersheds 

of this study. Variables included are as follows:

Hypsometric-analysis index: The index is a measure 

of the distribution of land surface within a 

basin—the relation of horizontal cross-sectional 

drainage area to altitude; a plot of these variables 

defines the hypsometric curve (Langbein and 

others, 1947, p. 140). Relative height, h/H (altitude of a given contour above basin outlet/ 

basin relief), is plotted against relative area, a/A-
METHODS OF DATA ANALYSIS 17

(area in basin above a given contour/total drainage area). The index used by Tatum (1965, p. 

886) is the relative height at the point a/A = 0.5. 

Mean ground-slope angle (eg) : Measured by Ferrell 

(1959, p. 67) as the average slope between successive contours, corrected for the relative proportion of total area occupied by each contour interval.

Mean stream length: The sum of stream lengths of 

all orders divided by the total number of streams. 

Data were obtained from Ferrell (1959) who 

utilized only streams delineated on topographic 

maps. Future determinations of stream length 

should be made by the contour method of 

Morisawa (1957) in which the drainage net is 

sketched by inserting streams wherever V-shaped 

contours are present.

Total stream length (2L): Total length of all 

streams in the watershed. The factor is highly 

area dependent but should more accurately reflect the degree of watershed dissection, implying 

greater sediment yield, than simple drainage area.

Mean bifurcation ratio (Rb): The mean of the ratios 

of the number of streams of each order to the 

number of streams of the next higher order. Ferrell (1959, p. 68) weighted his data by multiplying each bifurcation ratio of each successive pair 

of orders by the total number of streams involved 

in the ratio. Strahler (1957, p. 914) noted that, 

although the bifurcation ratio seems a useful dimensionless number to define a drainage system, 

it is highly stable except where powerful geologic 

controls exist.

Transport-efficiency factor (Ti): Defined as T1 = 

Rb x 2L, the product of the mean bifurcation ratio 

and total stream length. Found by Lustig (1965, 

p. 18) to be the factor showing the best correlation with sediment yield of any of the physiographic variables in his study of reservoir sedimentation in southern California. It is an adjustment of total stream length to reflect the character of the drainage pattern.

Sediment-area factor (SA ) : Another of Lustig^s 

more significant factors; defined as SA = A/cos ?«,, 

the ratio of drainage area to the cosine of the 

mean ground-slope angle. As suggested by Lustig 

(1965, p. 13) the ground-slope angle was determined at 100 points in each watershed, in this 

study by means of a grid overlay.

Sediment-movement factor (SM) '• The product, SM = 

S^Xsin Og, of the sediment-area factor and the 

mean of the sines of the ground-slope angles

(Lustig, 1965, p. 17). In theory it is a measure 

of the downslope forces acting on the weathered 

mantle of a watershed.

Mean basin exposure: The azimuth of the average 

direction of slope in the basin Ferrell (1959, p. 

67) outlined all major slope exposures, determined the slope direction of each, and summed the 

values proportionally by size of area to obtain 

the mean value. The range of values in the sample 

watersheds is probably too small to show the real 

effects of this variable.

Elongation ratio (ER): A ratio produced by dividing the diameter of a circle with an area equal 

to that of the watershed by the maximum watershed length measured in a straight line parallel 

to the main channel.

Relief ratio (Rr): The ratio of watershed relief and 

overall basin length (see Schumm, 1956, p. 612). 

Studies of Schumm (1954, p. 217) indicated 

strong correlation of relief ratio with sediment 

yields from small drainage basins on the Colorado 

Plateau. Relief ratio was selected by Ferrell 

(1959) on this basis as the physiographic variable to be used in estimation of sediment yields 

in Los Angeles County.

Area under the area-altitude (hypsometric) curve: 

The area beneath the hypsometric curve previously described.

Mean channel slope: Average slope of the drainage 

path between the highest and lowest points in a 

watershed, measured along the main drainage 

channel.

Ratio of surface area to planimetric area: The relation between the surface area and the drainage 

area as planimetered from a map. Surface area 

is the product of the plane area between contour 

lines and the secant of the slope between the same 

two contour lines (Farrell, 1959, p. 67).

Drainage density (Dd) : The ratio of the sum of 

channel lengths to planimetric drainage area. As 

noted by Strahler (1957, p. 916), stream density 

apparently increases in watersheds of greater 

erodibility and reaches a maximum in areas of 

badland topography.

Standard deviation from uniform slope: A measure 

of the variation of the slope of the main channel 

from a straight line drawn from the headwaters 

of a stream to its outlet. It is apparently similar 

to the coefficient of variation of flowpaths (Wallis, 1965, p. 50).

Elongation ratio and the area under the areaaltitude (hypsometric) curve are the variables that
18 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

TABLE 1.—Summary of physiographic variables, discharges, and sediment data for selected watersheds in eastern Transverse

Ranges of Los Angeles County

[Data in part after Ferrell (1959) and Tatum (1965)]

Drainage basin T, . TT (Number corre- Dramage Hypsosponds with area «M*nc: 

location in , •* "lalysis 

fi., o\ (n»is) index ng. £t)

Shields Canyon (8) ____ 0.23 0.51

Pickens Canyon (9) __ 1.70 .47

Hall Beckley Canyon (11) _ .68 .50

Birmingham Canyon (13) - .17 .42

Sunset Canyon (14) __ .43 .48

West Ravine (20) _ __ .25 .40

Little Santa Anita Canyon 2.39 .48

(26).

Mean 

groundslope 

angle 

Wg 

(degrees)

39.4

39.4

41.1

34.0

38.7

£i 97 I .Of\

36.2

Length of 

streams of 

order /i 

(ft)

2£i= 262

2Le = 399

2Ls= 761

21,4= 3,315

2Li— 345

2L2 = 345

2Ls= 1,535

21,4 = 4,439

21,5= 2,301

2Li = 287

2Le = 416

21,3= 751

21,4 = 3,000

21,5= 3,050

2Li = 166

21,2= 295

SL3 = 487

21,4= 650

2Ls= 2,196

2Li= 296

21,2= 486

2Ls= 983

2X,4= 1,382

21,5= 420

2Li — 205

21,2= 362

2Ls= 552

21,4= 1,590

21,5= 2,603

2Li= 337

2Z«= 698

2Ls= 1.266

21,4= 3,982

21,5=10,800

n/r Total Mean _ „<. Sedi- JVLean . , .* port>- _„ „ . stream bifur- ^ mentstream iength cation e area

(ft) ^ raii° factor factor

318 0.90 6.58 5.92 0.30

423 1.72 6.35 10.92 2.20

346 1.42 5.26 7.47 .90

203 .72 6.94 5.00 .20

350 .68 5.65 3.84 .55

260 1.01 6.52 6.59 .27

425 3.24 6.32 20.48 3.06

Sedimentmovement 

factor. 

Sit

0.19

1.40

.59

.11

.34

.12

1.81

Mean 

basin 

exposure 

(azimuth 

in 

degrees)

187.2

206.6

201.6

210.6

222 3

202.5

169.0

Elongation 

ratio 

ER

0.593

.783

.751

.673

.841

J.672

.632

1 Value for ER was determined by Farrell (1959) from topographic maps older and of smaller scale than modern editions and is not comparable to 

value used in a fallowing analysis. Included for consistency of data sources in this analysis.

explain the greatest amount of residual variance 

after discharge and vegetation factors are included 

in the regressions.

Elongation ratio is seen in the following sections 

to correlate positively with sediment yield; that is, 

the less elongate a watershed the higher the sediment yield. This conclusion is compatible with an 

element of sediment-transport theory—the likelihood of a given particle being eroded from a basin 

is generally inversely proportional to its distance 

from the basin outlet. In California, however, other 

data suggest that a low elongation ratio (an elongate watershed) reflects a degree of structural instability, as in a valley cut in directionally sheared 

or faulted bedrock. The highest sediment yields from 

large drainage basins in California are from elongate fault-line valleys in the Eel River basin. Also, 

the highest recorded yields from very small watersheds occur in elongate basins in the San Gabriel 

Mountains (for example, the Rainbow Drive.watershed; Scott, 1971). Geologic assessment of local 

basins in the size range of those in this study indicates, however, that few, if any, contain fault-line 

valleys.

The use of area under the area-altitude curve as 

a variable has been criticized because it is based 

on a single value rather than the entire population 

of a statistic (Wallis, 1965, p. 45). Criticism of its 

use is logical for areas of constructional topography, 

such as the volcanic terrain of the Wallis example.

SOIL ERODIBILITY

A major control of erosion rate in a watershed 

normally is the erodibility of the soils in that watershed. Soil erodibility, in terms of differences among 

the azonal mountain soils of watersheds that have 

high erosion rates, is not generally a part of soil 

mapping. Routine soil survey involves the categorization of soils by characteristics of subsurface 

weathering horizons that may have little relation 

to the erodibility of the surface layer. Analysis of 

soil erodibility, therefore, must be a special study.

Mapping of mountain soils on the basis of erodibility of the surface layer is not feasible. A valid 

but less time-consuming approach must be substituted. The method used here is to key soil credibilities to lithologic types; that is, by using values of 

the erodibility of soils developed on each rock type
METHODS OF DATA ANALYSIS 19

TABLE 1.—Summary of physiographic variables, discharges, and sediment data for selected watersheds in eastern Transverse

Ranges of Los Angeles County—Continued

[Data in part after Ferrell (1959) and Tatum (1965)]

Area 

Drainage basin under _. Ratio of D . Standard 

(Number cor- Relief af^ude chan^l are^ age iton"

fig. 2) ( Dd ) i ' curve area \"<*> slope

(in 2 )

Shields Canyon 0.445 44.78 36.5 1.30 29.6 16.6

(8).

Pickens Canyon .268 50.80 21.0 1.31 21.8 14.8

(9).

Hall Beckley .307 49.07 25.0 1.33 31.5 16.6

Canyon (11).

Birmingham .278 45.81 24.4 1.21 51.6 20.3

Canyon (13).

Sunset Canyon .333 51.47 28.9 1.28 39.3 17.2

(14).

West Ravine (20). .309 36.08 23.6 1.13 42.4 22.0

Little Santa Anita .291 48.07 20.6 1.24 43.4 14.1

Cajiyon (26).

Vegetation

index 

(VI)

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1952:

1954:

1955:

1955:

1938:

1943:

1952:

1938:

1943;

1952:

1938

1943

1954

12.04

14.89

16.22

11.30

17.76

14.31

12.79

19.41

: 22.16

1.62

; 1.62

: 3.10

17.00

18.00

: 19.00

16.66

: 20.13

: 21.27

: 19.12

: 20.08

: 13.32

Peak discharge

Qc f, 

(ft«/s)

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1952:

1954:

1955:

1955:

1938:

1943:

1952:

1938:

1943:

1952:

1938

1943

1954

82

82

37

503

632

218

223

241

; 101

2

2

: 4

106

138

: 67

76

: 90

: 29

: 743

: 707

: 219

Qc,

«ftVs

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1952:

1954:

1955:

1955:

1938:

1943:

1952;

1938 :

1943:

1952:

1938

1943

1954

'//mi 2;

358

358

160

296

372

128

328

354

149

12

12

: 23

247

321

; 155

305

: 361

: 117

: 311

:296

: 92

Sediment yield

> (yd"3 )

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1952;

1954;

1955:

1955:

1938:

1943:

1952:

1938:

1943

1952:

1938

1943

1954

31,000

4,940

; 7,770

141,000

41,000

7,760

: 79,600

; 39,000

: 13,200

: 5,370

: 5,590

: 2,670

6,620

1,180

: 3,440

; 30,500

: 6,870

; 3,230

: 61,700

: 22,000

: 55,400

Sediment 

yield per 

unit area

(erosion rate) SH ' 

(ydVmi 2 )

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1952:

1954:

1955:

1955:

1938:

1943:

1952:

1938:

1943:

1952:

1938:

1943:

1954:

135,000

21,500

33,800

83,000

24,100

5,500

117,000

57,400

: 19,400

31,600

32,900

15,700

15,400

2,750

: 8,000

122,000

: 27,500

: 12,900

: 25,800

: 9,200

: 23,200

and proportioning these values to the percentage 

of exposure of that rock type, a mean value of soil 

erodibility for a watershed can be determined.

Such an analysis ties erodibility to only one of 

the soil-forming factors—parent material. Because 

of tectonic activity, however, the Transverse Ranges 

are the penultimate in youthful terrain where 

parent material is logically the dominant soil-forming factor. Other workers (Anderson, 1954; Andre 

and Anderson, 1961; and Wallis, 1965) have found 

that erodibility indices related to geology correlate 

well with erosion rates in areas where the factor of 

parent material is of lesser importance. Andre and 

Anderson (1961, table 3) showed that parent material explained a larger amount of the variation in 

erodibility between samples than did any other soilforming factor for soils of northern California.

Soil erodibility is measured by a variety of indices, most of them applicable primarily to agricultural soils. Most of the indices applied to mountain 

terrain are proportional to the silt content of the 

soils or to the degree of aggregation of fine particles 

into larger aggregates. Two measures of erodibility 

shown to correlate significantly with erosion rates

in previous studies in mountain areas were measured for each major rock type in the study area. 

They are defined as follows:

Dispersion ratio (DR): The ratio, expressed as a 

percentage, of the percentage of measured siltand clay-size particles in an undispersed soil to 

the percentage of the same sizes after dispersion 

(Middleton, 1930, p. 3)

Surface-aggregation ratio (SA): The ratio, expressed as a percentage, of the surface area of 

particles coarser than silt divided by the value of 

aggregated silt plus clay. Surface area is obtained 

by treating the particles as spheres with a specific gravity of 2.65 and assigning mean diameters 

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

silt- and clay-size particles minus the percentage 

measured before dispersion (Anderson, 1954, p. 

272). Basically, this index is the ratio between 

the amount of surface area needing binding in 

order to form a cohesive, erosion-resistant soil 

to the amount of cohesive fine material available 

to accomplish the binding.
20 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

' Silt and clay content of the soils was measured 

by hydrometer, using techniques suggested by 

Bouyoucos (1936, p. 225-226) and Anderson (1954, 

p. 277). Size distribution of the coarser-than-silt 

fraction in each sample was then determined by 

wet sieving.

The number of samples collected from each rock 

type was proportional to the variability in lithology 

in each of the geologic units. For example, the 

metamorphic assemblages (pK), alluvium (Qal), 

and terrace deposits (Qt) were judged to be highly 

variable, and multiple samples of soils from these 

units were collected in both Ventura and Los 

Angeles Counties. Each sample consisted of three 

penetrations of the top 6 in (152 mm) of the surface layer with a coring device. At least three samples were collected per unit under conditions as 

nearly uniform as possible—natural vegetation on 

slopes of 20-30 percent facing south or southwest.

Both the dispersion ratio and surface-aggregation ratio were included in the final analysis. The 

values (table 2) compose a set of reference data 

that can be used to quantify soil erodibility throughout the area. The data are, of course, applicable 

only to the azonal soils of mountain and foothill 

areas.

SLOPE FAILURE

The extent of slope failure was calculated as the 

proportion of the drainage area underlain by failures in each of the 72 study watersheds. Talus and 

dry-sliding deposits were excluded; slides, slumps, 

block glides, and soil slips were included. Calculations were made with grid overlays on aerial photographs and geologic maps. Where landslide maps of 

adequate accuracy and scale existed, as in parts of 

the San Gabriel Mountains (see references in Morton and Streitz, 1969), this mapping was used as a 

standard for the delineation of map units. Elsewhere, criteria used for the field recognition (fig. 

8) and aerial-photograph interpretations of slope 

failures were those outlined by Ritchie (1958) and 

Liang and Belcher (1958).

The methods for determination of this variable 

are by nature subjective. Nevertheless, this objection is negated by applying the same standards to 

all the watersheds. Extension of the analysis to additional watersheds in the western Transverse 

Ranges should first involve study of maps or photographs of basins that have a large number of landslides and for which values of the slope-failure 

variable are given in this report. Watersheds in the 

Glendora area (East Hook Canyon, Harrow Canyon,

or Englewild Canyon), for which published or available maps exist (see references in Morton and 

Streitz, 1969), are ideal for this purpose.

Simple correlation of the slope-failure variable 

with the two retained physiographic variables and 

most of the other physiographic variables was at 

a relatively low level. It is possible that a slopefailure variable is a more sensitive indicator of 

erosion rates in areas where such rates are high. 

The logic (in addition to statistical significance) 

behind inclusion of this variable in the final analysis 

is established by the close association of extensive 

mass movement and debris-flow transport and, in 

turn, between debris-flow transport and high erosion rates.

HYDROLOGIC FACTORS

The selection of hydrologic factors related to erosion rates in southern California has been limited 

only by the number of studies. Therefore, the approach selected is in part pragmatic; as in the case 

of physiographic factors, variables selected will be 

those that correlate most significantly with actual 

sediment yields in the area.

Erosion rates are of greatest practical value if a 

recurrence interval can be established for majorstorm contributions. Unfortunately, the recurrence 

intervals of rainfall, discharge, and sediment yields 

rarely coincide. Because sediment records are much 

too sparse for determining actual recurrence frequencies, a precipitation or runoff variable for 

which a more reliable recurrence interval can be 

determined must be used to compare major-storm 

sediment yields. Values of variables for a 50-year 

design storm can then be compared with data for 

a storm like that of January 1969. The uncertainty 

in relating sedimentation, discharge, and rainfall 

frequencies is understood to apply but is pragmatically necessary.

PRECIPITATION

The true total rainfall over a basin is difficult to 

assess because of the interaction of orographic effects, wind, temperature, evaporation, geographic 

alinement of watersheds, and other basin characteristics. Extreme local meteorologic differences can, 

however, be negated by regionalization of data. 

Also, uncertainties will exist when a frequency is 

assigned to any actual storm, even when data are 

reliable. The true return period is not statistically 

established because of the short period of record 

at Transverse Range stations. Thus, a theoretical 

50-year precipitation may not be representative of
METHODS OF DATA ANALYSIS 21

TABLE 2.—Dispersion and surface-aggregation ratios as related to all geologic units in debris-producing areas of Ventura 

County and in selected drainage basins in Los Angeles County. Map symbols are those used by Kundert (1955)

Dispersion 

ratio (DR)

Surfaceaggregation 

ratio (SA)

Lithologic type Map 

symbol

Mean

Confidence 

liimts at

0.95 

probability

Mean

Confidence 

limits at

0.95 

probability

Quaternary alluvium—highly variable, poorly sorted sand

and gravel in or near modern stream courses ——————— Qal 56 

Quaternary terrace deposits—poorly sorted sandy gravel 

capping uplands or lateral to and above modern stream 

courses _____________________-_-___ Qt 50

Pleistocene nonmarine deposits—predominantly lacustrine

silt and clay with a few gravel lenses __— —— — —— Qc 57

Pleistocene marine deposits—medium- to well sorted sand 

and silt with thin intercalations of gravel; well exposed 

in foothills behind Ventura _________________— Qm 33 

Pliocene nonmarine sedimentary rocks—poorly indurated 

siltstone, sandstone, and minor conglomerate; erodes to 

badland topography, indicating areas of high sediment 

yield _________________________________ PC 53 

Middle and lower Pliocene marine sedimentary rocks— 

well-bedded siltstone, shale, and sandstone with conglomerate lenses; one of the predominant rock units underlying the Santa Clara River valley; locally synonymous with 

the Pico Formation, discussed in section on mass movements ________________________________ Pml 48 

Upper Miocene nonmarine sedimentary rocks—siltstone,

shale, sandstone, and conglomerate ______„___— Muc 44

Upper Miocene marine sedimentary rocks—thinly bedded 

siltstone, shale, and sandstone; better indurated than any 

younger rocks ________________________ Mu 52

Miocene volcanic rocks—intrusives and flows of variable

composition with agglomerates, tuffs, and breccias ____— Mv 52

Middle Miocene nonmarine sedimentary rocks—similar

lithology to Muc ________________________ Mmc 40 

Middle Miocene marine sedimentary rocks—lithology similar to Mu; a major rock type throughout Ventura County Mm 41 

Lower Miocene marine sedimentary rocks—lithology generally similar to Mm, Mu; more shale than in younger 

marine units __________________________ Ml 46 

Oligocene nonmarine sedimentary rocks—-generally thinbedded shale and sandstone distinguished by colorful 

shades of red and yellow; a major rock unit in western 

Ventura County _______________________ 0c 55

Eocene marine sedimentary rocks—interbedded shaly siltstone, and sandstone; sandstone predominates throughout 

much of the unit and forms resistant ridges with small 

potential for sediment yield _________________ E 34

Paleocene marine sedimentary rocks—poorly exposed shale

and sandstone __________________________ Ep 30 

Upper Cretaceous marine sedimentary rocks—massive indurated sandstone with minor interbeds of shale; moderately resistant with low erosion potential ________ Ku 38

Granitic rocks of Los Angeles County, age indefinite—mas*- 

sive intrusives of intermediate composition; high degree 

of deuteric alteration and postintrusion faulting and 

fracturing create potential for high sediment yields ___ gr 36 

Granitic rocks in Ventura County, of Jurassic age—lithologically similar to gr of Los Angeles County; less altered 

and fractured than equivalent rocks in Los Angeles 

County ________________________________ Jgr 40 

Pre-Cretaceous marine metasedimentary rocks of Los 

Angeles County—an assemblage of phyllite, schist, and 

gneiss, commonly containing small intrustives; high degree of alteration and fracturing create potential for 

high sediment yields _____________________——— pK 47

Pre-Cretaceous marine metasedimentary rocks of Ventura 

County—gneiss predominates in an assemblage also including phyllites and schists; a few small intrusives; less 

altered and fractured than equivalent rocks in Los 

Angeles County -_____________-______ pK 36

±18

±10

±6

±2 

±4

±5 

±3

±3 

±6 

±5 

±1

±3

±4

±2 

±10

±6

±1

±5

±4

132

111

103

103

121

79

69

66

140

66

62

70

75

77

57

59

121

108

139

145

±76

±27 

±25

±11 

±12

±8 

±9

±9 

±42 

±18 

±10

±8 

±12

±6 

±26

±35

±42

±30

±43
22 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

FIGURE 8.—Typical Transverse Range slump showing features used to identify slope failures, from top right to bottom left: 

Crown of slide (A); main scarp (B) ; transverse cracks (C) ; toe of slide (D). Road at left is offset along a longitudinal 

failure zone within the slump.

an actual storm, whereas a variable that expresses 

actual storm precipitation will be a valid physical 

descriptor, will be meteorologically realistic, and 

still be representative of a major storm, such as 

that of January 1969.

Any inventory of precipitation requires a knowledge of the meteorologic conditions that prevail during the corresponding periods of high discharge. It 

has been suggested that each storm is an entity in 

itself and will not be repeated; however, one storm 

pattern can be distinguished repeatedly in southern 

California coastal regions—the extratropical cyclonic storm previously described that develops off 

the Pacific coast and causes major floods as the storm 

moves inland. All major erosion-producing storms 

in the Transverse Ranges can be assumed to be of

this type. Comparable storms can be assigned a 

probable frequency and magnitude. Although large 

variations in rainfall occur and measurement deficiencies exist, storms of this type can be fitted 

reasonably well by a generalized isohyetal pattern. 

This pattern in January 1969 was found to coincide 

closely with the pattern of mean annual precipitation. Such coincidence should be expected because 

mean annual precipitation is influenced by the same 

factors affecting the major storms and a large proportion of the mean annual precipitation results 

from storms of this type.

Rainfall variables in the analysis included January 18-27, 1969, total-storm precipitation; 24-hour 

maximum January 1969 precipitation; 50-year 24- 

hour maximum precipitation; 50-year, 1-hour pre-
METHODS OF DATA ANALYSIS 23

cipitation; and mean annual precipitation. Values 

were obtained for each drainage basin from isohyetal maps prepared by local agencies, the National 

Weather Service, and the Geological Survey. Definition by the various sources was consistent throughout the region except for the Ojai area of Ventura 

County, where values of the 50-year 24-hour maximum and the 1969 24-hour maximum rainfall were 

poorly denned.

Mean annual precipitation was evaluated for each 

drainage basin on the basis of 90 years of record 

in Los Angeles County and 50 years of record in 

Ventura County. A comparison of map values of 

mean annual precipitation and the averaged yearly 

figures obtained from the National Weather Service showed good agreement.

Precipitation variables were studied to determine 

what intensities and durations would provide the 

best correlation. Sediment yields from several 

storms were plotted graphically against different 

precipitation variables, with corrections to eliminate 

the effects of watershed burns. The results indicated 

that 7- to 10-day total precipitation and 24-hour 

rainfall intensity are critical factors. The factor selected, K, is defined as 10-day x (24-hour precipitation) 2 and is analogous to the discharge function of 

Nelson (1970). The variable K is a measure of 

antecedent conditions as well as the peakedness of 

rainfall during the period in which sediment contributions by the unusual modes of transport described in previous sections are highest.

Frequency analyses of several long-term precipitation records were then evaluated (table 3) to determine a corresponding storm with a 50-year frequency of occurrence to adjust part of the 1969 

data.

PEAK DISCHARGE

A high correlation between sediment discharge 

and peak flow can be expected to exist. Sediment 

data from several large drainages in southern California indicate that 87 to 99 percent of the coarse 

material is transported during 1 percent of the 

time (written commun., C. G. Kroll, 1973). A comparison of 50-year and January 1969 peak discharges will aid in determining the degree of usefulness of the 1969 storm as a possible design storm.

Nelson (1970) has shown good results in relating 

annual suspended-sediment discharges to a variable 

that includes both annual runoff and annual peak 

discharge. Unless actually measured, however, peak 

discharge and annual runoff are extremely difficult 

to estimate for small basins like the study watersheds. An additional complication is that there is

TABLE 3.—Depth-duration comparison of 50-year precipitation 

and nearest actual precipitation at stations in the Transverse 

Ranges

Depth (in) and K factor

Station Duraname tion

Ojai 

Santa

_ _ ___ _ _ Monthly _- 

10-day __ 

24-hour — 

K .__...._

Paula —— Monthly — 

10-day __ 

24-hour __ 

K

Fillmore _ ___ _ Monthly _ 

10-day __ 

24-hour __ 

K

Santa Barbara -.Monthly — 

10-day ____ 

24-hour ___ 

K

Ventura _ __ -...Monthly _ 

10-day __ 

24-hour ...

K __ _. .

Mount Wilson ..10-day __ 

24-hour __ 

K ... ___

50- 

year 

precipitation 1

23.3 

20.6 

8.12 

1,270 

16.9 

14.7 

6.03 

480 

19.9 

18.0 

6.52 

690 

16.9 

14.0 

5.41 

362 

14.3 

11.1 

4.70 

216 

34.3 

13.8 

6,010

Nearest 

actual 

precipitation 

(year)

23.8 

20.1 

8.0 

1,430 

17.2 

14.7 

5.09 

404 

22.1 

20.53 

6.40 

705 

17.2 

13.3 

5.53 

388 

14.0 

12.7 

4.62 

202 

35.3 

12.0 

5,080

(1969) 

(1914) 

(1969) 

(1969) 

(1962) 

(1962) 

(1938) 

(1969) 

(1969) 

(1969) 

(1952) 

(1969) 

(1916) 

(1969) 

(1943) 

(1943) 

(1907) 

(1962) 

(1943) 

(1956) 

(1969) 

(1969) 

(1969)

January 

1969 

precipitation

23.8 

22.3 

8.0 

1,430 

17.6 

16.1 

5.01 

404 

22.1 

20.5 

5.86 

705 

15.6 

13.3 

3.96 

208 

8.7 

7.8 

2.75 

58.9 

35.3 

12.0 

5,080

1 Values obtained fi'om frequency curve.

no universally accepted method to estimate discharges in this area. The Pearson type III probability distribution and multiple regression have 

some acceptance. For example, Benson (1964) used 

multiple regression to relate peak discharge of a 

given frequency to assorted variables in order to 

estimate a flood-frequency curve. Anderson (1957) 

summarized a number of studies in which regression analysis was used to relate peak discharge to 

watershed variables.

Peak-discharge equations have been developed in 

southern California, but most have a limited range 

of application or are too general to define 1969 peak 

discharges for small watersheds. The 50-year peakdischarge equation with the smallest standard error 

(Crippen and Beall, 1970, p. 32) is:

Q5o = 40A°- 80MAP0- 85

where A is drainage area, in square miles, and MAP 

is mean annual precipitation, in inches. The standard error of estimate is 0.243 log units ( + 75, -43 

percent). Table 4 indicates the applicability of this 

equation to gaged watersheds in Los Angeles and 

Ventura Counties.

Combinations of variables were applied in this 

study to 89 gaging stations in southern California 

to determine the 1969 peak discharge at ungaged 

sites. The equation with the smallest standard error 

is:

Qcfs = 0.453A°- 84P°-"MAP1 - 22

where P is the January 18-27 storm precipitation, 

in inches, and other variables are as previously defined. The standard error of estimate is 0.265 log
24 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

TABLE 4.—Fifty-year peak discharges at selected stream-gaging stations in Los Angeles and Ventura Counties

Station 0 , ,. JJQ Station name

11-0805__ _ East Fork San Gabriel River near Camp Bonita _ 

11-0840 _ Rogers Creek near Azusa

11-0845 Fish Creek near Duarte _ _ _ _ _ ___

11-0865 __ Little Dalton Creek near Glendora __ _

11-0930 -Pacoima Creek near San Fernando

11-0980 _ Arroyo Seco near Pasadena _ _ _

11-1000 __ Santa Anita Creek near Sierra Madre _ _

11-1005 Little Santa Anita Creek near Sierra Madre __ _

11-1010 _ Eaton Creek near Pasadena _ _ _ __ _

11-1105 _ Hopper Creek near Piru __ _ _

11-1115 _ Sespe Creek near Wheeler Springs _____

11-1130 Sespe Creek near Fillmore _ _ _ _

11-1135 _ Santa Paula Creek near Santa Paula

11-1160 __ North Fork Matilija Creek at Matilija Hot Springs _ 

11-1175 _ San Antonio Creek at Casitas Springs _ _

11-1180 _ Coyote Creek near Ventura __

11-1185 Ventura River near Ventura _ _ _

Drainage

area 

A 

(mi 2)

85.0

6.6

6.4

2.7

28.5

16.6

9.7

1.8

6.5

18.0

23.6

49.5

251.0

40.0

15.6 

51.2

41.2

188

Mean 

annual 

precipitation 

MAP 

(in)

31

30

30

29

25

28

33

35

29

23

22

29

28

28

31 

23

25

27

Values of 

50-year 

peak 

discharge 

(frequency 

curve) 

(ftVs)

32,300 

2,920

2,810

1,870

9,140

7,530

5,010

362

1,910

8,650

9,950

14,800

54,300

19,100

7,920 

24,700

19,900

61,350

Calculated 

50-year peak 

discharge from 

equation of 

Crippen and 

Beall (1970) 

(ftVs)

21,700

3,210

3,160

1,560

9,000

6,430

4,820

1,340

3,120

5,805

6,940

15,900

56,500

13,000

6,670 

21,400

12,100

43,500

units ( + 84, -46 percent). The range of data used 

was: area, 4.65-644 square miles; mean annual precipitation, 10-35 inches; storm precipitation, 7.5- 

35 inches; and peak discharge, 280-68,800 cubic 

feet per second.

It was assumed that, if precipitation of the depth 

and duration experienced in the Transverse Ranges 

in January 1969 was repeatable in nature, the general equation for 89 stations might apply to a similar storm, the March 1938 storm. The results from 

24 stations with 1938 data were in error by an average of 61 percent. Table 5 compares 50-year peak 

discharges determined from frequency curves to 

1938 and 1969 peak discharges.

Final analysis of the hydrologic variables showed 

that neither 50-year nor January 1969 peak discharge as determined by multiple regression was

significant in estimating 1969 sediment yields. This 

conclusion may result from a bias population of 

high discharges. It may also result from the large 

initial error in the determination of peak discharge.

TOTAL TIME OF CONCENTRATION

The time required for water to flow from the most 

remote part of a basin to the outlet is the total time 

of concentration. Although the factor has numerous 

definitions and may not be precisely measurable, it 

can be useful if measured with a consistent procedure. It is a variable that may be helpful in explaining the hydraulics of channel networks.

At least nine definitions of time of concentration 

exist (Espey, Morgan, and Masch, 1966, p. 39), 

most of which employ a graphical measure of the 

time delay between the rainfall hyetograph and the

TABLE 5.—Comparison of 1938 and 1969 peak flow to the 50-year peak flow in Ventura County

S NoTM Station name

11-1085- — -Santa Clara River at Los Angeles-Ventura 

County line. 

11-1096 _ -Piru Creek above Lake Piru ___ ______

11-1100 _ _Piru Creek near Piru __ _ __ __

11-1115 _ Sespe Creek near Wheeler Springs _ _

11-1130 __ Sespe Creek near Fillmore _ _

11-1135 _ Santa Paula Creek near Santa Paula __

11-1145 — --Matilija Creek above reservoir, near Matilija 

Hot Springs. 

11-1155 __ Matilija Creek at Matilija Hot Springs __ 

11-1160 __ North Fork Matilija Creek at Matilija Hot 

Springs. 

11-1175 —— San Antonio Creek at Casitas Springs ____

11-1185 _ Ventura River near Ventura

Drainage 

area 

A 

(mi2 )

644

070

437

23.6

49.5

251

40.0

50.7

54.6 

15.6

51.2 

13.2

41.2

188

1938 peak 

discharge

(ftVs)

35,000

35,600

8,000

56,000

13,500

15,900 

5,580

11,500

39,000

Ratio to 

50-year 

peak 

discharge

0.68

1.04

.80

1.03

.71

.69

.70

.58

.64

January 

1969 peak 

discharge 

(ftVs)

68,800 

20,800

8,400

9,700

60,000

16,000

19,600

20,000 

8,440

16,200 

8,000

58,000

Ratio to 

50-year 

peak 

discharge

0.76 

.41

.84

.66

1.10

.84

.54

.87 

1.06

.66 .92'

.95
METHODS OF DATA ANALYSIS 25

runoff hydrograph. The definition of the Los Angeles 

County Road Department (unpub. data, 1969), 

when compared with observed velocities and indirect determinations of discharge in larger basins, 

is the most adequate in the Transverse Ranges. It is

/L3 y- 2 

TC = Z( - ) 

\HJ

where Z is a land-use coefficient; L is watershed 

length, in feet; and H is watershed relief, in feet.

FIRE EFFECTS

No analysis of erosion rates in southern California is complete without consideration of the effects of wildfires. The serious nature of the problem 

is indicated by the fact that watersheds discussed 

in this report, which are in national forests, have a 

Federal fire-control budget per unit area more than 

10 times the national average. And for the steep, 

fluelike mountain basins that debouch directly into 

habitated areas and therefore are in direct contact 

with human activity the costs are even higher.

The fires are a function of the characteristic 

summer dry season and dry fire-fanning winds 

caused by periodic reversal of the normal onshore 

flow pattern. Chaparral is rich in flammable resins 

and waxy leaf coatings which are consumed with 

an intensity that has been described as one of the 

most difficult wildland fire-control problems in the 

world. Although fires of human origin have tripled 

in 15 years, the overall rate of watershed burn has 

remained relatively constant in Los Angeles County 

since 1907 at about 1 percent per year, with a recurrence interval at an average point in brush cover 

of approximately 26 years. Improvements in firefighting technology have thus far kept pace with 

an increased number of fires by more effectively 

limiting their spread.

Existing fire records for the basins studied outside of national forests in Ventura County are of 

too short an interval (1963 to date) to define an 

accurate burn rate. Compilation of all data from 

this short period in the study watersheds indicated 

an average annual burn rate of 0.42 percent. Only 

three of the watersheds were touched by the widespread burns of September 1970. Longer records 

within national forest areas indicate that a substantially higher rate, above 1.00 percent, is more 

applicable.

A definitive study of the effects of wildfires on 

erosion rates by Rowe, Countryman, and Storey 

(1954) established relations between estimated discharges and sediment yields for many drainage 

basins in southern California. Tatum (1965, p. 888) 

used some of these data to obtain a general relation defining the progressive reduction of sediment 

yields in the 10-year interval following a burn. This 

relation, based upon data from flood-control reservoirs, indicates a ratio of the yield in the year following a total-watershed burn to the yield 10 or 

more years following the burn of 34.5. Records in 

debris basins in the study area show that this ratio 

is too high for the smaller basins for which sediment yields must be determined in the western 

Transverse Ranges. Unpublished U.S. Forest Service data (Rowe, Countryman, and Storey, 1949) indicate that the ratio ranges at least from 11.9 to 

35.0 for watersheds in the western Transverse 

Ranges in Ventura County.

This variability in response to burns led to the 

decision to include a fire factor as a separate independent variable, rather than apply a correction 

factor to data for either totally burned or unburned 

conditions. The effect of burns was approached in 

the following manner: For all watershed burns occurring during the 12 years preceding the storm 

catchment being analyzed, the percentage of nonrecovery of vegetative cover (100 percent of recovery) was multiplied by the percentage of watershed burned. This gave a variable correlating positively with erosion rate, with a theoretical range of 

0-100, and with a range of 0-88 in the watersheds 

with known erosion rates. Recovery rates were calculated from prorated area of burn and from vegetation type with relations developed by Horton 

(U.S. Forest Service, 1953, p. 8 and table 6) ; Rowe, 

Countryman, and Storey (1954, fig. 1) ; and Ferrell 

(1959, fig. E-2).

Watersheds with a fire factor of 88 included several with a 100-percent burn only 6 months before 

the January 1969 storm. Only a total burn immediately followed by a storm would yield an index of 

100.

The effect of fire is highly variable. Seasonal timing of both burns and storms, vegetative type, and 

the changing technology of fire- and erosion-prevention measures will all interact to prevent exact 

forecasts of burn effect on erosion rates. By using 

the results of previous studies of fire effects on discharge and yield of suspended sediment (see references by Anderson and Wallis, 1965), however, it 

should be possible to make a meaningful forecast 

based on the size of the burn and the time interval 

between burn and storm.

COVER AND LAND USE

The effect of vegetation type and cover density 

was assessed by means of variables indicating the 

proportion of drainage area in grass, in brush, and
26 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

TABLE 6.—Data for selected drainage basins with January 1969 sediment yields in Los Angeles and eastern Ventura County

Drainage basin (number 

corresponds with location 

in fig. 2)

Bell Canyon _ __ _ _ __

Limekiln Canyon _ — __ __

May Canyon (No. 1) _____

May Canyon (No. 2) _____

Rowley Canyon __________

Bluegum Canyon _ _ _ _

Blanchard Canyon __ ___

Snover Canyon _ _ ____

La Tuna Canyon ________

Childs Canyon ___________

Hillcrest Canyon _________

Deer Canyon ____________

Nichols Canyon _____ __

Lincoln Canyon _ ________

West Ravine ____________

Kinneloa Canyon (West) _

Kinneloa Canyon __ _ ___

Sierra Madre Villa __ ____

Bailey Canyon _ _ _______

Auburn Canyon _ _ __ _ ___

Little Santa Anita Canyon

Lannan Canyon _ _______

Bradbury Canyon _______

Spinks Canyon __________

Maddock Canyon ________

Hook Canyon (East) ____

Harrow Canyon _ _ ____ _

Englewild Canyon _______

Little Dalton Canyon ____

Morgan Canyon _________

Wildwood Canyon _____ _ _

Emerald Canyon ________

Drainage 

area 

A 

(mi 2)

_(!)__ 7.00

_(2)__3.69

_(3)__ .70

_(4)__ .09

_(5)__ .58

_(6)__ .19

_(7)__ .50

(10)__ .28

(12)__ 5.34

(15)__ .31

(16) __ .35

(17)__ .59

(18)__ .94

(19)__ .50

(20)__ .25

(21)__ .16

(22)__ .20

(23)__ 1.46

(24) __ .60

(25)__ .19

(26)__ 2.39

(27)__ .25

(28)__ .68

(29)__ .44

(30)__ .25

(31)__ .18

(32) __ .43

(33)__ .40

(34) —3.31

(35)__ .60

(36)__ .65

(37)— .16

Geologic units 

, exposed in 

drainage basin 

Symbols from 

Kundert (1955) 

Listed in order 

of predominance

Ku, Mm, Mu

Mm, Qc, Qt,

Mu, Qal

pK, Qc

Qc, pK

gr, Qal

gr, Qal

gr, Qal

gr, Qc, Qal

pK, gr, Qal

gr, Qal

gr, Qal

gr, Qal

gr, Mm, Mv,

Ku, E

gr, Qc, Qal

gr, Qal, Qc

pK

pK

gr, pK, Qal

gr, pK, Qal

gr, pK, Qal

gr, Qal

gr, pK, Qc, Qal

gr, Qal

gr, Qal

gr, Qal

gr, Oal

gr, Qc, Qal

gr, Qc, Qal

gr, Qc, Qal

Mv, Qc, Qal, Qt

Mv, Mm, Qc, Qal

Mv, Qc, Qal

Slope 

failures 

SF 

(acres/mi 2 )

19

10

32

20

43

55

55

51

19

49

20

39

7

53

44

63

43

61

32

15

72

30

42

19

22

165

82

70

114

80

20

15

Elongation 

ratio 

ER

0.727

.485

.656

.521

.646

.478

.654

.712

.795

.556

.804

.810

.664

.798

.495

.564

.567

.593

.575

.547

.632

.513

.705

.619

.705

.704

.974

.768

.548

.723

.650

.370

Surface 

aggregation 

ratio

SA

61

83

136

120

110

108

108

108

127

108

108

108

94

108

108

139

139

127

115

114

109

112

108

108

108

108

109

109

109

137

111

138

Dispersion 

ratio 

DR

40

48

50

53

41

40

40

41

45

40

40

40

41

42

41

47

47

46

42

42

40

41

40

40

40

40

42

41

41

52

48

48

Grass 

cover 

1968 photographs 

(acres/ mi 2 )

305

340

170

145

92

105

114

89

120

95

87

92

40

37

33

62

60

58

72

72

60

53

63

58

49

30

29

31

40

150

135

202

Brush 

cover 

1968 photographs 

(acres/ mi 2 )

113

66

274

315

272

399

425

445

373

430

444

406

215

450

430

421

433

410

440

445

412

463

455

460

453

397

375

386

450

380

295

375

Barren

1968 

photographs 

( acres/ mi 2 )

33

25

66

73

80

70

73

62

28

40

21

53

21

82

70

88

63

90

53

52

87

44

43

35

35

105

97

110

83

30

20

15

under barren conditions. Values were determined 

for the group of watersheds in Los Angeles County 

by means of grid overlay of aerial photographs 

taken late in 1968 so as to reflect conditions at the 

time of the January 1969 storms. All these variables 

were dropped from the final analysis because of a 

lack of statistical significance. The lack of significance logically was due to greater importance of 

the 'fire factor in determining cover density and 

erosion rates.

The chief change in land use, both observed and 

expected, is transfer of natural terrain to urban 

development. Lower parts of a number of the study 

basins in the eastern Transverse Ranges have been 

urbanized, sufficiently to evaluate the effect on erosion rates. The variable that was used to measure 

the amount of urbanization did not prove significant 

at a level at which the resulting regression was 

stable. The cause was obvious—because of slopestability problems only the lower parts of basins 

with moderate and high erosion rates can become

urbanized. By far the larger part of the sediment is 

derived, and will continue to be derived, from the 

steep, undeveloped upper parts of such watersheds.

RESULTS OF ANALYSIS

PREDICTIVE EQUATIONS

Sediment yields from the January 18-27, 1969, 

storm were selected as the dependent variable in 

the final regression analyses for predictive purposes. Reasons for restricting the analysis to this 

single storm are many. Primarily, data are more 

plentiful, more representative of future conditions, 

and include a greater range of conditions than are 

found in any other set of storm data. No other recent storm is comparable in the number of conditions which should be attached to a design storm in 

the area and which are satisfied by the January 

1969 storm. These include a typical antecedent condition, a typical storm pattern, a duration corresponding to the critical duration for maximum sedi-
RESULTS OF ANALYSIS 27

TABLE 6.—Data for selected drainage basins with January 1969 sediment yields in Los Angeles and eastern Ventura County

—Continued

Urbanization 

Drainage basin (Number 1968 

corresponds with location photoin fig. 2) graphs 

(acres/ mi 2 )

Bell Canyon __________ (!)__ 25

Limekiln Canyon _ __ __ _ (2) _ 122

May Canyon (No. 1) ____ (3)__ 

May Canyon (No. 2) ____ (4)__

0 

0

Rowley Canyon _____ _ (5)__215

Bluegum Canyon ____ __ (6) _

Blanchard Canyon ______ _ (7)__

Snover Canyon _ _ ___ (10) _

La Tuna Canyon ______ (12) __

Childs Canyon _______ (15) __

Hillcrest Canyon ____ ____ (16) _

Deer Canyon ____ ____ (17) _

0

23

32

25

5

13

11

Nichols Canyon _______ (18)__357

Lincoln Canyon ___ _ __ _ _(19)__

West Ravine ________ (20) __

Kinneloa Canyon (West) __(21)__

Kinneloa Canyon _ _ _ _ __(22)__

Sierra Madre Villa ____ (23) __

Bailey Canyon _ ___ ___ (24) __

Auburn Canyon ___ __ _ (25) _-

Little Santa Anita Canyon (26)__

Lannan Canyon __ _____ (27) __

Bradbury Canyon _ _ _ _ __(28)__

Spinks Canyon ___ __ _ (29) __

Maddock Canyon ______ (30) __

Hook Canyon (East) __ (31) __

Harrow Canyon (32)

Englewild Canyon _____ (33) __

Little Dalton Canyon __ (34)__

Morgan Canyon __________ (35) __

Wildwood Canyon _____ (36) __

Emerald Canyon __ ___ (37) __

5

60

6

0

52

29

3

0

8

0

10

0

0

0

0

15

6

6

7

Fire history

Date Percent 

latest of 

pre-1969 watershed 

burn burned

Oct.

Mar.

Nov. 

Nov.

Sept.

June

Nov.

Nov.

Oct.

Mar.

Mar.

Mar.

1967

1964

1966 

1966

1913

1964

1933

1933

1952

1964

1964

1964

No history

July

Oct.

1968

1935

No history

No history

Oct.

Oct.

Oct.

Dec.

Dec.

Oct.

Oct.

Oct.

Aug.

Aug.

Aug.

July

July

July

Aug.

1961

1961

1961

1953

1953

1958

1958

1958

1968

1968

1968

1960

1960

1957

1968

100

25

100 

100

100 <io

100

75

100

100

100

100

0

10

100

0

0

50

50

50

100

100

75

75

100

100

100

100

100

100

100

25

Fire 

factor 

January 

1969 

FF

65

3

52 

52

0

3

0

0

0

17

17

17

0

8

0

0

0

5

5

5

0

0

2

2

2

80

80

80

2

2

2

20

Total 

time 

of

tration J 

TC 

(min)

96.6

54.6

30.3 

25.0

48.1

43.1

52.2

43.6

76.3

29.8

26.3

39.3

21.4

24.7

34.5

24.0

25.9

31.4

30.2

25,9

32.4

22.9

28.8

28.2

24.6

25.4

38.6

33.5

46.1

35.5

38.0

44.1

January 1969 

precipitation

an. 18-27 

(in)

16.0

17.0

20.0 

20.0

20.0

25.0

25.0

28.0

15.0

17.0

17.0

19.0

16.0

25.0

27.0

27.0

27.0

26.0

28.0

28.0

32.0

27.0

23.0

22.0

22.0

21.0

25.0

25.0

28.0

19.0

18.0

17.0

K factor 

(10 day X (f)A l>m-.-»

Max. 24 hr precipi- 

(in) tation) 2 )

4.5

4.8

5.0

5.0

7.0

8.3

8.3

10.5

5.5

5.8

5.8

6.5

6.0

9.5

8.5

9.5

9.5

9.5

11.0

10.5

13.0

11.0

8.5

8.0

8.0

7.5

8.0

8.5

10.5

7.5

7.5

7.0

324

392

500 

500

980

1722

1722

3087

454

572

572

803

576

2256

1951

2437

2437

2346

3388

3087

5408

3267

1662

1408

1408

1181

1600

1806

3087

1069

1012

833

Mean 

annual 

precipitation 

MAP 

(in)

15.0

18.5

22.0 

23.0

22.0

23.0

24.0

26.5

19.5

18.5

18.5

19.5

17.5

23.5

25.0

25.0

25.0

26.0

25.0

26.0

27.5

23.0

25.0

24.0

25.0

23.0

23.5

24.0

27.0

22.5

22.0

22.0

Sediment 

yield 

Jan. 

18-27, 

1969

(yd*")

23,700

15,200

40,600 

2,900

2,000

1,060

9,780

8,600

37,300

3,760

8,000

30,400

3,450

16,600

13,700

16,500

13,500

106,000

27,600

6,830

102,000

3,420

39,100

13,400

7,800

1 25,200

52,600

44,800

256,000

9,900

7,890

790

1 Estimated.

ment yield (historical storms of duration longer 

than 10 days do not produce greater sediment 

yields), and a sufficient interval since the preceding 

major storm to allow a normal degree of sediment 

accumulation in channels.

Most importantly, the storm was of a magnitude 

for which the response of the watersheds to the 

complex variety of erosional processes could be assumed to be similar to that of a theoretical event 

of 50-year recurrence interval. Given this assumption, theoretical 50-year erosion rates for planning 

purposes can be estimated from the predictive equations by substitution of hydrologic variables of 

equivalent frequency.

It is necessary to establish a design fire condition when using the resulting equations for predictive planning purposes. For the general condition, a 

fire factor of 20 is assumed, corresponding to a 100- 

percent burn 4.5 years previous to the storm. An 

estimated post-burn interval of either 4 or 5 years 

has been used in previous hydrologic studies in the

area and is based on decision theory. The figure of 

4.5 years reflects the seasonal offset of burn and 

storm periods.

Data for watersheds with known erosion rates 

from the eastern Transverse Ranges in Los Angeles 

County are presented in table 6.

Regression equations resulting from stepwise 

analysis of the data of table 6 were selected from 

a large number of possibilities on statistical criteria. 

The equations included

log sv =-3.524 + 0.929 log A + 1.671 log ER 

+ 0.246 log SF + 0.249 log FF + 5.666 log MAP (1)

Sv is sediment yield of the January 1969 storm, in 

cubic yards; A is drainage area, in square miles; 

ER is elongation ratio; SF is area of slope failures, 

in acres per square mile; FF is fire factor; and 

MAP is mean annual precipitation, in inches. The 

standard error of estimate is 0.278 log units (+ 90, 

— 47 percent) ; the multiple correlation coefficient is 

0.894.
28 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

TABLE 7.—Data for drainage basins above proposed and selected existing debris-basin sites in Ventura County, Only the 

watershed variables shown in this report to be significantly related to major-storm sediment yields are included

Drainage basin 

(Number corresponds 

with location in 

fig. 3)

Drainage 

area

A 

(mi 2 )

Geologic units exposed 

in drainage basin 

Symbols from Kundert (1955) 

Listed in order of 

predominance

Slope 

failures 

SF 

(acres/ 

mi 2 )

Elongation 

ratio 

ER

Fire 

factor 

January 

1969 

FF

Total

time 

of 

concentration 

TC

(min)

K factor

(1969) 

(10 day X 

(24-hour 

precipitation) 2 )

K

K

factor 

(50 yr) 

K

Area 1

Cozy Dell Canyon __ ___ 

Stewart Canyon _ ___ _ 

Fox Canyon near Ojai__

(D— - 

(2)—— _ 

(3). —— 

(4) _ _

(5) ——

(6) ——

(7)- ——

1.71 

1.98 

.68 

.34

4.05

o 07

2.65.

E, 

E, 

E, 

E,

E,

E,

E,

E,

Qal 

Qt, 

Qt, 

Qal

Qal

Qal

Qal

'

Qal 

Qal

Qal

58 

83 

70 

90

qo

00

77

0.583 

.592 

.500

.656

.864

79O

r(»r

0 

2 

0 

0

0

0

0

0

55 

58 

49 

35

69

66

64

11920 

11372 

1 1056 

11056

11728

11274

11274

12913 

12601

12978

13267

13146

13146

Area 2

Fresno Canyon __ __ _

Sexton Canyon ____ __

Harmon Canyon _ _ ____

Peppertree Canyon __ __

Aliso Canyon __________

Adams Cayon _ , __ ____

Fagan Canyon _____ —— _

Mud Creek Canyon ____

Orcutt Canyon __

Keefe Ditch _- ._

Jepson Wash _ _ _______

Pole Creek Canyon ____

Real Canyon _____ ___

Warring Canyon ______

(9) ——

(10) ____

(ID ——

(12) ——

(13)——,

(14) ——

(15) _ __

(16) ——

( 17)

(18)

(19) __

(20) ——

(21)_ ——

(22) ——

1.27

2.67

3.03

2.22

11.37

8.36

3.01

2.43

3.38

.61

1.31

7.65

.25

1.08

Mm, Mu, Pml, Qal,

Pml, Pu, Qm, Qal

Pml, Pu, Qm, Qal

Pu, Pml, Qal

Pml, Pu, Mu, Mm,

Pml, Mu, Mm, Pu,

Pml, Qc, Pu, Qal

Pml, E, Qt

Pml, Qt, E, Qc, Pu,

QC, - E Qt Qnl

-1-1 , ^Ptl', \9Cdl

Qc, E, Qal, Qt

Mm, Qal, Ml

Mm, Pu, Qal

Mm, Pu, Pml, Qal

Qt

Qal

Qc, Qal

Qai al 

6

13

13

13

6

13

19

96

51

77

64

6

45

45

.537

.645

.664

.657

.773

.600

.675

.596

COO 

• 9£t>

.506

.471

.616

.627

.782

0

4

0

0

0

0

0

64

64

0

15

3

21

47

67

74

72

73

96

111

81

58

68

40

53

99

33

50

828

240

324

425

726

1098

792

971

792

929

971

666

469

469

1294

714

784

833

1614

1835

1590

2164

2200

1704

1781

1433

992

992

Area 3

Fox Canyon near Somis_ 

Gill Barranca _ — ______

Coyote Canyon ________

Mahan Barranca __ __

Long Canyon __ _ _ _

South Grimes Canyon _

Gabbert Canyon _ _ _

Alamos Canyon __ ____

Brea Canyon

Dry Canyon ___ _______

Tapo Hill Diversion

(West) _ _____________

Tapo Hill Diversion

(East) ______ ________

Tapo Canyon _ _________

(23) —— _ 

(24) ____

(25) ——

(26) ——

(27) ——

(28) ____

(29) ——

(30) —— _

(32)——.

(33)—— _

(34)' ____

2.41 

1.06

1.02

1.54

2.71

3.86

3.81

4.80

1.92

1.20

.16

.22

(35) __ 17.60

Pml, Qc, Mm, Mv, Pu, Ml, Qal 

Qc, Pml, Qal

Pm

Qc,

Qc,

Qt,

Qt,

Ml,

Qal

0C,

0C,

<t>c,

Qc,

1, Mm, M

Qt, Pml,

Qt, Pml,

Qc, Qal

Qc, Qal

0c, Qal,

'Qal <bc

Qal

Qal

Mm, Qt,

1, Mv, Qc, Qal

Qal

Qal

Mm

<pc, Qal, Mm, Mu

32 

19

26

6

19

19

32

32

19

19

13

13

6

.517 

.572

.509

.473

.603

.495

.482

.706

.575

.647

.836

.630

.841

65 

61

55

9

0

0

3

2

10

62

55

55

7

74 

67

60

78

81

99

98

85

77

58

33

35

116

232

188

209

192

205

192

192

224

150

159

168

168

304

905 

693

1072

1035

1215

1035

830

1035

712

712

712

712

1325

1 Values relatively less well defined.

log- Sv = 1.244 + 0.828 log- A + 1.382 log- ER

+ 0.375 log- SF+ 0.251 log- FF+ 0.840 log- K. (2)

K is the storm-precipitation factor, and other variables are as defined for equation 1. The standard 

error of estimate is 0.324 log units (+ 111, -52 percent) ; the multiple correlation coefficient is 0.854.

log Sv =- 0.981 +1.132 log A - 1.059 log TC „

+1.322 log ER + 0.363 log SF+ 0.250 log FF 

+ 4.847 log MAP. (3)

TC is total time of concentration, and other variables are as defined for equation 1. The standard 

error of estimate is 0.252 log units ( + 79, -44 percent) ; the multiple correlation coefficient is 0.918.

Although these equations achieve more of a reduction in standard error than similar previous 

analyses, the value of this analysis is its applicability to the more variable conditions in the western 

Transverse Ranges.

AREAL VARIATION IN EROSION RATES

The observed differences in erosion rates throughout the Transverse Ranges were confirmed by estimation of January 1969 and 50-year sediment yields 

using the predictive equations (table 7). Three distinct groups of watersheds delineated on the basis 

of erosion rates correspond closely with the three 

major drainages that reach the Pacific Ocean in 

Ventura County (fig. 3).

Area 1: High erosion rates.—The following study 

watersheds are in the drainage of the Ventura 

River:

Cozy Dell Canyon 

Stewart Canyon 

Fox Canyon near Ojai 

Dron Canyon 

Gridley Canyon 

Senior Canyon
RESULTS OF ANALYSIS 29

TABLE 7.—Data for drainage basins above proposed and selected existing debris-basin sites in Ventura County. Only the 

watershed variables shown in this report to be significantly related to major-stm^m sediment yields are included—Continued

Drainage basin (Number 

corresponds with 

location in fig. 3)

Mean 

annual 

precipitation 

MAP 

(in)

Calculated January 

1969 sediment yield 2 

per unit area (erosion 

rate)

S,/

(yd 3/ Source 

mi 2 ) equation

Calculated 1969 

sediment yield per 

unit area with 

FF of 20

S,/

(yd 3/ Source 

mi 2 ) aquation

Total designstorm (1969) 

sediment 

yield

(yd 3 )

Calculated sediment 

yield per unit area 

with 50-yr K factor 

and FF of 20 8,'

(yd V Source 

mi 2 ) equation

Total designstorm (50-yr) 

sediment 

yield 

8, 

(yd s)

Area 1—Continued

Cozy Dell Canyon ___ ___ 

Stewart Canyon __ __ _ 

Fox Canyon near Ojai _ 

Dron Canyon _. _ _

Gridley Canyon _ _ ____ 

Senior Canyon _ ____ _ 

Horn Canyon __ ——— __ 

Wilsie Canyon _ _ _ _

(1) __ 27.0 

(2) __ 28.0 

(3) __ 24.0 

(4) 24.0

(5) __ 27.0 

(6)____27.0 

(7) __ 27.0 

(8) __ 26.0

29,900 

47,500 

14,700 

24,100

30,700 

39,000 

38,100 

23,800

3 

3 

3 

3

3 

3 

3 

3

63,200 

84,300 

31,200

65,200 

82,500 

80,700 

50,600

3 

3 

3 

3

3

3 

3 

3

108,000 (3 ) (3) 

167,000 (3 ) (3) 

21,200 ( 3 ) ( 3) 

17,300 (3 ) (3)

264,000 ( 3 ) ( 3 ) 

462,000 ( 3 ) ( 3 ) 

264,000 (3 ) (3) 

134,000 ( 3 ) ( 3 )

Area 2—Continued

Fresno Canyon ———— _

Sexton Canyon __ —— _

Harmon Canyon _ _ __

Peppertree Canyon ____

Aliso Canyon ___ _ _ _ _

Adams Canyon _ _ _

Fagan Canyon _______ _

Mud Creek Canyon ____

Orcutt Canyon _ _____

Keefe Ditch ___________

Jepson Wash ___ _ _ ____

Pole Creek Canyon _ _

Real Canyon __ ______

Warring Canyon ______

Fox Canyon near Somis_

Gill Barranca _ __ _____

Coyote Canyon _ _ ______

Mahan Barranca __ ___

Long Canyon _ _ _______

South Grimes Canyon _

Gabbert Canyon _ ____

Alamos Canyon _ _ ____

Brea Canyon ___ _ _ ____

Dry Canyon _ —— _ __

Tapo Hill Diversion

(West) _____________

Tapo Hill Diversion

(East) ______________

Tapo Canyon _ _______

(9) __ 20.0

(10) __ 21.0

(11) __ 22.0

(12) __ 22.0

(13) __ 22.0

( 14 ) 21 0

(15) __ 21.0

(16)_ , _ 21.0

(17) __ 21.0

(18) __ 22.0

(19) __ 22.0

(20) __ 21.0

(21) __ 19.0

(22) __ 20.0

(23) __ 18.0

(24) __ 17.0

(25) __ 18.0

(26) _i 17 0

(27) __ 18.0

(28) __ 16.0

(29) __ 16.0

(30) __ 16.0

(31) __ 14.0

(32) __ 15.0

(33) __ 15.0

(34) __ 15.0

(35) __ 19.0

3,940

2,990

2,770

3,610

4,010

5,630

6,910

4 37,400

19,700

11,800

17,900

3,840

18,300

23,800

6,180

5,550

5,700

1,630

1,940

1,310

2,030

3,400

2,640

5,620i

9,940

6,360

5 3,280

2

2

2

2

2

2

2

2

2

2

2

2

2

2

Area

2

2

2

2

2

2

2

2

2

2

2

2

2

8,350

4,490

5,870

7,660

8,500

11 900

14!700

4 27,900

14,600

25,100

19,300

6,180

18,100

19,100

3 — Continued

4,560

4,180

4,40|0

1,990

4,100

2,800

3,280

6,060

3,140

4,210

7,690

4,950

5 4,270

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

10,600

12,000

17,800

17,000

96,500

99,800

44,200

4 67,900

49,500

15,300

25,300

47,300

4,530

20,600

11,000

4,430

4,490

3,070

11,100

10,800

12,500

29,100

6,030

5,050

1,230

1,090 •'75,100

12,200

11,200

12,300

13,500

16,600

18,400

26,400

'54,700

34,600

41,800

32,100

11,800

34,000

35,900

14,400

12,500

17,400

8,200

18,300

11,500

11,200

21,900

11,600

14,800

25,900

16,600

•' 14,700

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

15,500

29,900

37,400

29,900

189,000

154,000

79,400

4 133,000

117,000

25,500

42,000

90,100

8,490

38,800

34,600

13,200

17,700

12,600

49,700

44,300

42,700

105,000

22,300

17,800

4,150

3,650 •'•259,000

2 Under conditions existing in January 1969.

3 Preferable equation for these basins does not include K factor (see text).

* No useful estimate of sediment yield is possible (see text).

5 Yields for basins where independent variables significantly exceed range of variables used to compute equations.

Horn Canyon 

Wilsie Canyon

This group of watersheds forms the south slope 

of Nordhoff Ridge where bedrock is the Eocene 

sequence of the Matilija overturn, a deformed 

structural feature associated with recently active 

thrust faulting. Sediment yields are high relative 

to other parts of the Transverse Ranges. Erosion 

rates from the January 1969 storm ranged from an 

estimated 31,200 to 84,300 yd3 /mi2 (9,210 to 24,900 

m3/kni2 ) corrected for a uniform fire factor of 20 

(table 7).

Area 2: Moderate erosion rates.—The following 

study watersheds are in the drainage of the Santa 

Clara River, with the exception of Fresno Canyon, 

which is in the Ventura River basin:

Fresno Canyon 

Sexton Canyon

Harmon Canyon

Peppertree Canyon

Aliso Canyon

Adams Canyon

Fagan Canyon

Mud Creek Canyon

Orcutt Canyon

Keefe Ditch

Jepson Wash

Pole Creek Canyon

Real Canyon

Warring Canyon

The watersheds of this group are formed in the 

thick Miocene-Pliocene section of the Santa Clara 

River valley. The Fresno Canyon watershed is likewise underlain by this sequence where it strikes 

across the lower part of the Ventura River valley. 

Erosion rates from the January 1969 storm ranged 

from an estimated 4,490 to 27,900 yd3 /mi2 (1,330
30 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

to 8,240 m3/km2 ) corrected for a uniform fire factor 

of 20 (table?).

Area 3: Low erosion rates.—The following watersheds are part of the Calleguas Creek drainage:

Fox Canyon near Somis

Gill Barranca

Long Canyon

South Grimes Canyon

Gabbert Canyon

Alamos Canyon

Brea Canyon

Dry Canyon

Tapo Hill Diversion (West)

Tapo Hill Diversion (East)

Tapo Canyon

Watersheds in this area are formed in sedimentary rocks which are similar to those exposed in area 

2 but which are generally finer grained. Lower 

erosion rates in this area are also a function of 

lesser relief and less pronounced recent uplift. Erosion rates from the January 1969 storm ranged 

from an estimated 1,990 to 7,690 yd'/mi2 (587 to 

2,270 m3/km2 ) corrected for a uniform fire factor 

of 20 (table 7).

EROSION RATES FOR PLANNING AND DESIGN

Two approaches to the selection of appropriate 

erosion rates for planning and for design of debris 

basins have been used in the Transverse Ranges.

The first approach is to recognize the difficulties 

in attaching any meaningful frequency to a sedimentation period and to select a major storm and 

designate it as the design storm. Design is then 

based on the erosion rates of that storm. This approach was used locally with the 1938 storm in 

southern California.

The second approach is to adjust a known or estimated erosion rate to a design frequency by using 

as an adjustment factor either rainfall or peak discharge of the selected frequency. Uncertainty introduced by extrapolation of events of design frequencies may be considerable, from both the standpoint 

of establishing meaningful frequencies for sedimentation periods of this magnitude and the statistics 

of the situation.

Both methods are used in this report, depending 

on the similarity of the 1969 storm to the desired 

design storm in a given area. The predictive equations indicate what the January 1969 ' sediment 

yield at a site would have been if a debris basin 

of standard design had existed. Where 1969 precipitation was significantly different from that of

the design storm, substitution of 50-year rainfall 

variables in the equations can be made, assuming 

that the watershed response would be the same in 

both cases. Extension of the prediction equations is 

based on the general comments of Wallis (1967).

The January 1969 storm is locally an excellent 

design storm. Rainfall and discharge parameters 

were of the same general magnitude as those of a 

50-year storm in many watersheds in the eastern 

Transverse Ranges of Los Angeles County. Westward in Ventura County the degree of similarity 

was more variable. In parts of the area the intensity factors were similar, but in other sections of 

the county there were marked differences. The following discussion notes these differences and suggests planning and design criteria.

Area 1 (watersheds north of Ojai in drainage of 

the Ventura River).—In this area, with as high an 

erosion potential as is known to exist in Ventura 

County, the January 1969 storm and a 50-year storm 

were similar (see table 3; K values of table 7 exaggerate differences). Rates from equation 3 were 

selected because that relation gave the value closest 

to the estimated actual January 1969 sediment yield 

in the existing Stewart Canyon debris basin (table 

8). Suggested design yields for debris basins in this 

group of watersheds can be found in the column of 

table 7 entitled "Total design-storm (1969) sediment yield." Even though substitution of values of 

50-year rainfall in equation 2 (equation 3 does not 

contain a storm-precipitation variable) may give 

higher rates, equation 3 best defines the erosion response of these watersheds. In light of the poorly 

defined distribution of storm precipitation in this 

area, planners may best rely on a relatively welldefined set of data—the rates that would have occurred in these basins in 1969 with a fire factor of 

20, with the important check of an actual yield 

figure.

The difference between the figure of 167,000 yd 3 

(128,000 m 3 ) in table 7 and the actual maximum 

capacity of 300,000 yd 3 (230,000 m 3 ) in the Stewart 

Canyon debris basin may be considered as a safety 

factor or as volume that can be utilized for debris 

storage if there has been no burn in the preceding 

4.5 years.

Area 2 (watersheds in the Santa Clara River valley, plus Fresno Canyon in the Ventura River drainage).—Southward within Ventura County, the intensity of the January 1969 storm decreased relative to the theoretical 50-year storm. In this area 

of variable but generally moderate sediment yields, 

the difference was enough to cause a difference in
RESULTS OF ANALYSIS 31

TABLE 8.—Data and comments on estimated January 1969 sediment yields per unit area (erosion rates) in selected existing

debris basins in Ventura County

Drainage

basin

( number

corresponds

with 

location 

in fig. 3)

Drainage 

area

A 

(mi 2 )

Fire 

factor

FF

(January 

1969)

Estimated

actual 

January

1969

sediment

yield per

unit area

(erosion

rate) 8,'

(yd Vmi 2)

Calculated

January 1969

sediment yield per

unit area 

(erosion rate) Comments

(yd

Source 

equa-J 

tion

Stewart Canyon (2) 1.98 52,700 47,500

Jepson Wash (19) 1.31 15 20,400 17,900

Maximum capacity was available 

at start of 1968-69 season. 

Berm of storm debris left at 

time of 1969 cleanout was 

approximately equaled in 

volume by amount of original 

bed material removed at time 

of 1969 cleanout.

Cleanout in 1969 removed some 

original bed material; figure 

corrected by an approximation 

of this amount. Small amount 

of coarse debris passed 

spillway.

calculated rates by more than a factor of two. Equation 2 gave the best results when calculated 1969 

values were compared with estimated actual 1969 

yield in a debris basin in Jepson Wash (table 8). 

Design sediment yields for this group of watersheds 

may be found in the column of table 7 designated 

"Total design-storm (50-yr) sediment yield." The 

figures were calculated with equation 2 by substitution of the appropriate storm-period variables.

Area 3 (watersheds in the Calleguas Creek drainage).—The 1969 storm was inappropriate as a design storm in this area of low sediment yields because of proportionally lower rainfall intensities in 

the southern part of Ventura County. Equation 2 

was used to calculate probable sediment yields resulting from 50-year precipitation in this area, and 

the results appear in table 7 under the column 

labeled "Total design-storm (50-yr) sediment yield." 

Unlike areas 1 and 2, there was no key watershed 

with a suitable existing debris basin from which to 

compare calculated and actual 1969 yields. This lack 

was not critical because there was little difference 

in yields determined with each of the three equations. In addition, because sediment yields in this 

area are less than elsewhere, sediment-retention 

structures may not be necessary in some watersheds 

of this group.

An additional factor suggests that actual yields in 

watersheds of area 3 will be substantially less than 

the volume indicated in table 7. This factor is the 

fine-grained nature of the bed material in the area. 

Fifteen field counts (50 points per locality) indicated an average silt- and clay-size content of 43 

percent in the bed material of these watersheds.

Catchments in areas with sediment this fine grained 

will have substantially lower trap efficiencies; that 

is, more sediment will pass the catchment and most 

should traverse a well-designed system of floodcontrol channels without excessive deposition. Because of this factor, less capacity will be needed 

but the reduction can only be estimated. A conservative approach might deduct 20-30 percent from 

the values of table 7.

This latter conclusion was confirmed during the 

reconnaissance of dry stock ponds, check dams, and 

small-scale debris basins in area 3 in 1970. Many 

structures, especially those with functioning outlet 

towers, retained little of the sediment inflow to the 

reservoirs during the 1969 storms. Much sediment 

clearly passed the structures by remaining in suspension.

ACCURACY OF EROSION-RATE ESTIMATES

It is important to realize the limitations to any 

indirect determinations of erosion rates—the practical uncertainty of the estimates. It is statistically 

impossible, even with the comprehensive data available to this study, to assess all the factors that control erosion rates. Side-by-side watersheds, identical in terms of the factors discussed in this report, 

may still show a difference of 50 to 100 percent in 

major-storm erosion rates. Past records of existing 

debris basins in southern California make this point 

abundantly clear. The chief reason for this lack of 

predictability is the extreme variability of the 

watersheds, which results in a situation where differences in a factor impossible to assess may cause
32 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

significant differences in erosion rates. Examples of 

such unassessable factors include the many and differing mechanisms of landslide triggering, the intensity of grazing in the months immediately before 

a storm, or the efficacy of reseeding programs after 

a burn.

The calculation of individual sediment-yield volumes, based on the characteristics of that specific 

watershed, is a considerable advance over the use 

of the so-called debris-production curve. In the 

latter technique, a plot of sediment yield against 

size of drainage area is applied to all watersheds of 

a given area. The position of the curve is estimated 

according to degree of similarity with areas of 

known erosion rates. Application of the technique 

to the group of closely similar watersheds in area 1 

north of Ojai is illustrated in figure 9. In this case, 

the curve is located from a control point—the value 

calculated for Stewart Canyon, site of a debris 

basin with an estimated actual January 1969 yield 

close to the corresponding calculated yield (see 

values in tables 7 and 8). The appropriate predictive equation 3 for Stewart Canyon was selected on 

the basis of the estimated actual yield in the Stewart 

Canyon basin. The shape of the curve was based on 

well-established relations between yields and size

of drainage area, including many measurements in 

Los Angeles County (Ferrell, 1959). It is similar to 

the relation S/^A- 0 - 15 , in which sediment yield per 

unit area is inversely proportional to basin area. 

Brune (1948, p. 15) and Langbein and Schumm 

(1958, p. 1079) developed this relation in the midwestern United States, and a similar relation has 

been shown to apply to watersheds with flood-control reservoirs in the Transverse Ranges (Scott, 

Ritter, and Knott, 1968, p. 29).

With a plot of the individual calculated yield 

values (fig. 9), it is clear that the figure for Stewart 

Canyon is a poor index for other watersheds in the 

group. The individual values based on equation 3 

are plainly preferable. Because of an unusual distribution of basin characteristics, lower yield rates 

rather than larger are indicated in the smaller 

basins. The effect of basin size alone on sediment 

yield per unit area is, of course, to reduce yields 

per unit area from the larger basins.

The inconsistency illustrated in figure 9 occurs in 

as uniform a set of basins as exists in areas of high 

erosion rates in the Transverse Ranges. Although 

the curve is based on a good control—a major-storm 

yield from a basin of intermediate size—it indicates 

the poor results that would be obtained from inDRAINAGE AREA, IN SQUARE KILOMETERS

10

500,000

°- 100,000

10,000

Dron Canyon

Hypothetical debris-production curve 

using only Stewart Canyon yield as 

' control

Hnrn Canyon Senior Canyon Stewart Canyon '——•———- •

^Cozy Dell Canyon .Gridley Canyon 

Wilsie Canyon

Fox Canyon

' I i i i i ' I i i i I I I I I ! I I

- 100,000

10,000 - 

Q

5 10 

DRAINAGE AREA, IN SQUARE MILES

FIGURE 9.—Sediment yields for watersheds near Ojai (area I) compared with debris-production curve based on estimated actual

yield in Stewart Canyon.
RESULTS OF ANALYSIS 33

discriminate use of debris-production curves in the 

area.

The reasons for the general inapplicability of 

debris-production curves in the area are, first, the 

high magnitude of erosion rates and the consequent 

sensitivity to minor variations in basin and storm 

characteristics, and second, the variability of the 

watersheds. This general situation has been the 

curse of designers of debris basins and flood-control reservoirs throughout the Transverse Ranges. 

There are a number of impoundment structures, 

constructed by many different agencies, which overestimate the yield by as much as 500 percent or underestimate the yield by as much as 80 percent. This 

variation is a function of the complexity of the 

problem, not of incorrect approaches, and was only 

readily apparent for the first time after the storms 

of 1969. In fact, estimates of major-storm yields 

within 50 percent of the ideal value verge upon extrasensory perception. Studies that indicate an accurate predictive technique for historical annual 

sediment yields over an extended period in a single 

watershed do not necessarily define major-storm 

yields there or elsewhere with remotely similar accuracy.

Most design has proved to be overdesign, because 

of the natural designer's bias in this direction, but 

also because true underdesign is not always apparent. Debris basins filling to near capacity show 

a remarkable ability to retain coarse material owing to loss of flow competence with reduction in 

gradient caused by the wedge of sediment deposited 

behind the impoundment. The coarse sediment displaces eroded finer grades of sediment, some of 

which may be transported through the flood-control 

system without causing serious problems. In the 

case of overdesign, the large trap efficiencies of the 

debris basins will cause retention of nearly all 

detritus, including fine sediment that could traverse 

the system without difficulty. Thus, the apparent 

degree of overdesign is lessened. In short, there is 

a natural apparent fit of sediment yields to the 

capacity of debris basins, regardless of overdesign 

or underdesign.

Overdesign is, of course, not the serious problem 

it first may appear to be. Excess capacity serves as 

debris-storage space which, in a time of disappearing disposal sites and escalating haulage costs, is 

of considerable value. It also allows greater flexibility in cleanout schedules after a major storm or 

a large burn. At some sites where excess capacity 

can be added at little additional cost it may be economically justifiable to add excess capacity as a

safety factor. Additional capacity in cases of underdesign can normally be added by excavation below natural stream gradient, provided a stabilized 

inlet structure is included to prevent upstream 

scour.

As debris basins are constructed throughout the 

Transverse Ranges, the additional sediment data 

collected will require the modification of the erosion 

rates in this report. It is certainly true that any 

study of erosion rates is immediately outdated by 

the next significant storm.

LONG-TERM EROSION RATES 

ESTIMATION OF RATES

The average annual rate at which sediment is 

eroded from a watershed is the sum of the large 

contributions from major storms like those of 1938 

and 1969 plus the individual contributions of lesser 

storms and periods of low flow, all divided by the 

number of years of record. It is, with a correction 

for trap efficiency, a good measure of the rate at 

which the land surface of the Transverse Ranges is 

undergoing denudation. Long-term erosion rates are 

of practical value in the planning of cleanout costs 

over a long period, the selection of debris-disposal 

areas, and related problems of sediment management. Long-term rates that reflect natural conditions will be a useful index for assessment of possible future increases in erosion related to environmental changes.

Figure 10 is a plot of average annual sedimentyield rates for a group of watersheds in Los Angeles 

County for periods of 25 to 42 years. Watersheds 

that are the sites of channel-stabilization programs, 

involving the construction of check dams, were excluded from this group. Fire conditions for the 

watersheds plotted were typical of the Transverse 

Ranges; nearly all the watersheds had been burned 

at least once during the period of record. The unexpected feature of this plot is that the apparent decline in erosion rate with increase in basin size is 

substantially greater than is empirically the case in 

single-storm yields, as shown by the trend of a typical design curve (fig. 9). Although part of this 

anomaly is caused by sluicing of some sediment 

from the largest basins, it is likely that long-term 

curves are actually steeper than single-storm curves, 

reflecting a relation between impoundment trap 

efficiency and watershed size. Trap efficiencies are 

less over the long term because the finer grained 

erosional products of lesser storms more readily 

pass the outlet structures. And, predominantly be-
34 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

50,000

C/3 10,000

5000

DRAINAGE AREA, IN SQUARE KILOMETERS

10 

—————I————I———I——I——I 'l I |——I——I—T—T -i———i——i——i—i 'i i I——i—I—r

0,000 uj

1000 D 

0

1 5 10 

DRAINAGE AREA, IN SQUARE MILES

FIGURE 10.—Long-term sediment yields at selected sites in Los Angeles County.

cause of selective sorting (Scott, 1967, p. 315), 

grain size is finer at the outlets of larger watersheds, thereby increasing the effect of reduced trap 

efficiency on the sediment yields of larger basins.

No method of computing trap efficiency (see 

Brune, 1953) is entirely satisfactory when applied 

to debris basins. Plotting of debris-basin capacities 

against drainage areas and a quantitative consideration of the difference in grain size of deposited and 

discharged sediment indicates that approximately 

40 percent of the total silt and clay, 94 percent of 

the sand, and 99 percent of the grades coarser than 

sand are retained in a properly designed debris 

basin with a watershed 1-2 mi 2 (2.6-5.2 km 2 ) in 

area and a functioning outlet tower. If the average 

size distribution is likewise estimated, an average 

estimated trap efficiency of 85-90 percent is probable over the long term, with the lower figure applicable to larger watersheds (2-10 mi 2 , 5.2-26 

km 2 ) and the higher figure more likely for smaller 

basins (<2 mi 2 , <5.2 km 2 ).

With a correlation for trap efficiency of the above 

amount, the rates in figure 10 can be applied with 

confidence as average long-term erosion rates, based 

on the 25-40 year period prior to 1970. The rates 

shown would apply to watersheds under 10 mi - (26 

km 2 ) in size in areas of high erosion rates like that

near Ojai (area 3) and in the frontal watersheds 

of the San Gabriel Mountains. Lower rates in other 

areas would be scaled down proportional to the reduced size of major-storm rates in such areas.

An additional means of estimating long-term 

rates is the comparison of January 1969 storm 

yields, where they were similar to 50-year design 

yields, with long-term rates in the same basins. The 

same factor, percent of design yield equivalent to 

average annual rate, could logically be applied to 

other watersheds in the same general area. For 

watersheds with records exceeding 25 years, the 

average annual rate was 2 to 17 percent of the January 1969 yields modified to the design burn condition. Closer inspection of the data and elimination 

of basins where sluicing of sediment was a factor 

or where burns were unusually common during the 

period of record indicated that a more probable 

range was 8 to 13 percent, with the lower range of 

8 to 10 percent applicable to watersheds of 1 to 5 

mi 2 (2.6 to 13 km 2 ) in size, and the range of 10 to 

13 percent useful for watersheds from 5 to 10 mi 2 

(13 to 26 km 2 ) in size.

IMPLICATIONS OF LONG-TERM EROSION RATES

Figure 10 indicates that a watershed in an area 

of high erosion rates with a drainage area of 0.5
LONG-TERM EROSION RATES 35

mi 2 (1.3 km 2 ) has been eroded at an approximate 

rate of 7,700 ydVyr (5,900 mYyr) for the last 40 

years. This rate is equivalent to a net denudation 

rate of 7.5 ft (2.3 m) per 1,000 years, or nearly 

1 in (25.4 mm) each 11 years. Although high, this 

rate is less than maximum rates elsewhere that reflect gullying of deposits such as loess. The above 

rates in the Transverse Ranges probably approach 

the maximum for areas in which consolidated bedrock is being eroded directly. It should be emphasized that such a rate does not reflect uniform denudation over the surface of a watershed.

As indicated by Schumm (1963, p. 3), rates like 

the above tell us that considerably less time is required for erosion of an uplifted area than was 

formerly thought. The above rate of 7.5 ft (23 m) 

per 1,000 years indicates that Gilluly's estimate 

(1949, p. 570-571) of 5,000 ft (1,520 m) of erosion 

from the Ventura anticline in a period of 1 million 

years (5 ft, or 1.5 m, per 1,000 years) is possible.

Modern rates of uplift in tectonically active areas 

of California are as much as 25 ft (7.6 m) per 

1,000 years (Gilluly, 1949 and Schumm, 1963). A 

rate of 17 ft (5.2 m) per 1,000 years has been recorded for Mount San Antonio, the highest peak in 

the San Gabriel Mountains (Stone, 1961). Stone 

also found that level-line data indicated that the 

south flank of the San Gabriel Mountains was undergoing uplift at a rate of 20 ft (6.1 m) per 1,000 

years. Both the Holocene and the historical uplift 

along active faults described earlier clearly define 

rates on the order of 25 ft (7.6 m) per 1,000 years 

and locally suggest rates that are even higher. 

The measured rate of erosion, scaled down as necessary if applicable to such large areas, still is no 

more than a fraction of the rate of uplift.

The importance of this comparison is that it is 

indeed probable that major landforms and watershed characteristics in the Transverse Ranges are 

influenced by both tectonism and erosion. A measure of reaction to uplift more sensitive than any 

physiographic factor yet proposed is the key to accurate correlation of erosion rates in the Transverse 

Ranges. Whether the use of the extent of slope failures as described above is an answer or not, future 

studies of erosion and sedimentation in the study 

area and similar parts of California should treat 

this problem as a prime consideration.

PRESENT STAGE IN CYCLE OF ALLUVIATION AND 

CHANNEL ENTRENCHMENT

The latest period of widespread alluviation in the 

Transverse Ranges has been at least temporarily

reversed during historical time. The deposits of 

many headwater areas in area 3 have been deeply 

entrenched within valley floors. At least a few channels are now incised in bedrock. Elsewhere, fill surfaces and alluviated hillslopes have not suffered 

localized erosion such as gullying. Vegetation is intact except where heavily grazed or recently burned.

Whether erosion is now increasing or decreasing 

in intensity is of practical interest. Decisions based 

on present trends must be qualified with the consideration that such trends may be only short-term 

perturbations about a long-term trend or equilibrium condition and, as such, may be broken at any 

time. To determine the trend of alluviation or channel entrenchment, parts of Tapo Canyon (fig. 3), 

the most widely alluviated of the 72 study watersheds, were studied in detail.

Evidence from exposed root structures of perennial plants showed that hillslopes in Tapo Canyon 

are undergoing sheet erosion of moderate and 

areally uniform intensity. Rates of erosion appeared 

to vary expectably according to topographic position and soil erodibility; however, several episodes 

of channel cutting have occurred in historical time 

in Tapo Canyon, and the latest is apparently increasing in intensity at present. Entrenchment of 

the channels is similar to that described by Bull 

(1964, p. 117-125) from channels on fans along 

the eastern Coast Ranges in Fresno County.

Present stream channels in Tapo Canyon are confined to steep-walled trenches in the silt-rich valley 

fills (fig. 11). The trenches range from 8 to 35 ft 

(2.9 to 10.7 m) in depth and, as in Fresno County, 

contain well-developed remnants of paired terraces 

5 to 14 ft (1.5 to 4.3 m) above the present channel. 

Bull (1964, p. 121-122) was able to make a clear 

correlation between the two periods of trenching 

in Fresno County and periods of high annual rainfall and high frequency of large daily rainfall in 

1875-95 and 1935-45.

At least the younger period of trenching in Tapo 

Canyon is a probable equivalent to that in Fresno 

County. Preservation of a debris-retention structure 

set with lengths of oil-well casing on the pairedterrace remnants dates the second episode of entrenchment as post 1928-33. The now-suspended 

base of the structure is 10 ft (3 m) above the 

present channel bottom (fig. 12). The same 1935- 

45 period of high annual rainfall seen in Fresno 

County is evident in many local records. No correlation can be established for the older of the two 

historical periods of channel cutting, but the possibility of correlation is clear.
36 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

FIGURE 11.—Typical silt-rich valley fill in tributary of Tapo 

Canyon. Individual points to in-place stump of fossil California coastal live oak (Quercus agrifolia).

FIGURE 12.—Undercut debris-retention structure in tributary 

of Tapo Canyon. Individual standing in bottom of wash provides scale.

The latest period of trenching- is being continued 

or renewed by the latest post-1965 wet period. As 

much as 1.5 ft (0.5 m) of net scour occurred during 1969 storm runoff alone.

TIME VARIATION OF EROSION RATES

The recency and severity of the channel cutting 

in Tapo Canyon is testimony to the sudden changes 

in erosional factors that can take place in the study 

watersheds. It emphasizes the sensitivity of the 

watersheds to change. An indirect cause of this 

sensitivity is the presence of easily erodible fills 

adjacent to active stream channels. These fills create 

a future potential for erosion rates higher than 

those measured, with relatively minor changes in 

climate or land-use factors. No such fills exist in 

the watersheds of area 1; few of any consequence 

are found in area 2; however, they are widely developed and constitute a significant unknown in the 

erosional regime of area 3.

Over the short term, it is unlikely that the rates 

of table 7 will be markedly affected by the activities of man, because of the rugged, nearly inaccessible nature of the watersheds with moderate and 

high erosion rates—those of areas 1 and 2. One exception to this generality is a change in wildfire 

rate. The resultant trend of more common but more 

effectively controlled burns is not yet clear.

Grazing intensity will be a chief cause of change 

in the relatively low rates of area 3. Watersheds of 

area 3 will also be those most susceptible to minor 

changes in hydrologic or land-use factors because 

of the widespread alluvial fills in those watersheds.

With the assumption that the climate of the past 

40 years will be typical of the future climate, erosion rates based on the existing historical records 

can be extended. Estimates of annual precipitation 

back to 1769, 100 years before measurements began, 

were made for the Los Angeles area by Lynch 

(1931) on the basis of notes by Spanish missionaries. His graph, combined with subsequent data, 

reveals a downward trend from what was probably 

the peak of a wet period in 1769 to a major trend 

reversal in 1884. Since then a pronounced series of 

wet and dry periods has occurred, of roughly equal 

magnitude and duration. During the period of sediment records a dry period extended from 1923 

through 1934, a wet period from 1935 through 1944, 

a dry period from 1944 through 1964, and a wet 

period from 1964 through at least 1970. The period 

of record is therefore representative of several wet 

and dry intervals.
REFERENCES CITED 37

Extrapolation of past rates, either those based on 

long-term records or the assumed frequencies of 

single events, should be done with caution. There 

is no guarantee that future data will be similar to 

those of the past, especially when considerable 

changes in erosion rates can be produced by minor 

variations in a number of factors.

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38 TRANSVERSE RANGES, SOUTHERN CALIFORNIA

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