Flood Surge on the 

Rubicon River, California 

Hydrology, Hydraulics 

and Boulder Transport

GEOLOGICAL SURVEY PROFESSIONAL PAPER 422-M

Prepared in cooperation with the 

California Department of 

Water Resources
Flood Surge on the 

Rubicon River, California 

Hydrology, Hydraulics 

and Boulder Transport

By KEVIN M. SCOTT and GEORGE C. GRAVLEE, JR. 

PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 422-M

Prepared in cooperation with the 

California Department of 

Water Resources

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

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 45 cents (paper cover)
CONTENTS

Abstract_ _-__-----_---_-__--_-__________-________

Introduction-_--_---_-_-_----_------_-______-______

Acknowledgments. __________________________________

Physical setting--__---______----_-_________________

Location and physical features-__________________

Climate and vegetation..._-_____-_._____--______

Bedrock geology._______________________________

Geomorphic history and stratigraphy of the Rubicon 

canyon._---___--______._______._..__________

Embankment failure and release and downstream 

passage of surge._-___-_-___-_-___________-_-_

Flood of December 21-23, 1964, in the Rubicon River 

basin.___________________________________________

The storm._--_-____-___---__-_-_-______-----_-

The flood._..____._._._.__.______....._..._

Inflow and outflow at Hell Hole Dam._________

Peak discharge of the surge downstream from 

Hell Hole Dam_-__----_------._______.

Attenuation of surge wave.______-___-----_-_

Erosional effects of the surge.________________________

Effects on channel morphology.________-____----___--

Mass movements caused by the surge.________---___-_

Depositional features..______________________________

Methods of study.._.___..______________--_-____-

Page 

Ml

1

2

2

2

3

4

8

8

8

10

12

12

12

13

14

17

17

Depositional features Continued Page

Lateral berms or terraces_________-______________ M18

Boulder bars_-_---_-_-_________________._ 19

Terrace accretion.-_--__--__-____---.___________ 19

Boulder fronts____________________________ 20

Episodic boulder movement______________________ 23

Pool and riffle pattern.__________________________ 25

Bed-material forms and internal sedimentary structures_____.________________________________ 26

Transport and deposition of bed material in reach downstream from damsite-_________________________ 27

Downstream changes in patterns of bars and berms_ 27

Composition. __________________________________ 27

Particle size..__________________________________ 27

Distribution of particle size._____________________ 29

Competence_ _________________________________ 31

Roundness.____________________________________ 32

Cause of downstream decline in particle size- ______ 33

Macroturbulence. ____--__-__________--____.____ 34

Effects of the surge relative to normal stream processes. _______________________________________ 35

Summary and conclusions.__________________________ 36

References cited.___________________________________ 37

Index___________________________ 39

ILLUSTRATIONS

Page 

FIGURE 1. Index map showing drainage areas in study area..______________-_______----_--_-____------------_-- M3

2. Longitudinal profile of the flood route____________-_____-____--_-___-_--__--__--_--------------- 4

3. Map showing location of physical features in the reaches below the Hell Hole damsite pertaining to the study _ 5

4. Composite stratigraphic column of the deposits in the upper Rubicon River canyon.____________________ 6

5. Cross section of the completed Hell Hole Dam and the stage of construction at the time of failure._______ 7

6. Photograph showing breaching of the rockfill embankment at the Hell Hole damsite near time of peak discharge.

7. Graphs showing hourly precipitation at stations nearest the Hell Hole damsite for the period December 18-26, 1964___.__...__.___________.___.___________-__________----.----------.---------_-- 9

8. Graphs showing flow and storage data associated with the failure of Hell Hole Dam in December 1964.___ 11

9. Photograph showing lateral supply of sediment to the flood channel from a deposit of terrace gravel and till,

3.3 miles downstream from the damsite-._--_-_-__-___-_---_---_---__------------_--------------- 13
IV CONTENTS

Page 

FIGURE 10. Photograph of trees showing maximum abrasion level 3-5 feat above present ground surface_____________ M14

11. Prefailure and postfailure cross profiles of the channel at the site of the first gaging station reached by the

surge ___________________________________________________________________________________ 15

12-16. Photographs showing 

12. Base of the largest slide in the Rubicon gorge_____________________________________________ 16

13. Boulder berms preserved 0.7 mile downstream from the damsite____________________________ 17

14. Section of berm parallel to stream channel in reach below Hell Hole damsite__________________ 18

15. Lobate bar formed downstream from a bedrock projection into channel_____________________ 20

16. Large gravel bar formed 1.2 miles below Hell Hole damsite___________________________________ 21

17. Map showing radial pattern of flow and deposition at the downstream end of Parsley Bar__ ___________ 22

18. Photograph showing view across flood channel of boulder front on Parsley Bar___.---___________-_-____ 23

19. Map of the part of Parsley Bar containing the largest boulder front-__________________________________ 24

20. Map of reach showing sequence of bedload movement-_-_-_-__-_--____-__-___-_-_-____---______-____ 25

21. Photograph shdwing contribution of sediment to the flood channel by a landslide, 2.5 miles upstream from

the junction of the Rubicon River with the Middle Fork American River.___________________________ 26

22. Graph showing relation of bed-material composition, in terms of the percentage of diorite, to the channel

distance downstream from the damsite_-____---_---_--------------------_------_-------------__- 27

23. Photograph showing bed material of Parsley Bar, extending from 1.6 to 2.9 miles downstream from the

Hell Hole damsite-___________________________________________________________________________ 28

24-33. Graphs showing relation of 

24. Mean particle size to distance downstream from damsite-____________________________________ 28

25. Mean particle size of samples in pools and rimes to distance downstream from damsite._________ 28

26. Diameter of larger cored boulders to distance downstream from damsite_________--____-____-_ 29

27. Dispersion in particle size to distance downstream from damsite___--___-__-_______-__________- 30

28. Skewness to distance downstream from damsite.----_------__-__--____---_________-___-_-_._ 30

29. Indirectly measured tractive force to distance downstream from damsite_______._...___-_____-_ 31

30. Maximum particle size to indirectly measured tractive force._________________________________ 32

31. Particle roundness to distance downstream from damsite.____________________________________ 33

32. Roundness of cored boulders to distance downstream from damsite--_-__-_______---____-______ 33

33. Mean particle size to indirectly measured tractive force.----_-------_-_--________-____.______ 34

TABLES

Page 

TABLE 1. Summary of flood stages and discharges.__________________________________________________________ M10

2. Description of channel cross profiles__-_-_--_-_--_------------------_-----_----------------_--_-__- 15

3. Summary of sediment data in channel below damsite..______________________________________________ 29

4. Summary of maximum particle-size data___-___-_--_____---_------_---_--__-_-_--_--_---___--_-____ 32

5. Size and height above present thalweg and distance from point of origin of boulders deposited by macroturbulence.._ ____-__-_________-_-_______-____-__--__-----_-------__---_---__-__--_---_____--___- 35

SYMBOLS

A Cross-sectional area of channel

0,4, Phi skewness

Di,D3 Quartile size

d Depth of channel

K Conveyance

Mz Graphic mean diameter

N Size

n Manning roughness coefficient

Q Discharge

R Hydraulic radius

S Slope of water-surface gradient

SKt Inclusive graphic skewness

S0 Trask sorting coefficient

y Specific weight

07 Inclusive graphic standard deviation

0$ Phi or graphic standard deviation

r Shear stress
PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA HYDROLOGY, HYDRAULICS, AND

BOULDER TRANSPORT

By KEVIN M. SCOTT and GEORGE C. GRAVIJGE, JR.

ABSTRACT

The failure of the partly completed Hell Hole Dam December 

23, 1964, released a surge with discharge greatly in excess of 

any recorded flow on the nipper part of the Rubicon River, a 

westward-drainage of the Sierra Nevada. Extensive erosion of 

glacial-outwash terraces along the steep, bedrock course of the 

stream indicates that the surge was probably greater than any 

post-Pleistocene discharge. Such a unique event, during which 

more than 700,000 culblc yards of rockflll from the breached dam 

embankment was washed downstream, permits documentation 

of the sedimentologic and geomorphic effects of a single catastrophic flow.

A torrential rainfall, 22 inches in the 'basin upstream from the 

damsite during the 5 days preceding the failure, produced 

dramatic and unprecedented runoff. Natural peak discharges in 

the region were generally equivalent to the maximum discharges 

attained in previous floods for which there is record. The surge 

release, however, produced peak discharges substantially in 

excess of previously recorded flows along the entire 61-mile 

route downstream from the damsite, along the Rubicon, Middle 

Fork American, and North Fork American Rivers. The discharge was still 3.3 times the magnitude of the 100-year flood on 

the Middle Fork American River, 36 miles downstream from the 

damsite. Average velocity of the flood wave was approximately 

22 feet per second. Erosion of detritus from steep, thickly 

mantled canyon walls resulted in thalweg aggradation at five 

cross-profile sites. Stripping of the lower valley side-slopes may 

have triggered a period of increased mass movement in the gorge 

of the Rubicon River.

Depositional forms and flow dynamics were strongly influenced by sediment sources that included colluvium, terrace 

remnants, till, and landslides directly triggered by the surge. 

Terracelike boulder bennis, probably associated with macroturbulent transport of boulders in suspension, formed in backwater 

areas in the uppermost Rubicon River canyon. Boulders were 

piled to a depth of 5 feet on a terrace 28 feet above the thalweg 

at a peak stage of 45 feet. Boulder fronts as much as 7 feet high 

that formed lobate scarps transverse to the channel in an expanding reach indicate that locally bed material moved as 

viscous subaqueous rockflows. Movement of coarse detritus in 

large gravel waves may also have occurred.

The diorite rockfill in the dam embankment acted as a point 

source of boulders distinguisJhaible downstream, analogous to a 

natural point source of coarse particles, and allowed determination of downstream changes in sedimentological parameters. 

Rouudness changes were rapid owing to the extreme coarseness 

of the material. Transition from angular to subangular occurred

almost immediately 'after initiation of movement, and change 

from suibrounded to rounded took place within approximately 

1.5 miles of transport from the damsite. Pronounced downstream 

decrease in mean particle size was mainly due to progressive 

sorting. The effects of abrasion and breakage were relatively 

minor and caused less than 10 percent of the overall size decline 

in the section of channel from 0.4 to 1.3 miles below the damsite. 

Sorting improved irregularly downstream with respect to the 

damfill components.

Competency of the flow was approximated by a method in 

which tractive force is determined indirectly at the deepest 

point in a cross section. Tractive force was then plotted against 

the mean diameter of the 10 largest boulders deposited at each 

point where 'observations were made to extend the relation between tractive force and particle size into the range of extremely 

coarse detritus. Competency generally decreased downstream 

but fluctuated greatly. Great quantities of coarse material were 

swept laterally into the channel by the surge, but were carried 

only short distances downstream, as 'shown by variation in competence, marked decrease in size of boulders of upstream lithologic types downstream from a geologic contact, and by distance 

of transport of material from the dam embankment. Maximum 

distance of transport of any identifiable particle from the damsite was 2.1 miles.

Assessing the effects of the surge passage in terms of a 

natural geomorphic event, catastrophic floods, possibly resulting from landslide or ice damming as well as from unusual 

runoff, may strongly influence the morphology of mountain 

streams yet be relatively unimportant in terms of total sediment 

transport. Such rare floods may set the modify pool-riffle patterns, cause cycles of increased mass movement, and trigger 

mass flow of coarse detritus in the channel. They may cause 

extensive lateral supply of extremely coarse sediment to the 

channel where, however, 'the material is dispersed by flows of 

lesser magnitude in combination with the size-reduction effects 

of weathering in place.

INTRODUCTION

On December 23, 1964, impoundment of runoff from 

an intense and prolonged storm caused the failure of a 

partly completed rockfill dam on the western slope of 

the Sierra Nevada. A surge was released which traveled 

down the Eubioon Eiver into the Middle Fork American River and into North Fork American River before 

final containment in Folsom Lake. Emphasis in this 

study is on the hydraulics of the surge flow, the moveMi
M2 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

ment of bed material, the resulting depositional forms, 

and erosional and other geomorphic effects in short, 

the events following release of the surge at the Hell 

Hole damsite.

A description of the precipitation intensity and distribution of the storm that caused the failure, synthesis 

of hydrographs at the damsite based on available flow 

and storage records in the basin, and a description of 

the failure of the rockfill are included to give as full a 

documentation of the event as possible. What happened 

both before and after release of the flood wave is of 

interest from the standpoint of dam planning and 

construction on the many rivers of the West with precipitous, rock-walled canyons and steep gradients. Progressive downstream breaching of dams could effect considerable destruction, whether caused by catastrophic 

natural floods or possible wartime action.

The passage of the flood wave also furnished the opportunity to study the effects of an unusual hydrologic 

event and the role of such extraordinary floods in determining stream morphology, sedimentation, and landscape evolution. Although such after-the-fact studies of 

floods are necessarily limited in scope to largely qualitative observations, several aspects of this event are noteworthy. The surge on the Kubicon Kiver was greatly in 

excess of any recorded natural flood discharge throughout its entire course and may have been the largest postPleistocene discharge to traverse the upper reaches of 

the Kubicon River canyon. The distinctive lithology of 

the rockfill from the partly completed dam provided 

a tracer for study of bed-material movement and 

changes in sedimentological parameters resulting from 

particle movement from a point source by a single large 

floiw.

ACKNOWLEDGMENTS

J. C. Brice, Washington University, St. Louis, Mo., 

accompanied the senior writer on a preliminary reconnaissance trip into the Rubicon River canyon and made 

several helpful suggestions as to the conduct of the 

study. G. O. Balding, U.S. Geological Survey, provided 

valuable field assistance during most of the 18 days of 

fieldwork during August and September 1965.

The writers are also grateful to McCreary-Koretsky 

Engineers, supervisors of the construction of Hell Hole 

Dam, for providing reservoir-stage readings and estimates of the volume of rock washed from the breached 

embankment. The cooperation of I. L. Van Patten, assistant resident engineer at Hell Hole damsite, greatly 

expedited the fieldwork. W. A. Scott, sheriff-coroner of 

Placer County, provided a log of officers' observations of 

the downstream passage of the surge. Unpublished precipitation data were supplied by the Sacramento Municipal Utility District. Moore and Taber, Inc., permitted use of seismic measurements of one of the largest 

landslides triggered by the surge in the Rubicon River 

canyon.

The manuscript benefited from criticism by J. C. 

Brice, Washington University; R. K. Fahnestock, University of Texas; and George Porterfield and S. E. 

Rantz of the Geological Survey.

The study was part of a cooperative program with the 

California Department of Water Resources.

PHYSICAL SETTING 

LOCATION AND PHYSICAL FEATURES

The Rubicon and American Rivers drain a part of 

the western flank of the Sierra Nevada in Placer and El 

Dorado Counties, Calif, (fig. 1), at about the latitude 

of Lake Tahoe. The Rubicon rises northeast of Pyramid 

Peak in El Dorado County at an elevation of 8,700 feet 

above mean sea level and flows through a broad glaciated 

valley before reaching the meadow known as Hell Hole, 

which was the site of dam construction and release of 

the flood wave. Hell Hole is 65 airline miles northeast 

of Sacramento. The surge traversed 61.1 channel miles, 

descended 3,800 feet (fig. 2), and finally debouched into 

Folsom Lake, a reservoir 20 miles northeast of Sacramento.

During its downstream movement, the flood wave 

passed initially through the upper glaciated part of the 

Rubicon River canyon, which has a length of 7.0 miles 

as measured from the damsite (elev. 4,250 ft) to the 

lowest probable Pleistocene ice terminus (3,650 ft) and 

an average slope of 86 feet per mile. This section of the 

canyon has an average width of about 2 miles and a 

depth of approximately 1,800 feet below the undulating, 

dissected erosion surface that forms the gently westsloping flank of the Sierra Nevada. Downstream from 

the glaciated reaches, the flow entered the gorge of the 

Rubicon, a steep-walled V-shaped incision 22.7 miles in 

length, as much as 2,400 feet deep, and commonly less 

than 2 miles wide. The channel descends at a rate of 

110 feet per mile between canyon walls having slopes 

greater than 90 percent.

Both relief and valley side-slope diminish rapidly 

downstream from the junction of the Rubicon with the 

Middle Fork American River. Several bedrock meanders 

of large amplitude and small radius are immediately 

below the junction. Canyon depth in this part of the 

flood route diminishes from 2,200 to 800 feet in a distance of 26.5 channel miles, and the channel slope averages 23 feet per mile. The last channel entered by the 

surge was the North Fork American River which slopes 

19 feet per mile in the final 5.0-mile passage into Folsom 

Lake.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M3

120°30'

121°00'

EXPLANATION

Route of surge

a 

Dam or damsite 39°00'

Drainage basin boundary

U.S. Geological Survey streamgaging stations

1 Rubicon River at Rubicon Springs

2 Rubicon River below Hell Hole Dam

3 Rubicon River near Georgetown

4 Rubicon River near Foresthill

5 Middle Fork American River near 

Foresthill

6 Middle Fork American River near 

Auburn

Recording precipitation

1 Blue Canyon Airport (U.S. Weather 

Bureau)

2 Union Valley Reservoir (U.S. 

Weather Bureau)

3 Picket Pen Creek near Kyburz 

(U.S. Geological Survey)

4 Kyburz- Strawberry (U.S. Weather 

Bureau)

Auburn '

FIGURE 1. Drainage areas in study area.

CLIMATE AND VEGETATION

Because of strong relief, the local climatic variations 

of the western Sierra Nevada are great, but some generalizations are possible. The area is characterized by 

a pattern of summer drought, punctuated by thunderstorms, and winter rain related to changes in the general 

circulation caused by migrating Pacific Ocean pressure 

centers. Kapid increase in precipitation with elevation 

results from rising and cooling of moist airmasses having prevailing west-to-east movement as they reach the 

great Sierra Nevada barrier.

Mean annual precipitation ranges from about 25 

inches in the vicinity of Folsom Lake to about 60 inches 

at Hell Hole. The difference in precipitation total reflects orographic effects in the region. Although there is 

a general relation between precipitation and elevation, 

the orientation and exposure of an area with respect to 

the direction of air movement likewise affect the quantity of precipitation received, and consequently, departures from the general precipitation-elevation relation 

are common. Most of the total annual precipitation occurs during the period November through March. Above 

an elevation of 5,000 feet the precipitation is usually in 

the form of snow, most of which is stored in mountain 

snowpacks. Consequently, annual peak discharges for 

streams draining the higher elevations generally occur 

in late spring during melting of the snowpack. The peak 

discharges of late spring usually are not excessively 

large. The severe floods that infrequently occur do so 

during heavy winter storms when meteorologic conditions are such that the freezing level is 8,000 feet or 

higher, with the result that rain rather than snow occurs over most of the mountainous terrain. The catastrophic flood of December 1964 occurred under such 

conditions.

The vegetation pattern is highly irregular owing to 

strong relief and associated microclimatic changes. The 

canyon bottom traversed by the surge is in the Arid
M4 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

Glacial _ . . _ Va||ey Rubicon Gorge

 86 -- 110 -

 riDAniriMT IN FCTT PTR Mil F 

 North*

Fork 

American 

River

 19 

I 

10

1

15

I 

20 25 30 35 40 45 

DISTANCE DOWNSTREAM FROM DAMSITE, IN MILES

FIGURE 2. Longitudinal profile of the flood route.

Transition life zone above an elevation of about 3,000 

feet, which is substantially lower than the regional Sonoran Transition boundary because down-canyon drainage of colder air admits intrusion of higher elevation 

vegetation into the lower life zone. A striking difference 

in vegetation between opposing canyon walls was noted 

and is a function of isolation differences. The southerly 

shaded side of the canyon supports a much larger percentage of high-elevation flora.

BEDROCK GEOLOGY

Bedrock exposed in the immediate vicinity of the 

damsite and along the first 6 miles of channel downstream from the dam is part of a typical sierran hybrid 

diorite pluton. The texture of the rock is variable but 

generally is fairly coarse and devoid of phenocrysts. Local phases of granodiorite, gabbro, and diabase were 

observed. Within the reach that extends 4 miles from 

the damsite to the south end of Parsley Bar (fig. 3), a 

general increase in the quantity of mafic minerals and 

the number of mafic inclusions occurs. Downstream, the 

pluton becomes more gneissic and passes gradationally 

into a small roof pendant of gneiss and sericite schist, 

probably part of the Calaveras Formation of late Paleozoic age that forms the bedrock floor of the channel for 

approximately 100 yards 2.5 miles below the damsite. 

Downstream from the pendant, the rock is a biotite 

quartz diorite. Platy flow structure, schlieren, and alined 

mafic inclusions and segregations are present throughout the plutonic mass.

All the material used in the dam was excavated from 

the spillway immediately adjacent to the damsite and 

is a uniform massive fine- to medium-grained light-gray 

diorite. It is possible to differentiate the fresh hackly 

pieces of fill diorite washed downstream at the time of 

failure from the rock types naturally present as clasts 

in the channel. The clasts are normal dioritic alluvial 

boulders, substantially better rounded than the dam 

material, and have a white weathering patina. Angular 

blocks of bedrock which are derived from valley sides 

can be differentiated by the presence of iron-stained 

sheared surfaces locally crossed by dikelets.

Downstream from the junction with the South Fork 

Rubicon River (fig. 1), the channel is cut mainly in 

schist, slate, and metasandstone of the Oalaveras Formation. Below the confluence of the Rubicon River 

with the Middle Fork American River, the channel is 

incised into highly sheared metasedimentary rock of
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M5

i Partly completed dam

Maximum macroturbulence effects

Bed material not disturbed below 

this point

End of nearly continuous berm 

formation

Evidence of flow episodes

Farthest downstream berm of

dam fill

- Site of channel cross profile 

(previous stream-gaging station)

Terrace accretion

End of en masse movement of 

dam fill

Boulder fronts

Farthest downstream boulder of 

dam fill

Bar with radial flow pattern

Glacial striations maximum 

demonstrable extent of Pleistocene ice

1000 2000 3000 

__I__

CONTOUR INTERVAL 200 FEET 

DATUM IS MEAN SEA LEVEL

4000 FEET 

__I

FIGDEE 3. Location of physical features in the reaches below the Hell 

Hole damsite pertaining to the study. Evidence of flow episodes 

shown on figure 20 ; site of channel cross profile, figure 11; bar with 

radial flow pattern, figure IT.

293-922 O 68

the Calaveras Formation of late Paleozoic age and, 

farther downstream, into the Mariposa Slate of Mesozoic age which is complexly faulted against amphibolite, 

chlorite schist, and assorted greenstones of the Amador 

Group of Mesozoic age.

Major structural trends are northwest-southeast and 

are part of the Mother Lode belt in the western half 

of the area, but are more nearly east-west in the reaches 

close to the dam. Shear zones and fractures trend 

slightly north of west and show no determinable direction of slip. A conjugate set of joints and extension 

fractures into which aplite and pegmatite dikes have 

been introduced is apparently the basic structural control of the channel trends. Conspicuous joint sets trend 

approximately east-west and north-northeast.

GEOMORPHIC HISTORY AND STRATIGRAPHY OF THE 

RUBICON CANYON

The Eubicon and Middle Fork American Rivers are 

bedrock gorges incised within a west-sloping erosion 

surface of Tertiary age, remnants of which form the 

divides on the west flank of the Sierra Nevada. Incision 

that accompanied the Pliocene and Pleistocene uplift 

and tilting of the range allowed the major trunk 

streams, such as the Rubicon, to maintain a westerly 

course normal to the dominant structural lineaments. 

Minor tributaries, however, show a high degree of structural control, particularly along the western flank of 

the range where their flow is parallel to the north and 

northwesterly shear system. Some apparent bedrock 

meanders in upper reaches of the Rubicon are attributed 

to the combined influence of regional slope and transverse joint sets. Various strath terrace levels on gorge 

sides represent fluctuations in the process of valley 

incision.

A poorly defined remnant of what Matthes (1930, 

p. 32) calls the "mountain-valley" stage of Sierra 

Nevada development is present as a series of matching 

slope inflections that probably represent terrace remnants at a level of approximately 1,600 feet above the 

river and 800 feet below the plateau surface in the vicinity of the downstream end of the Rubicon River. Within 

the upper section of the Rubicon are remnants of two 

former levels, one at 15-30 feet and one at 45-60 feet 

above the present channel thalweg. The lower of these, 

and possibly the upper as well, represent valley widening by glaciation, followed by cutting of the notch that 

locally confines the present channel.

The Rubicon River canyon was glaciated downslope 

at least to an elevation of 3,975 feet, three-fourths of a 

mile below Parsley Bar, as indicated by glacial striations and glacially shaped asymmetrical bosses on the
M6 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

bedrock surface. The valley form and gradual transition 

from a broader to a narrower channel suggest glaciation 

to an elevation of 3,650 feet. Striations are also present 

on bedrock spurs at the head of Parsley Bar and 700 

feet below its downstream end. The lowest of the glacial 

benches is marked by glacial flutings and large, meltwater-'scoured potholes.

Parsley Bar, an alluviated valley flat 1,900 feet in 

maximum width, is the most notable of many small areas 

of glacial and glaciofluvial deposition present in wider 

reaches. A composite stratigraphic column of the deposits in the cutbanks of Parsley Bar and the rest of 

the glaciated upper part of the flooded channel is presented in figure 4. These deposits contain an abundance 

of extremely coarse material, supply of which has been 

a controlling factor in the depositional pattern resulting 

from the flood surge.

LITHOLOGY

0

0-20

THICKNESS,

IN 

FEET

0-3

0-10

0-3

0-15

0-18

DESCRIPTION

Cobble-boulder gravel, sand matrix; 

commonly horizontally stratified, 

some large-scale crossbedding; 

coarsest material at top; scour 

channels, sand lenses.

Mudflow or till, similar to lower till; 

thinner weathering rinds; great 

lateral extent.

Pebble-cobble gravel, rust-colored; 

sand matrix; inclined and contorted bedding, some crossbedding, sand lenses.

Granule gravel, rust-colored, 

crossbedded.

Sand and silt, white to rust, poorly 

consolidated, relatively well 

sorted; torrential and trough 

crossbedding.

Till resting in pockets on glacially 

scoured, striated surface; graygreen mud and silt with angular 

dispersed clasts. Granitoid clasts 

are highly decomposed.

FIOURE 4. Composite stratigraphic column of the deposits in the upper 

Rubicon River canyon. Maximum thickness exposed at any site is 58 

feet.

EMBANKMENT FAILURE AND RELEASE AND 

DOWNSTREAM PASSAGE OF SURGE

Hell Hole Dam, a major component of the multipurpose Middle Fork American River Project, is an inclined-core rockfill dam, 410 feet high and 1,570 feet long 

at the crest, which required an estimated 8,800,000 cubic 

yards of material. The events resulting in failure of the 

partly completed structure and release of the flood surge 

were documented by an engineering consulting board in 

1965 at the request of the construction supervisors, 

McCreary-Koretsky Engineers. The following description is based on that documentation and data supplied 

by W. A. Scott, sheriff-coroner of Placer County, I. L. 

Van Patten, assistant resident engineer at the damsite, 

and local newspaper coverage of the event.

A cross section of the stage of completion at the time 

of failure (fig. 5) indicates that the downstream rockfill had been completed to an elevation of 4,470 feet, 220 

feet above the streambed. The impervious earth core, to 

act as a seal on the upstream side of the main supporting 

rockfill, had been placed to an elevation of 4,300 feet, 

approximately 50 feet above the streambed, at the time of 

the storm. Impoundment of storm runoff had reached the 

level of the core crest by 1:30 p.m., December 21,1964, 

and only 34 hours later the water level had moved an 

additional 72 feet up the face of the pervious rockfill. 

Water began flowing through the rockfill shortly after 

the core was overtopped, and flow increased as the level 

rose.

During the early morning darkness of December 23, 

erosion of the toe of the structure began and was accompanied by progressive raveling of the downstream face 

of the embankment. Water was still rising on the upstream face and at 9:30 a.m. had reached a depth of 150 

feet against the entire structure and 100 feet against the 

unprotected rockfill. At this time, continuous slumping 

had sufficiently reached the downstream face of the dam 

for water to top the eroded crest (fig. 6). Once overtopped, the loose rockfill was rapidly reduced, and the 

surge was expelled in 1 hour at an average flow of 

260,000 cfs (cubic feet per second).

As the massive surge began its passage through the 

61 miles of uninhabited canyon, officers of the Placer 

County Sheriff's Office were dispatched to access and 

observation points. They reported a slow initial rise, 

followed by a "terrific amount" of debris, mainly logs 

but including trailer houses, a shack, and assorted tanks 

and drums, followed by the wave, rising at about 2 feet 

per minute. Four of the five downstream bridges collapsed. The State Highway 49 girder bridge broke up 

rapidly and dropped into the river, but, just downstream, the unused Portland-Pacific Cement Co. bridge, 

constructed in 1910 and one of the first multiple-arch.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M7

Elev. 4400 ft.

Water level at time of failure

Elev. 4300 ft.

Upstream rockfill Core Elev. 4250 ft.

0 100 200 300 400 500 FEET

I_____i_____i_____i_____i_____i

FIGURE 5. Cross section of the completed Hell Hole Dam (dashed lines) and the stage of construction at the time of failure (solid lines).

FIGURE 6. Breaching of the rockfill embankment at the Hell Hole damsite near time of peak discharge. View at 9:45 a.m., 

December 23, 1964. Note angularity of freshly quarried diorite, left. Photograph courtesy of McCreary-Koretsky Engineers, 

San Francisco, Calif.
M8 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

spans on the west coast, did not fail. Four of the five 

stream-gaging stations were also obliterated. The surge 

was finally contained in Folsom Lake at about 1:30 p.m., 

having traversed the route from the dam at an average 

speed of 22 feet per second (15 miles per hour).

At the damsite, 726,000 of the 2,100,000 cubic yards 

of emplaced downstream rockfill (fig. 5) had been 

washed down the channel. Approximately 130,000 cubic 

yards of this detritus was subsequently recovered from 

the reach extending approximately 1,800 feet downstream from the dam where the material had accumulated to a depth of 30-40 feet. The partly completed 

core, however, was not eroded. Sedimentological measurements were made starting at the point in the channel below which rock-recovery operations had not destroyed the continuity of the deposits.

FLOOD OF DECEMBER 21-23, 1964, IN THE RUBICON 

RIVER BASIN

THE STORM

The flood-producing rains of late December in the 

Rubicon River basin began with fairly heavy precipitation early on December 19, under meteorological conditions that gave no indication that storms of unusual 

intensity would follow. The Pacific high, a high-pressure airmass, occupied most of the ocean area between 

Hawaii and Alaska and effectively blocked the migration of moist tropical air to the west coast. Because the 

storm track lay around the north side of the Pacific 

high, from the Gulf of Alaska to Oregon and northern 

California, the storm of December 19-20 was accompanied by low temperatures that brought snow to the 

higher elevations.

On December 20, the Pacific high was undergoing 

progressive erosion in the subtropics northeast of Hawaii. This erosion allowed subsequent storms to move 

across the Pacific Ocean at successively lower latitudes 

before turning toward the west coast. A storm track, 

500 miles wide, was thus established from the western 

Pacific near Hawaii to Oregon and northern California. 

Concurrently an extemely cold mass of air from the 

Arctic region met the warm moist air about 1,000 miles 

west of the coast and intensified the storm systems as 

they moved rapidly toward the mainland. The combined effect of moist unstable airmasses, strong westsouthwest winds, and mountain ranges oriented nearly 

at right angles to the flow of air resulted in torrential 

rainfall from December 21 to 23. During this period 

temperatures rose sharply. The freezing level rose to 

elevations of 10,000 feet, and therefore almost all precipitation over the Rubicon River basin was in the form 

of rain.

There are no precipitation gages in the basin, but 

from precipitation records for stations in the surrounding area (fig. 1),, it is estimated that the basin upstream 

from the damsite received 22 inches of precipitation in 

the 5-day period, December 19-23. Figure 7 shows 

hourly precipitation for four stations in the general region of the Rubicon River basin. From these data and 

generalized elevation-precipitation relations for the 

Sierra Nevada, it is deduced that the basin upstream 

from Hell Hole Dam received the following maximum 

amounts of precipitation in the periods indicated:

Duration period (hours) 

1 _____________

12 ________.

24 ________. 

48 ________.

Maximum precipitation (inches) 

.___________ 0.6 

___________ 4.5 

___________ 8 

___________ 14

THE FLOOD

Antecedent conditions were favorable for heavy runoff from the storms of late December. Precipitation had 

been greater than normal during the month of November, and the occasional rains that occurred during the 

first half of December maintained soil moisture at high 

levels. The potential for rapid runoff was further increased by the low temperatures of mid-December that 

caused freezing of the ground at the higher elevations.

A substantial snowpack had accumulated in the mountains prior to the heavy precipitation of December 21- 

23, but melting snow is believed to have had only a minor 

effect on the flood peaks that resulted from the December storms. On December 20, about 20 inches of snow lay 

on the ground at the 6,000-foot level. On that day, the 

U.S. Weather Bureau reported 67 inches of snow on the 

ground at Twin Lakes, elevation 7,829 feet, south of the 

Rubicon River basin. By December 24, the depth of the 

snowpack at Twin Lakes had diminished to 42 inches. 

No data are available concerning the water content of 

the snowpack, but much of the 25 inches of reduction of 

depth at Twin Lakes was probably due to compaction. 

Preliminary analysis of the Sierra Nevada floods by the 

U.S. Army Corps of Engineers and by the California 

Department of Water Resources has indicated that over 

most of the snow-covered area the heavy rainfall and 

associated meteorological conditions during the storm 

generally supplied only enough heat to ripen the snowpack. This ripening means that the snowpack became 

isothermal at 32°F and became sufficiently dense to retain a small percentage of free water in the capillary 

spaces in the pack. The net result was that most of the 

snowpack had little effect on the peak runoff, in that it 

contributed little melt and offered little delay to the 

rain passing through it. However, at lower elevations 

where the snowpack was too light to resist melting by 

the rain, the melt augmented runoff. On the other hand, 

at the higher elevations where the snowpack was very
0.2

CO °'4

LU

I

o

? 0.2

0.0

FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA

DECEMBER

M9

DAILY TOTALS

Union Valley Reservoir

DAILY TOTALS

DAILY TOTALS

0.0

DAILY TOTALS 

FIGDRB 7. -Hourly precipitation at stations nearest the Hell Hole damsite for the period December 18-26, 1964.

heavy, the pack absorbed rain and actually reduced 

runoff.

The runoff in response to the intense rain of December 21-23 was dramatic, and streams everywhere rose 

rapidly. This was the third time in 9 years that the Rubicon River basin had been deluged, and, if it were not 

for the primitive state of development of most of the 

region, damage on each occasion would have been catastrophic. The first of the three floods occurred in December 1955, but the peak discharges of that flood, the 

greatest recorded until that time, were exceeded in the 

flood of January 31-February 1,1963. The flood of December 1964 had peak discharges that were roughly 

equivalent to those of the flood of 1963 in the general 

area. Three- and five-day periods of maximum storm 

runoff for the 1964 flood were 109 and 146 percent of the 

1955 values, the largest previously recorded. Peak stages 

and discharges for the three floods at gaging stations 

on the Rubicon River mainstem and lower Middle Fork

American River are listed in table 1. Also included in 

table 1 are estimated recurrence intervals for the floods 

at these five stations, as determined from a statewide 

flood-frequency study by Young and Gruff (1966).

Of the three gaging stations on the Rubicon River 

mainstem (fig. 1), only that at Rubicon Springs is upstream from Hell Hole Dam. This station was established after the 1955 flood, and consequently no 

hydrograph is available for that flood, although its 

peak discharge had been computed from floodmarks at 

the site. Complete hydrographs are available, however, 

for the floods of 1963 and 1964. The Rubicon River 

station near Georgetown was operative during the 1955 

flood, but was destroyed both in 1963 and 1964, and 

consequently, hydrographs are not available for the 

last two floods. The Rubicon River station near the 

river mouth (near Foresthill) was established after 

the 1955 flood, and it too was destroyed both in 1963 

and 1964.
M10 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

TABLE 1. Summary of flood stages and discharges

Maximum flood

Stream and place of 

determination

Recurrence

Gage Dis- interval 

Date height charge (times>100- 

(ft) (cfs) yr flood)

Rubicon River

4280....

4310..-.

4332....

Rubicon River at Rubicon 

Springs.

Rubicon River near 

Georgetown.

Rubicon River near 

Foresthill.

Dec. 1955.. 

Feb. 1963- 

Dec. 1964...

Dec. 1955- 

Feb. 1963- 

Dec. 1964..

Feb. 1963- 

Dec. 1964..

13.0 

14.28

18.76 

25.8 

71. 1±_

35

78

9,270 

11,500 

U1.300

51,000 

58,000

83,000

1.20 

1.49 

1.47

1.46 

1.66

1.54

Middle Fork American River

4333....

4335....

Middle Fork American

River near Foresthill.

Middle Fork American

River near Auburn.

Feb.

Dec.

Dec.

Feb.

1963-

1964-

1955-

1963-

1964- .

38

61

33.9

43.1

60.4

113,000

2 310, 000

379,000

3 121,000

. « 83, 600

 253,000

1.20

3.30

.34

1.06

.38

2.22

1 Adjusted for storage and diversion.

2 1964 discharge conputed by slope-conveyance method.

a Old site; about 1,000 ft upstream from present site.

* Natural flow prior to surge.

' Surge.

Of the two gaging stations on the Middle Fork 

American River, that near Foresthill, just 'below the 

mouth of the Eibucon Eiver, was established after 1955 

and destroyed in 1964. Consequently, the only one of 

the three flood hydrographs available for this station 

is that for 1963. The station on the Middle Fork 

American Eiver near Auburn operated during the 

floods of 1955 and 1964, and flood hydrographs are 

therefore available for those years. The station was 

destroyed, however, by the flood of 1963.

Because there are no concurrent hydrographs for 

any of the three major floods at successive gaging stations on the Eubicon Eiver mainstem and lower Middle 

Fork American Eiver, flood-routing procedures could 

not be developed to compute the hydrographs for the 

flood wave of December 1964 at the destroyed gaging 

stations. Furthermore, the lack of precipitation stations in the affected basins precludes the use of rainf allrunoff relations for reliable computation of inflow into 

the reaches between gaging stations, for use with any 

flood-routing procedures that might be developed. Some 

rather crude methods were used, therefore, to compute 

the inflow and outflow hydrographs at Hell Hole Dam 

and the peak discharge at stations downstream from the 

damsite.

INFLOW AND OUTFLOW AT HELL HOLE DAM

The only data available for computing inflow and 

outflow at Hell Hole Dam were a record of storage 

of impounded water behind the dam during the flood 

period (fig. 8) and a chronology of the dam failure, 

both supplied by McCreary-Koretsky Engineers. It was

essential that the inflow hydrograph be synthesized, 

because with an inflow and storage record, the outflow 

hydrograph could be computed from the equation for 

conservation of mass:

Outflow=Inflow ± change in storage.

The obvious method of computing the inflow hydrograph would be by means of a rainfall-runoff relation, 

such as the unit hydrograph, which could not be used 

because of the lack of rainfall records in the basin. The 

method finally used to compute inflow involved a relation between natural discharge of the Eubicon Eiver at 

Eubicon Springs (31.4 sq mi) and at Hell Hole Dam 

(120 sq mi). This relation, in turn, was derived from a 

relation between natural discharge of the Eubicon Eiver 

at Eubicon Springs and discharge of the Eubicon Eiver 

above the mouth of South Fork Eubicon Eiver (135 sq 

mi). The discharge of Eubicon Eiver above the mouth 

of South Fork Eubicon Eiver was obtained by subtracting the gaged discharge of South Fork from the 

gaged discharge of the Eubicon Eiver near Georgetown. The net result of the analysis was the. equation:

Inflow at Hell Hole Dam=3.5 X (discharge at Eubicon Springs, adjusted for storage and diversion)

From this equation, the inflow hydrograph at Hell Hole 

Dam was computed (fig. 8), its peak discharge being 

39,000 cfs.

A check of the inflow hydrograph was made by examining the area under the hydrograph for the storm 

period December 20-27. A basic premise implied in the 

derivation of this hydrograph was that storm runoff, in 

inches, of the Eubicon Eiver at Hell Hole damsite was 

closely equivalent to the storm runoff of the Eubicon 

Eiver near Georgetown. Furthermore, runoff records 

for the past 15 years show that during major storms, the 

storm runoff at the Georgetown station was 1.25 times 

as great as the storm runoff of Silver Creek at Union 

Valley Dam (fig. 1). For the period December 20-27, 

1964, a storm runoff of 16.97 inches for Silver Creek 

indicates a corresponding storm runoff of 21.21 inches 

for the Eubicon River near Georgetown. This runoff 

value for the Georgetown station closely checks the volume of inflow 21.24 inches under the hydrograph for 

Hell Hole Dam.

The computed inflow hydrograph and the record of 

storage at Hell Hole Dam were used, as explained 

above, to compute the outflow hydrograph. The outflow 

hydrograph, also shown in figure 8, shows that the discharge attained a maximum 1-hour mean discharge of 

260,000 cfs when the embankment was breached.
40,000

35,000

30,000

S 25,000

20,000

15,000

10,000

5000

FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA

320,000

Mil

jxFor peak surge see graph at right

DECEMBER 

A. Inflow and outflow hydrographs at Hell Hole Reservoir

280,000

Q

O 240,000

200,000

160,000

z 120,000

40,000

I Hole outflow hydrograph, 1964

21 23

DECEMBER

B. Hydrograph of peak surge outflow at Hell 

Reservoir

Hole

28,000

24,000

20,000

16,000

12,000

8000

240,000 

Q

Z

0

0

UJ 

0.

LJ

U.

0

5° 120,000

o

z

o tr

T.

Q

0

_ -^- -

20

_/

21

A J*

//-

//

22

IA

^^

Da

 <1 ' \N

23

surge caus

m. Comp

-1955 A1964

v^

24

ed by failu

uted volun

~^ra.^-_^

25

re of Hell

ie 24,800 'c

_s->

26

Hole

ere -ft.

V> C^= ^

27

DECEMBER

C. Record of storage in Hell Hole 

Reservoir

DECEMBER 

D. Hydrographs at Middle Fork American River near Auburn

FIGURE 8. Flow and storage data associated with the failure of Hell Hole Dam in December 1964.
M12 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

PEAK DISCHARGE OF THE SURGE DOWNSTREAM FROM HELL HOLE DAM

In the absence of a reliable procedure to route the flood 

wave from Hell Hole Dam to downstream gaging stations, the slope-conveyance method was used to compute peak discharge. The basic equation in the method 

is the Manning equation:

Q=KS», (1)

where Q is peak discharge, in cubic feet per second,

K is conveyance, in cubic feet per second, and 

S is the slope of the energy gradient.

The conveyance, K, is determined from the geometry 

and roughness of the channel, by means of the equation:

(2)

n

where n is the Manning roughness coefficient,

A is cross-sectional area, in square feet, and 

R is hydraulic radius, in feet, or cross-sectional 

area divided by cross-sectional wetted 

perimeter.

In the slope-conveyance method, the assumption is made 

that the energy slope at a site is constant for all extremely high stages. Therefore, if the discharge at some high 

stage (Qi) is known, the discharge at some higher stage 

(Qz) can be computed from the ratio of the conveyances at the two stages, where

Q,=Ql(Ka/Kl). (3)

If there is no overbank flow at either of the two discharges, the values of nr and nz will probably be equal 

to each other in which case equation 3 can be further 

simplified to :

j*). (4)

Equation 4 was used to compute the 1964 peak discharge 

(Qz) at the gaging station on the Middle Fork American Kiver near Foresthill (table 1) which was destroyed 

by the flood. At this station, the 1963 .peak stage and 

discharge (Qi) were known, as well as the 1964 peak 

stage.

The gage on the Middle Fork American Kiver near 

Auburn operated throughout the flood and strikingly 

shows the surge caused by the failure (fig. 8) . The computed volume of water in the surge is 24,800 acre-feet 

at this station.

ATTENUATION OF SURGE WAVE

Numerous approximations of stage, made by hand 

level throughout the flood course, indicate that the 

height of the flood wave decreased at a fairly constantrate between the dam and Parsley Bar and again in the 

reaches between the Kubicon gorge and Folsom Lake.

Wave Height increased below Parsley Bar, reaching a 

maximum in the confining narrows of the Rubicon 

gorge and reflecting the gradual narrowing of the channel downstream from Parsley Bar.

The peak discharge of the surge also decreased downstream. The peak on the Middle Fork American River 

near Foresthill, computed by the slope-conveyance 

method, was 310,000 cfs (table 1). Downstream, peak 

flow near Auburn was 253,000 cfs, including a natural 

flow of approximately 60,000 cfs in the river just before 

the surge. Trial application of the slope-conveyance 

technique at the three upstream gaging stations which 

were destroyed by the surge suggested that larger discharges occurred there. There is no way, unfortunately, 

of estimating the momentary peak release at the damsite which was certainly substantially greater than the 

maximum 1-hour average flow of 260,000 cfs (fig 8).

Because breaching of the dam was not instantaneous, 

release from the reservoir was sufficiently retarded so 

that the surge did not immediately attain its peak discharge. There was apparently no development of a bore 

or breaking front to the wave during any part of the 

downstream flow. Average velocity of the wave, approximately 22 feet per second (or 15 miles per hour) 

over the entire route, was determined by map measurement of longitudinal distance in the channel thalweg 

and from the time that the main mass of forest debris 

uprooted by the surge entered Folsom JLake.

EROSIONAL EFFECTS OF THE SURGE

Sedimentary deposits in and along the channel were 

severely eroded by the surge. Deposits affected by the 

flood wave include, in approximate order of magnitude: 

(1) Alluvial deposits, of both Pleistocene and Recent 

age, (2) colluvium, including material from landslides 

triggered by the surge, (3) soil, and (4) bedrock. Erosion of the terrace gravel of Pleistocene age was dominantly by lateral cutting rather than surface scour, 

probably because the surface of the deposits was armored by a mat of vegetation (fig. 9). A surprisingly 

small amount of bedrock plucking occurred, but notable 

expanses of bare scoured bedrock surfaces, particularly 

in the gorge of the Rubicon, give the impression of 

extensive removal of joint-bounded blocks. Wherever 

preflood control is present, as at bridge and gagingstation sites and as evidenced by comparison of preflood 

photographs with the present channel, only a minor 

amount of bedrock removal can be proved. Striations 

and percussion marks formed by cobbles and boulders 

carried by macroturbulence are common on the bedrock 

surfaces up to the high-water mark in the interval
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M13

FIGURE 9. Lateral supply of sediment to the flood channel from a deposit of terrace gravel and till, 3.3 miles downstream from the

damsite.

from the damsite to approximately 1,200 feet down the 

channel.

Stands of mature yellowT pine, cedar, and Douglas-fir 

wTere removed by the surge, the maximum loss of timber 

occurring in the Parsley Bar area. Many of the trees 

were 250-400 years old. A large yellow pine and a cedar 

felled by the flood wave were 351 and 390 years old, 

respectively. Jams of timber within the channel were 

relatively rare. Several formed in the 4-mile reach below 

Parsely Bar, but most of the vegetal debris traveled 

the complete route to Folsom Lake. A few trees in the 

upper reaches were killed by bark removal. The maximum abrasion of tree trunks by suspended rock fragments was generally in a zone from 3 to 5 feet above the 

present aggradational surface (fig. 10). Below this 

level, the trunks may have been protected by temporary burial. Even in the lower reaches of the flood course, 

the discharge was clearly of a high recurrence interval. 

Possibly late 19th century placer tailings wTere removed 

from Poverty Bar, Cherokee Bar, and uppermost 

Oregon Bar, all on the Middle Fork American River.

293-922 O 68 3

EFFECTS ON CHANNEL MORPHOLOGY

The five gaging-stations sites along the flood course at 

which channel cross profiles had been previously measured were resurveyed. Changes in channel morphology 

wrought by the surge decreased downstream as discharge and recurrence interval of the surge decreased. 

Comparison of twT o presurge surveys at each of two of 

the cross-profile sites shows that the bulk of the observed 

change wT as in fact related to passage of the surge and 

not to normal annual fluctuations.

A summary of the channel modifications and stage at 

each station is presented in table 2. Change is most evident at the station farthest upstream wThere the pronounced aggradation in the thahveg is related to downstream movement of rockfill from the dam (fig. 11). 

Deposits on the glacial-outwTash terrace at this station, 

however, contain very little material derived from the 

dam, and therefore occurred at an early stage of the 

surge. The other cross profiles exhibit thahveg aggradation that is unrelated to deposition of the dam rockfill.

The cross profiles and field observations suggest that 

much colluvium, locally containing very coarse material,
M14 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FIGURE 10. Trees showing maximum abrasion level 3-5 feet above present ground surface. Perched boulder is 3.4 feet in intermediate diameter. View Is looking downstream, approximately 4 miles below the damsite.

has been stripped from the base of steep valley sideslopes adjoining the bedrock channel which probably 

have not been awash since glacial recession. Slumping of 

boundary terrace deposits into the channel resulted 

from undercutting. Most of this locally derived sediment was apparently introduced into the flow on the 

receding stage when competency was continuously decreasing. Therefore, far more material was supplied to 

the channel than could be transported, and much of the 

extremely coarse material supplied from the sides of 

the channel was either not moved or moved only short 

distances. Thus, a chief effect of this short-period catastrophic surge was to introduce coarse sediment from 

lateral deposits into the active stream channel with a 

resulting net aggradation in the thalweg.

In short, the effect in terms of cross profile was to 

modify a V-shaped channel to a more nearly U-shaped 

channel. The surge altered the channel radically in confined reaches and moved, at least for short distances, a 

large proportion of the material previously present in 

the channel. Although only five cross profiles are a 

meager record on which to generalize, deposition in the

thalweg may have been nearly continuous along the 

entire flood route.

MASS MOVEMENTS CAUSED BY THE SURGE

The surge triggered massive landslides as it progressed downstream and undercut valley side-slopes. 

Most of the slope failures were at the outer bank of 

channel bends where velocity and water depth were 

greatest. Large slides were confined to the steep-sided 

gorge of the Rubicon, which is cut in slaty metamorphic 

rocks of the Calaveras Formation. The steepness of these 

slopes, which have a deep regolith, is attributed partly 

to the nearly vertical attitude of the bedrock. At the 

time of surge passage, the slopes had been exposed to approximately 48 hours of continuous rain and were saturated so that conditions were ripe for slope failures. 

Although evidence of soil creep is general, there are 

virtually no recent scars of previous major earthslides in 

the canyon.

The slope failures are commonly triangular in plan 

view and the largest has a maximum toe width of 630 

feet. Slide scars reach a maximum elevation of 500 feet
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M15

TABLE 2 Description of channel cross profiles

Location of stream-gaging 

station

Distance 

downstream

site (miles)

Peak stage (ft)

1963 1964

Description of flow cross profile 

and channel

Approximate 

change in Changes in cross profile 

thalweg 

elevation 

(ft)

Rubicon River below 1. 3 21. 5 53. 5 

Hell Hole Dam. 

(See fig. 11.).

Rubicon River near 8. 9 25. 8 71. 1 

Georgetown.

Rubicon River near 26. 3 35. 0 78. 2 

Foresthill.

Middle Fork American 35. 6 38.0 61. 1 

River near Foresthill.

Middle Fork American 54. 5 1 43. 1 60. 4 

River near Auburn.

Lateral bedrock control, 

675 ft wide; alluvial 

fill, 375 ft wide. Glacialoutwash terrace comprises left half of fill.

Lateral bedrock control, 

400 ft wide; alluvial fill, 

250 ft wide. Left bank 

cut in glaciofluvial and 

colluvial deposits.

Lateral bedrock control, 

255 ft wide; alluvial fill, 

135 ft wide and probably 

quite shallow.

Lateral bedrock control, 

378 ft wide; alluvial fill, 

260 ft wide.

Only basal part of section 

was resurveyed.

+ 7. 0 Aggradation in thalweg; 125-ft 

lateral movement of thalweg; 

accretion on terrace surface; 

probable bedrock removal 

on right bank.

+ 1. 5 Aggradation in thalweg; 50-ft

lateral movement of thalweg; 

deposition of large boulder 

bar on right side of channel. 

No erosion of bedrock.

+ 2. 5 Aggradation in thalweg; no

lateral displacement of thalweg; addition to boulder bar 

on left side of channel. Possible slight removal of bedrock.

+ 3.0 Aggradation in thalweg; 20-ft 

lateral displacement of thalweg; addition to coarse bar on 

left side of channel. No removal of bedrock.

+ . 2 Slight aggradation in thalweg 

but as much as 3.0 ft of aggradation in other parts of 

channel, addition to coarse 

bar on left side of channel. No 

determinable removal of bedrock.

1 Old site; about 1,000 ft upstream from present site.

EXPLANATION

Pine tree with scar of cable attachment

Bedrock

o

Control point 

On prefailure surface

 

Control point

On postfailure surface

FIOUBK 11. Prefallure and postfailure cross profiles of the channel at the site of the first gaging station reached by the surge, Rubicon River

below Hell Hole Dam, 1.3 miles downstream.
M16 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FIGURE 12. Base of the largest slide In the Rubicon gorge, 0.5 mile downstream from the mouth of Little Grizzly Canyon, 18.7 

miles downstream from Hell Hole damsite. Note scour due to surge at right, the downstream direction. Helicopter, right, 

indicates scale.

above the channel. The slide that involves the greatest 

volume of displaced material is linear in plan view and 

has a nearly constant width of about 300 feet. It originated in a spoon-shaped rupture approximately 400 feet 

above the bottom of the gorge (fig. 12). In addition to 

the large slides, nearly continuous sliding and slumping 

of colluvium on a smaller scale occurred along channel 

banks within the gorge.

Slides are identified as the slow-earthflow and the 

debris-avalanche type described by Varnes (1958). At 

the heads of several slides, individual blocks of bedrock 

moved by slip, but such block slumping was transitional 

to movement by earthflow and debris avalanche in the 

lower part of the slides. Most of the slides began as 

mass movements of semiconsolidated material and were 

soon transformed downslope to flows.

The largest slides occurred after the surge peak and 

thus did not contribute significantly to the sediment 

load of the flood. Movement of some slides probably 

began before arrival of the flood peak, and the removal 

of detritus from the toe of the slide allowed renewal of 

movement. Most of the smaller slides were probably directly triggered by erosion near the flood peak and were 

virtually coincident with passage of the surge. Presence 

of highest scour lines on some slides at levels substantially below the high-water mark shows that sliding continued during and after flood recession. Virtually all 

the landslips were closely related to the surge passage, 

and most of the slides were directly triggered by the 

flood wave. Probably comparable saturation conditions 

resulting from storms of 1955 and 1963 produced little 

or no sliding. As discussed on page M36, the destruction 

of slope equilibrium by the surge of 1964 also resulted 

in renewed sliding a year later during the next rainy 

season.

Even the largest slide did not appreciably dam the 

channel. The present low-water channel cuts through as 

much as 7 feet of slide detritus and lateral levees 

of slide detritus (fig. 12) are present downstream 

from the slide. Measurements of floodmarks immediately upstream showed a maximum depth of 62.3 feet 

and those downstream, 63.5 feet, in a comparable channel. Thus, the peak flow apparently was not affected by
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M17

the slide, and most of the movement was during the recession phase.

Determination of the surface area of three of the 

largest slides and estimation of the depth, for slumped 

parts of slides as described by Philbrick and Cleaves 

(1958), permitted an estimate to be made of the volume 

of material in each slide. Seismic measurements of depth 

were available for one of the three slides. Extension of 

these estimates to 29 other slides, on the basis of relative size as measured on aerial photographs, yielded an 

order-of-magnitude approximation of the total amount 

of material involved in landslides resulting from the 

flood 800,000 cubic yards a surprisingly small figure 

and an estimate subject to large sources of error.

Only a small part of this volume was actually contributed as sediment to the flood. The time relation of sliding to the flood crest, armoring of the debris avalanches 

by extremely coarse material at the surface, and coherence of the mud snouts of the flows prevented much erosion by the flood. The percentage of material eroded 

from each slide by the surge was estimated principally 

on the basis of aerial photographs and, for about a third, 

on the basis of field observations. Less than 30 percent of

the total slide volume was removed by the flood. Eroded 

colluvium and the inestimable quantity of detritus supplied by continuous small-scale sliding that occurred in 

steep-walled reaches were almost certainly more important as sources of sediment. Virtually all the latter material was moved by the flood.

DEPOSITIONAL FEATURES

METHODS OF STUDY

Depositional forms were studied in the field and by 

analysis of large scale (1:1,200) aerial photographs, obtained for the 4 miles of channel downstream from the 

damsite. Additional photography was made of the remainder of the flood channel at a scale of 1: 6,000. The 

photography was made between August 28 and September 10, 1965. A comparison with two sets of preflood aerial photographs was possible, one flown in 

September and October 1948, at a scale of 1:42,000, and 

another taken during July and August 1962, at a scale 

of 1:20,000. Approximately 12 miles of the poorly accessible canyon was covered on foot and the rest was 

traversed by helicopter. Channel forms from the dam

&&&sae&!*&®3@it&

FIGURE 13. Boulder berms preserved 0.7 mile downstream from the damsite. Note greater coarseness of material in center of

channel. Flow Is from left to right. Photograph by J. C. Brice.
M18 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

to a point 5 miles downstream were studied and mapped 

by use of the large-scale photographs and T^-minute 

topographic maps enlarged three times as base maps.

LATERAL BERMS OB TERRACES

The most striking depositional forms resulting from 

passage of the flood surge are terracelike berms composed of boulders of diorite from the dam rockfill (fig. 

13). The nearly flat-topped berms occur continuously 

on both sides of the channel in the uppermost reach, but 

downstream they are confined to protected areas behind 

bedrock projections or the inner bank of bends. Berms 

composed of rockfill derived at the damsite are restricted 

to the upper part of the flood route, which ends 1.1 miles 

downstream from the dam toe.

The berm surface is as much as 28 feet above the present channel thalweg and decreases generally, but not 

regularly, downstream. Although subsequent artificial 

removal of material prevents exact determination, the 

level of the berms probably rose upstream nearly to the 

level of the dam core, 50 feet above the channel. At a 

point approximately 1,600 feet downstream from the 

dam, a pronounced inflection in the berm surface occurs.

The surface does not correlate with a change in slope 

in the present thalweg, which is extensively aggraded 

in that reach, but may correspond to a slope change in 

the original channel or to a constriction in the temporarily clogged channel. In transverse section, the surface 

of the berms usually slopes gently toward the present 

channel, and where berms are present on both sides of 

the channel they occur at approximately the same level.

Another characteristic of the berms is the presence of 

the coarsest detritus at or near the surface (fig. 14). 

They are similar in this respect to the glacial-outwash 

terraces in the Kubicon Kiver basin. This reverse grading of the deposits relates to the depositional dynamics, 

possibly to dispersive stress forcing larger particles to 

the surface during movement (Leopold and others, 1964, 

p. 211) rather than to occurrence of the coarser particles 

remaining as a lag deposit after removal of fines.

Generalization on the relation of the coarseness of the 

berms to the coarseness of material in the channel is not 

possible. In the reach immediately downstream from the 

dam, coarsest debris occurs in the center of the channel. 

Through the middle of the area of berm formation, in 

which the pool-riffle pattern becomes more pronounced,

FIGURE 14. Section of berm parallel to stream channel in reach below Hell Hole damsite. Rucksack is at the base of the berm 

section. Note the coarse material at the top of the berm surface and the contrast in size with coarser material in center of 

channel, foreground. All material is diorite from the damsite.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M19

berms may be either coarser or finer than the adjacent 

channel material, but all the berms are coarser than pool 

components. The berms farthest downstream are distinctly finer than the channel material.

The terracelike berms are superficially analogous to 

common alluvial terraces, formed by channel aggradation followed by incision of a lower channel. Generally, 

alluviation followed by erosion is a lengthy process. The 

fact that the terracelike berms were formed by a single 

flood event and that the bed material is almost entirely 

of boulder size, and thus, once deposited, would be unlikely to be eroded at a later, necessarily lower, stage 

suggests another origin for the coarse terracelike 

deposits.

Macroturbulent conditions transported boulders in 

suspension well above the berm surface; the evidence of 

such transport, perched boulders of rockfill diorite, is 

most conspicuous in the uppermost reaches of the flood 

channel which show extensive berm formation. The material composing the berms, however, was deposited 

from bedload, which locally moved as thick subaqueous 

debris flows or a series of gravel waves, in this section 

of the channel. The top of the continuous berms may 

represent the approximate level of bedload flow or wave 

movement attained in the main channel rather than 

any static aggradational surface. The degree to which 

the present channel below the berm surface represents 

incision by erosion at a later stage must remain an 

unknown, as must the configuration of the surface, 

whether static or dynamic, represented by the berms. 

The berm surface probably was continuous across the 

channel at one period during the surge. Bedload movement may have been continuous in the center of the 

channel and, as the material moved out of the reach, the 

present low-water channel could have been left as basically a depositional rather than an erosional feature. In 

terms of the time sequence of events only, the situation 

may be similar to that occurring in the formation of 

mudflow levees in which, just after a confined mudflow 

has peaked, continuing flow in the center of the channel 

moves most of the flow out of the reach, leaving terraces 

marking the peak on each side of the watercourse. Local 

presence of the coarsest material in the center of the 

channel is due to lagging of the largest particles well 

behind the peak volume of bedload movement. At least 

in the lower reaches of berm formation, both formation 

and incision or removal of the berms probably occurred 

during the receding stage. This probability is evidenced 

by relations at the cross-profile site (fig. 11) where material carried by the surge peak and then deposited on a 

high-level terrace included very little rockfill detritus.

All the berms described above are composed of damfill material. Downstream, similar but discontinuous

isolated ridges of sand formed in areas of low velocity 

during the surge, particularly on vegetated banks. Such 

distinctly finer grained berms may be present within 10 

feet of the high-water mark where the stage was approximately 50-60 feet. Also in a downstream direction, 

local terracelike berms formed by the surge and composed of normal alluvial gravel, including some detritus 

of boulder size, occur on point bars and behind bedrock 

projections on the Middle and North Forks of the 

American Eiver. Thus, the berm-f orming process is not 

solely related to the supply of sediment provided by the 

rockfill, but does relate generally to the large volume of 

sediment transported by the surge.

BOULDER BARS

The terracelike berms are transitionally replaced 

downstream by lobate bars. The bars are in part flat 

topped, consist of boulder-size material, and are the 

dominant depositional forms downstream. The upstream 

pattern of the bar trends is linear, parallel to the 

straight reaches between the damsite and Parsley Bar, 

and the apexes of the bars are directly downstream from 

bedrock projections within or lateral to the channel 

(fig. 15). As the channel becomes sinuous, the pitch 

of the surfaces of the bars alternates from one side 

of the channel to the other, as does the low-water channel (fig. 16).

In part of the uppermost section of the channel and 

at the downstream end of Parsley Bar, there is a medial 

bar or ridge of coarse material. The bar terminates in a 

frondescent or radial pattern with flow lines deviating 

as much as 50° on each side of the axis of the channel. 

The flow lines are not visibly related to channel configuration, a slightly contracting reach, but are marked 

distinctly on the aerial photographs by felled and 

lodged trees, textural changes, and ridges of bed material (fig. IT).

TERRACE ACCRETION

Addition of material on the surface of a terrace remnant near the head of Parsley Bar suggests that the 

flood had a greater magnitude than flows, presumably 

of Pleistocene age, that were responsible for transport 

of the terrace fill. As much as 5 feet of boulder gravel 

was added to the terrace remnant on the left bank at 

the site of the former gaging station, Eubicon Eiver 

below Hell Hole Dam, 1.3 miles downstream from the 

dam (fig. 11). The flood did not aggrade the extensive 

terrace deposits of Parsley Bar or any terrace farther 

downstream.

The gravel added to the terrace upstream from Parsley Bar consists almost entirely of reworked terrace 

gravel derived from the margins of that terrace and
M20 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FIGURE 15. Lobate bar formed downstream from a bedrock projection into channel at right. Central part of bar is mainly reworked 

terrace and recently deposited alluvial detritus. Secondary bermllke formation in foreground is predominantly rockflll from the 

dam embankment. View looking downstream 1.1 miles below the damsite.

from others upstream. However, two cored boulders 

of dam fill were observed in the deposit. The head of 

the terrace was eroded and a cutbank having a maximum height of 12 feet was formed, at the base of which 

the present channel fill is composed of diorite derived 

from the dam. The comparison of aerial photographs 

suggests that as much as 20 feet of lateral erosion occurred along the terrace margin.

BOULDER FRONTS

An arcuate boulder front 3-7 feet high extends 250 

feet across the flood channel in the middle of Parsley 

Bar (figs. 18,19). The front represents a point to which, 

at a late stage of the surge passage, boulders as much as 

10.5 feet in intermediate diameter were transported, but 

beyond which little coarse material was moved. The 

boulder deposit transgressed across and only partly replaced a sand layer 2-8 feet thick that was deposited 

during an earlier interval of surge recession. Numerous 

small trees in growth position protrude through the 

sand layer which was molded into longitudinal and arcuate transverse dunes. Similar boulder fronts on a

smaller scale occur at other localities on Parsley Bar.

The term "boulder front" is used here to indicate 

a scarp or face of boulder gravel normal to the flow direction. The deposits are not similar to the bouldery 

snouts of mudflows, to which the term has been applied, 

in that the entire deposit in this case is composed of 

boulder gravel.

Like the berms, the main boulder front apparently 

is characterized by presence of the coarsest clasts at the 

surface, although the coarseness of the deposit precluded thorough excavation. The front occurs in a rapidly expanding part of the Parsley Bar reach. During 

the flood recession, the front was incised at the position 

of the present low-water channel, and coarse material 

was moved a small distance downstream. The single 

cored boulder of dam fill observed beyond this point 

was probably transported by macroturbulence during 

the surge peak.

The front represents the point in the channel at 

which the critical tractive force fell below the value 

necessary to move the entire flow, because of reduction 

of depth associated with the gradual but pronounced
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M21

FIGCEE 16. Large gravel bar formed 1.2 miles below Hell Hole damslte. Riffle in foreground is parallel to stream course. Bed 

material is predominantly rockfill from the dam. Width of channel is approximately 200 feet.

expansion of the reach and reduction in slope. A stage 

measurement showed that maxium depth of water at 

this point in the channel was 5-6 feet above the top of 

the deposit and was probably substantially less at the 

time the boulder now reached this point. Competency 

necessary to move the mass was clearly much less 

than that required to move the component particles 

individually.

Two alternative hypotheses for formation and movement of the boulder front are proposed. According to 

one, the boulder front represents the front of a type 

of subaqueous viscous now in which the bedload moved 

as a churning mass fluid, yet sufficiently viscous to 

move as a discrete unit. Fronts that are possibly in part 

analogous have been reported on subaerial gravelly 

mudflows by Sharp and Noble (1953, p. 551) and 

Fahnestock (1963, p. 20) and on flood gravels by 

Krumbein (1942, p. 1364) who applied the term "boulder jams." According to Krumbein, longitudinal 

segregation results in concentration fronts of very 

coarse debris which dam the channel.

Alternatively, the boulder fronts may represent a 

large wave of gravelly material analogous to a sand

29S-922

wave or delta front. Movement is by rolling and saltation of the component material at the surface of the 

deposit until the foreset part or avalanche face of the 

form is reached, whereupon particles cascade down the 

front and are at least temporarily deposited owing to 

the sharp reduction in competence at the crest of the 

front. Migration of the dune form or delta takes place 

continuously by erosion of the upstream surface and 

deposition on the front. Tractive force necessary is 

that required to move the component particles as 

normal bedload.

Dune or delta movement obviously requires particles 

with similar hydraulic behavior, and dune materials 

are thus generally fairly well sorted. Consideration 

of this mode of transport is based on two lines of evidence. The morphology of the front is remarkably 

similar to that of generally smaller scale sand waves or 

migrating microdeltas (PettiJohn and Potter, 1964, pi. 

74). However, strongest evidence for this mode of 

formation is the presence of probable crossbedding of 

an amplitude similar to the height of the boulder front 

and exposed in a cutbank in glaciofluvial deposits of 

Pleistocene age at the same position in Parsley Bar.
M22 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

Q.

G)

CD 

O.

CD

EXPLANATION

Scarp of sand or gravel

Marked change in 

grain size

o 

Boulders (X 2 scale

size)

Current direction 

(fallen trees)

Flow lines

Approximate highwater mark

Cutbank in terrace 

deposits

Riffle in low- water 

channel

Boundary of low- water 

channel

CD

Field sketches applied to 

an aerial-photograph 

base. Boulders are 

shown twice the actual 

scale size and each is 

the coarsest particle in 

its immediate vicinity.

100

I

200

I

300 FEET

FIGURE 17. Radial pattern of flow and deposition at the downstream end of Parsley Bar.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M23

M^~

"*""*^^S''^"S-'^^'^S^ '^^S* '^^^^ik'J^^

. 4<te *'.** '^^5 .iiair^a^yfite.8'^^"

r-'«i^.S&r5--- '^^^^gt^mm>i

._ »»r; ^^ *%K.- Tv^Wr."a»*i "if »-*«kfcr-»»... «(__iw

 *=j

FIGURE 18. View across flood channel of boulder front on Parsley Bar, Maximum height of the front is 7.2 feet. Note trees tilted

in a downstream direction, to the right.

Crossbedding, inclined stratification in most places 

truncated at the upper surface, is preserved evidence 

of the migrating front or foreset area of dune or microdelta. Thus, wave movement with large amplitude 

probably occurred at that point on the bar during 

Pleistocene time. The crossbedded unit, which will be 

described below, contains a few clasts of boulder si&e 

but is domjnantly a sand, pebble, and cobble gravel.

The material making up the 'boulder front deposit in 

the f oreset region has a median diameter (74 mm) that 

is much in excess of any reported value for crossbedded 

deposits and sorting (07=2.34; $0 =3.36) that is substantially poorer than any previously recorded for 

deposits whose texture has not been altered by induration. A surge released by failure of a natural gravel fill 

formed large dunes with a modal class of granules (2-4 

mm) in the f oreset region (Thiel, 1932, p. 456), but the 

resulting deposits were substantially 'better sorted than 

the boulder fronts on Parsley Bar. The Parsely Bar example contains some extremely coarse boulders with 

diameters equal to the probable thickness of the deposit. 

The authors conclude therefore, that the deposit and the 

boulder front primarily represent the sudden cessation 

of in-mass viscous flow of bed material. Movement of

this type has not been directly observed, and its general 

importance in fluvial transport cannot be evaluated at 

present. This species of subfluvial debris flow can 'be 

considered transitional between mudflow and normal 

bedload movement by sliding, rolling, and saltation. 

The large-scale crossbedding and smaller, dunelike 

fronts composed of better sorted material indicate that 

local movement of material in the form of dunes in 

which the front represents an avalanche face probably 

accompanied the mass flow. Both forms of movement 

probably occurred simultaneously.

EPISODIC BOULDER MOVEMENT

A series of three berm surfaces 1 mile below the damsite shows that a series of boulder movements occurred 

during the surge. These surfaces are underlain by rockfill from the dam (fig. 20). Adjacent to the channel 

wall at this site is a very coarse deposit in which no 

lateral variation in size was noted. This deposit abuts 

against a distinctly finer bermlike mass characterized 

by a marked fine-to-coarse gradation toward the center 

of the channel. In the central channel the material is 

distinctly finer than in the adjacent berm, but is similarly coarser toward the channel axis. Thus, during
M24 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

EXPLANATION

Boulder front 

Scarp of sand or gravel 

Dune forms

Marked change in 

particle size

Boulders (X 2 scale 

size)

Current direction 

(fallen trees)

Approximate highwater mark

x

Cutbank in terrace 

deposits

Boundary of low-water 

channel

Field sketches applied to 

an aerial-photograph 

base. Boulders are 

shown twice the actual 

scale size and each is 

the coarsest particle in 

its immediate vicinity. 

Bed material adjacent 

to the front downstream 

is sand

FIGURE 19. Part of Parsley Bar containing the largest boulder front.

each of three episodes, successively finer materials were 

deposited, but within each deposit the grain size probably coarsened toward the center of the channel. The 

relative difference in particle size between berms and 

channel is thus partly explained. In the steep-walled 

confining reach nearest the dam and at the downstream 

terminus of rockfill movement, only a single episode of 

movement is preserved and is recorded by deposits in

which the coarsest material is in the middle of the 

channel.

The sequence of berms noted in the dam-fill material 

and the boulder front on Parsley Bar both suggest that 

movement of bed material was episodic in response to 

a single flood wave. The possibly irregular supply of 

rockfill by periodic caving of the dam embankment may 

explain the berm sequence. According to an eyewitness,
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M25

however, failure of the structure was gradual and continuous. Movement of bed material as viscous debris 

flows and dune forms created longitudinal concentrations of boulders moving down the channel. In any 

reach of channel other than Parsley Bar, the boulder

100 200 FEET

EXPLANATION

Predominantly rockfill from damsite

Deposits are numbered in order of deposition

Bedrock

Contact

Dashed where approximately located

Relative size of boulders 

Scarp

Riffle in low-water 

channel

FIGURE 2,0. Reach showing sequence of bedload movement. See figure 3 

for location.

fronts would have migrated to a point where locally 

increased competence, at a channel bend or constriction, 

would have removed material from the flow or w^ave 

front at the same rate it was supplied, and coarse material would tend to accumulate and form a riffle analogous to a kinematic wave (Leopold and others, 1964, 

p. 212). Movement in waves or as viscous flows in response to extraordinary floods may be a factor in controlling the pool-riffle pattern in boulder-bed streams.

POOL AND RIFFLE PATTERN

Most stream channels show longitudinally alternating deeps and shalknvs, described as pools and riffles. 

Pools are the quiet stretches of relatively smooth water, 

and riffles occur in steeper parts of the channel with 

more rapid flo\v. Such sequences are scarce in the upper 

section of the Rubicon River above Parsley Bar, and the 

comparison of preflood and postflood aerial photographs 

shows that there are now fewer rapids in this upper, 

heavily aggraded part of the flood course than were 

there before the flood surge. In the sinuous section of 

channel immediately above Parsley Bar, the typical 

pool-riffle pattern is well developed. Riffles are located 

where the low-water flow moves across the bar and impinges against the bedrock wall of the channel. In general, the preflood pattern of pools and riffles has not 

been greatly changed. The preflood pattern of Parsley 

Bar has been modified and pool-riffle sequences are now 

few. Creation of a few new riffles resulted from supply 

of coarse detritus by terrace erosion during the receding 

stage of the surge and from cessation of movement of 

the boulder fronts.

The normal pattern of alternately pitching bars and 

pools and riffles predominates in the channel between 

Parsley Bar and the downstream end of the Rubicon 

River. Demonstrable modification of a pool and riffle 

has occurred at the site of the Rubicon River-Foresthill 

stream-gaging station where a riffle just downstream 

from the bridge at that location has been removed and 

is now the deepest part of a pool. The poor resolution 

of both sets of preflood aerial photographs in that part 

of the canyon prevents precise documentation of other 

changes in the pattern. Definite new riffles were formed 

by lateral supply from mass movements of material 

too large to be moved by the flood (fig. 21). The comparison of aerial photographs suggests, however, that 

there are now fewer riffles in that section of the surge 

channel. The flood removed numerous deltaic contributions of sediment by tributary streams and small preflood mass movements that had created riffles.

No noticeable modification of pools and riffles occurred on the Middle and North Forks of the American 

River, although a fe\v minor changes in configuration
M26 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FIGURE 21. Contribution of sediment to the flood channel by a landslide, 2.5 miles upstream from the junction of the Rubicon River 

with the Middle Fork American River. Because the slide occurred after passage of the surge at a stage during which the 

competence was insufficient to transport the coarses detritus of the slide, a riffle sequence was formed. Material with intermediate 

diameters in excess of 10 feet remains in the channel. Base of the slide is 535 feet across.

occurred. Mass movements directly resulting from the 

flood are also relatively minor along that part of the 

channel.

BED-MATERIAL FORMS AND INTERNAL 

SEDIMENTARY STRUCTURES

Deposits of the flood surge in the upper reaches are 

noticeably lacking in preserved sedimentary structures 

owing to the extreme coarseness of the deposits. Stratification could not be observed in the exposed berm sections. Surficial forms in the sand deposits consist of 

longitudinal ridges, flat-topped bars which in some 

places are associated with fronts as much as 2.5 feet 

high, and a series of crescentic convex-upstream waveforms as much as 20 feet across, visible on aerial photographs but not readily noticeable on the ground.

The fronts associated with the sandbars do not display internal stratification, either plane or crossbedded. 

Horizontal stratification, which is evidence of a planebed form, becomes abundant in the sand and gravel 

deposits of the lower reaches, particularly on the Middle 

and North Forks of the American Eiver. Sand deposits

of the lower reaches shown abundant crossbedding and 

ripple lamination, as well. Sets of crossbeds are tabular, 

with approximately parallel cutoffs and nontangential 

f oresets, and have an amplitude of as much as 10 inches. 

Current-ripple lamination consists of climbing-ripple 

forms, as much as 0.8 inch in amplitude, of Walker's 

type 1 and 2 (Walker, 1963, p. 175) in cosets as much as 

4 inches in thickness interbedded with plane-bedded 

strata. Abundance of preserved bed forms in a downstream direction in both gravel and sand deposits reflects 

a general decrease in grain size downstream and more 

sustained flow in the lower reaches. Lack of structures 

in the sand deposits of the upper reaches, such as 

Parsley Bar, may reflect rapid deposition. Plane beds, 

preserved as horizontal stratification, are usually interpreted (Gwinn, 1964, fig. 1) as evidence of the upper 

rapid-turbulent flow regime of Simons and Kichardson 

(1961) ; dunes and ripples, preserved as types of crossbedding, as evidence of the lower, tranquil-turbulent 

flow regime.

The cutbanks of the glacial terrace deposits show a 

variety of crossbedding. Crossbedding with an ampli-
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M27

tude of as much as 7 feet occurs in the deposits of 

Parsley Bar. The inclined strata of these largest crossbeds consist of highly variable layers of sandy pebble 

and cobble gravel and are at the base of the youngest 

unit of the alluvial stratigraphic succession (fig. 4). 

Trough crossbedding, preserved evidence of crescentic 

dunes, is abundant in the units of white sand and silt 

and rust-colored granule gravel occurring above the 

main body of till.

TRANSPORT AND DEPOSITION OF BED MATERIAL IN 

REACH DOWNSTREAM FROM DAMSITE

DOWNSTREAM CHANGES IN PATTERNS OF BARS 

AND BERMS

Berms of dam-fill diorite are present immediately 

below the dam; they grade downstream, at progressively 

lower elevations, into deposits in the present channel 

thalweg. The pattern of berms that sharply abut against 

valley walls is gradually replaced by a pattern of local 

flat-topped and lobate bars. The bars and the terraceaccretion deposits seem to have been molded of locally 

derived stream alluvium by the initial flood peak which 

had not entrained much coarse dam-fill material. There 

was a distinct time lag between formation of the lateral 

bars of stream alluvium and arrival of the bulk of the 

dam fill which predominates within the channel itself.

Upon reaching Parsley Bar, flow expanded from the 

confining bedrock channel, cut into terrace deposits, and 

formed boulder fronts and a large medial bar with 

radial flow patterns. Movement of the dam fill ended 

abruptly at the head of the bar, 1.6 miles below the 

damsite. The dam fill moved as a nearly undiluted mass 

because the surge had virtually cleared the channel. 

Beginning with the bedrock channel at the lower end of 

Parsley Bar, the normal pattern of alternately pitching 

gravel bars occurs.

COMPOSITION

The percentage of diorite in streambed materials is 

plotted against distance from the original toe of the 

dam in figure 22. The sharp contrast between the bed 

material in the reach above Parsley Bar and in Parsley 

Bar itself is evident from a comparison of figures 14 

and 23. A large majority of the diorite boulders in the 

deposits which contain more than 90 percent diorite 

(fig. 14) originated from the dam embankment. Even 

at the downstream terminus of the in-mass movement 

of fill where only 8 percent of other rock types dilute 

the deposit, probably less than 6 percent is diorite derived from within the channel. This conclusion is based 

on the proportion of diorite to other rock types in the

90

or»

70

60

50

30

10

n

1 ^"""""1 EXPLANATION

 

Mainly diorite from damsite

o

Mainly normal stream

alluvium

1

"»*-l>4-«

Lateral

1 1 1

S-. I »"N»

_

,-Terrace accretion

b^rs_o o ° ° °

0 0 ° ~~

0

_

0.5 1.0 2.0 3.0 

DISTANCE FROM DAMSITE, IN MILES

5.0

FIGURE 22. Relation of bed-material composition, in terms of the percentage of diorite, to the channel distance downstream from the damsite. All measurements, except those noted, were made in or near the 

thalweg.

bars composed of locally derived material that were 

formed during an earlier stage of the surge. A small 

proportion of the diorite in those bars was derived from 

the dam fill, in all probability less than 10 percent.

PARTICLE SIZE

The size of particles moved by the flood was measured 

to obtain some idea of variation in dynamics along the 

stream channel and possible variations in competence 

in a downstream direction. The method described by 

Wolman (1954), in which the intermediate diameter of 

100 clasts is measured in the field, was used because of 

the extreme coarseness of the deposits. A preliminary 

evaluation suggested that samples involving traverses 

perpendicular to the channel would show great variation. Therefore, a tape was used to construct a sampling 

grid in the area of each reach containing the coarsest 

particles.

Several sampling problems occur in measurement of 

bed materials as coarse as those present in the surge 

channel. In some samples the intermediate axis must be 

approximated, but placement in the correct size class for 

calculation of a size distribution is generally possible. 

Boulders having a diameter greater than 9 011 the phi 

grade scale (512 mm) are difficult to maneuver into a 

position where the true intermediate axis can be measured. Where large boulders are partly buried, the exposed minimum diameter was used as an approximation 

of the intermediate diameter. When the grid point fell 

over a crevice in boulder gravels with an openwork 

texture, not even an approximation of the size of the 

selected particle was possible. Such sampling points 

were excluded.
M28 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FIGURE 23. Bed material of Parsley Bar, extending from 1.6 to 2.9 miles downstream from the Hell Hole damsite. Very little 

rockflll moved this far. The material is predominantly reworked terrace gravel. Compare with figure 14. Weathered diorite, 

granodiorite, and granite are the main rock types.

Figure 24 is a plot of the graphic mean diameter 

(Folk, 1964, p. 44) at localities sampled in the upper 

reaches, calculated from the formula:

where subscripts represent the percentage of material 

larger. The median diameter of each sample is also included in table 3. Eelative to the median, the graphic 

mean is a more valid measure of overall size of the sample and closely approximates the computed mean.

8

PARSLEY 

BAR

1 1 EXPLANATION

 "^ Mainly diorite

0

Mainly normal 

stream alluvium

1 1

1 1 1 1

^V^

\ \ \

1^

o 

o

0

o

0 Boulder front

1 1

0.1 0.5 1.0 2.0 3.0 5.0 5 

DISTANCE FROM DAMSITE, IN MILES

FIGURE 24. Relation of mean particle size to distance downstream 

from damsite.

The size of the dam-fill detritus systematically decreases downstream. Additional measurements were obtained to compare the relative coarseness of sediment in 

several pool and riffle deposits (fig. 25). Each of the 

riffles contains coarser material than does the upstream 

pool. To determine change in particle size with depth, 

a pool deposit in a heavily aggraded part of the reach

1 1 1

EXPLANATION

 

Sample location in a pool

O 

Sample location on a riffle

1 1 1

1 1 1 1 

o 

o

 

1 1 1

o

o

 

05 1.0 

DISTANCE FROM DAMSITE, IN MILES

32

FIGURE 25. Relation of mean particle size of samples in pools and 

riffles to distance downstream from damsite.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M29

TABLE 3. Summary of sediment data in channel below damsite

Sample

1 -- . .

2 .- .

3...-.... ....

4-..... ....

5.. ....

6 -..- -_..

7

8 ~ . 9..... .......

10.. . ..-

11- .

12 ..

13 . 

14

15... . ......

16. 

17 .

18. .

19. 

20 . .

21. .

22. .

23 

Pi..... 

Bi.. .. .....

Pi... ...

R,.. 

P3 .. 

B,___ _

P4.. .....

B4 .

D istance M edian 

from dam- b-axis 

site particle 

(miles) size (nun)

0.42

.51

.56

.61

.76

.81

.99

1.10

1.16

1.27

1.32

1.45

1.48

1.56

1.60

1.67

i on

1.99

2.32

2.50

3.10

3.55

4.10

.56

.59

.70

.73

1.09

1.15

1.23

1.27

338 

338 

416 

315 

416 

274 

239 

274 

147 

147 

208 

169 

239 

208 

89 

169 

74 

112 

119 

137 

512 

588 

388 

194 

416 

208 

315 

89 

256 

91 

194

Graphic Trask Phi Inclusive Phi Inclusive 

mean sorting standard graphic skewness graphic 

b-axis coefficient deviation deviation a<t> skewness 

particle So <r<t> <n SKi 

size Mz

-8.40 

-8.53 

-8.30 

-8.27 

-8.57 

-7.80 

-7.93 

-8.13 

-7.07 

-7.17 

-7.57 

-7.30 

-7.77 

-7.70 

-6.30 

-7.17 

-5.93 

-6.60 

-6.50 

-6.83 

-8.77 

-8.93 

-8.10 

-7.53 

-8.50 

-7.40 

-8.13 

-6.67 

-8.07 

-6.50 

-7.67

1.79 

1.79 

2.06 

1.74 

1.86 

2.07 

1.98 

1.56 

2.08 

1.47 

1.63 

2.14 

2.14 

1.57 

1.51 

2.22 

3.36 

2.73 

2.37 

2.37 

2.13 

2.07 

2.15 

1.51 

1.51 

1.63 

1.68 

1.87 

1.95 

1.57 

1.46

1.40 

1.30 

1.70 

1.15 

1.30 

1.75 

1.45 

.95 

1.60 

1.20 

1.00 

1.75 

1.70 

.90 

.95 

2.05 

2.40 

2.00 

2.00 

1.90 

1.75 

1.70 

1.75 

1.00 

1.00 

1.05 

1.15 

1.30 

1.35 

1.00 

.90

1.49 

1.30 

1.91 

1.28 

1.23 

2.08 

1.57 

.96 

1.78 

1.15 

1.00 

1.71 

1.73 

.95 

.99 

2.22 

2.34 

2.03 

2.00 

1.89 

1.79 

1.74 

1.73 

1.03 

1.06 

1.08 

1.11 

1.20 

1.38 

1.02 

.95

0.0 

-.15 

.35 

.04 

.15 

.26 

-.03 

-.05 

.13 

.08 

.20 

.09 

.12 

.0 

.16 

.17 

.17 

.15 

.30 

.21 

.40 

.24 

.43 

.10 

.30 

.43 

.22 

-.31 

-.09 

.0 -.11

0.08 

.06 

.41 

.11 

.18 

.33 

.11 

-.03 

.27 

.13 

.21 

.13 

.16 

.08 

.20 

.27 

.17 

.18 

.30 

.19 

.37 

.24 

.36 

.18 

.27 

.44 

.21 

-.29 

-.01 

-.03 -.10

Slope Maximum Tractive 

S (ft stage force l 

per ft) (ft) (Ib per 

sqft)

0. 0304 

.0289 

.0283 

.0281 

.0214 

.0208 

.0207 

.0175 

.0172 

.0169 

.0165 

.0158 

.0158 

.0125 

.0125 

.0121 

.0058 

.0062 

.0061 

.0068 

.0134 

.0080 

.0085

58.0 

63.3 

47.6 

44.0 

31.1 

35.5 

37.0 

40.3 

42.5 

43.0 

41.0 

37.2 

36.0 

30.5 

25.0 

22.5 

23.8 

22.7 

29.6 

25.3 

34.0 

47.6 

44.3

110.0 

114.1 

84.1 

77.1 

41.5 

46.1 

47.8 

44.0 

45.6 

45.4 

42.2 

36.7 

35.5 

23.8 

19.5 

17.0 

8.6 

8.8 

11.3 

10.7 

28.4 

23.8 

23.5

i Assuming 7=62.4 Ib per cu ft.

between the dam and Parsley Bar was excavated during 

a period of subsurface flow. Only a slight increase in 

coarseness was observed to a depth of 4 feet. Thus, the 

difference in grain size between the material in pools 

and that in riffles is probably not a surficial effect.

Downstream size decrease is also shown by the intermediate diameters of clasts marked by shothole coring (fig. 26) which positively identifies such boulders as 

being supplied to the surge at the damsite. The relative 

coarseness of the cored boulders and the size of the bed 

material confirms the interpretation that the bulk of 

the detritus in the channel downstream as far as Parsley 

Bar consists of dam fill. The sharp end to the tongue of 

dam fill at the head of Parsley Bar, the marked reduction in competence owing to slope and stage reduction at 

that point, and the apparent lack of dilution of Parsley 

Bar bed materials by dam fill all suggest that coarse 

fill was not transported beyond the head of the bar, 1.6 

miles downstream from the dam. However, a definite 

boulder of fill, having an intermediate diameter of 1.4 

feet and containing a shothole, was found near the middle of Parsley Bar, 2.1 miles from the damsite, and 

downstream from the boulder front. The boulder was 

probably one of a very few pieces of detritus transported 

that distance during conditions of maximum turbulence, 

with or shortly after the surge crest. The noticeable 

rounding of the specimen suggests that it was not rafted 

to this point by vegetation.

DISTRIBUTION OP PARTICLE SIZE

The distribution of particle size of clastic elements 

at each sampling locality is presented in table 3 in 

the form of a sorting coefficient S0 (Trask, 1932, p. 71), 

the phi or graphic standard deviation &$ (Inman, 1952, 

p. 130), and the inclusive graphic standard deviation 

crx (Folk, 1964, p. 45). The measures of dispersion or 

sorting are calculated as:

_i/ _i_ a i Yzcr^ -\- r- #95 0sPARSLEY 

I BAR

z ^'u

_

°- 11.0

z"~

cc

LJ

<

a

<

a

LJ

I -8.0

LJ

t1 1 1

0

n

^

EXPLANATION

0 "1

1 1 1

0

0 ° 0

D n

0

O

Boulders in or near thalweg

Boulders on lateral bars o 

terraces

1 1 1

r

1 1 1 1

1 1

0

0

0 °

,

1 1

4096

2048 5

01 i-* ro

0.1 0.5 1.0 2.0 3.0 

DISTANCE FROM DAMSITE, IN MILES

FIGURE 26. Relation of diameter of larger cored boulders to distance 

downstream from damsite.
M30 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

where D\=diameter, in millimeters, at which 25 percent of the material is larger, and #3=the diameter at 

which 75 percent is larger. Other subscripts also denote 

the percentages of material larger. For each of the 

measures, values are inversely proportional to the 

degree of perfection of sorting.

Figure 27 is a plot of the inclusive graphic standard 

deviation against distance downstream from the damsite. All the coarse deposits, regardless of location, can 

be characterized as poorly sorted relative to gravel 

found in most other geological environments, all values 

of 07 being above 0.9. The degree of sorting is also on 

the low side relative to other published values of sorting of fluvial gravel (Emery, 1955, p. 47). However, an 

irregular increase in degree of sorting is evident in the 

dam fill distributed along the stream course. Material 

in the blasted fill was probably originally distributed 

according to Rosin's law, the geometrical relation between quantity of material in each size class produced 

by crushing. Some natural sediment sources, such as 

talus, glacial till, and mechanically and chemically 

weathered igneous rocks, approximate the distribution, 

in which each size interval contains about half the 

weight of material in the next larger interval (Krumbein and Tisdel, 1940, p. 301-304). Thus, the changes 

in size and sorting in the distributed dam fill should 

approximate the results of transportation away from a 

natural point source of sediment supply. The values 

should be only slightly affected by contamination as 

shown in figure 23.

Previous studies show that sorting commonly does 

not vary according to distance of transport when fairly 

short distances of transport are involved (Plumley, 

1948, p. 548; Brush, 1961, p. 152). Normal stream gravel 

is the product of many probably short increments of 

movement accompanied by mixing with material in the 

channel, in-place weathering, and contributions from

^ 2.5

t\3 'o

£ ?

* t

o z

- 0.5

PARSLEY 

I BAR |

1 1 1

EXPLANATION 

Mainly diorite

O 

Mainly normal 

stream alluvium

1 1 1

1 1 1 1

i

* 

1 1 1 1

1 1 1 OBoulder front 

O

0 n

0

 ~ o £> oo

 * / 

1 1 1

0.1 0.5 1.0 2.0 3.0 5.0 

DISTANCE FROM DAMSITE, IN MILES

FIGURE 27. Relation of dispersion in particle size to distance 

downstream from damsite. A decrease in the inclusive 

graphic deviation reflects increased sorting.

colluvium and tributaries. However, the effect on each 

individual supply of sediment, as shown in figure 28, is 

to increase the degree of sorting of the material as it is 

transported downstream.

Sorting is not, as would be expected, better in the 

riffles than in the pools. The beds of both pools and riffles 

composed of fill material are presently of openwork 

texture. Accumulation and infiltration of fines would be 

expected in the pools but not in the riffles, where velocity is greater, but infiltrated fines were not included 

by the surficial measurements.

Skewness, or degree of asymmetry of the distribution 

curve, was calculated as the phi skewness 0,4, (Inman, 

1952, p. 130) and the inclusive graphic skewness SKr 

(Folk, 1964, p. 46), as follows:

,/ 

 

(016 + 084) ~050. - 

016+084 205Q. 05 + 095 

2(084 016)

Departure from 0.0 indicates the degree of asymmetry 

of the distribution, absolute limits ranging between 

+ 1.0 and 1.0. Most of the deposits are positively 

skewed, a characteristic of coarse fluvial gravel (table 

3).

A plot of the inclusive graphic skewness (fig. 28) indicates approximate uniformity of skewness in a downstream direction. The apparent slight tendency for increase of positive skewness downstream may reflect only 

the fact that the deposits of normal stream gravel are 

more positively skewed than the deposits of fill material. 

No definable change in skewness resulted from sedimentation of the dam fill. The material in the fill was originally positively skewed if distributed according to 

Rosin's law.

PARSLEY 

I BAR |

'i 0.4

V

« °-3

CO

£ 0.2 

5 w

UJ K n ,

«§

o _ o.o 

I'-o,

§zo ~~ 2 °'2

_l

O n A

1 1 1

- EXPLANATION 

Mainly diorite 

from damsite

o 

Mainly normal

stream alluvium

1 1 I

I 1 1 1

 

 

i

1 1 1 1

1 1 1

0 0

 0

o

o QS-^ 

« "^Boulder front

1 1 1 -0.5 - 

0.1 0.5 1.0 2.0 3.0 5. 

DISTANCE FROM DAMSITE, IN MILES

FIGURE 28. Relation of skewness to distance downstream from damsit
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M31

COMPETENCE

The transporting power or ability of moving water 

to transport debris may be expressed as competence, a 

measure of the maximum size of particles of a certain 

specific gravity that a current is able to move. Competence is commonly expressed in terms of the intermediate diameter of the largest particle that can be 

transported at a given flow velocity. Because of the likelihood of unstable flow and the laborious nature of indirect velocity determinations, which could have been 

made for only a few suitable reaches, tractive force was 

used as an alternative measure of competence (Leliavsky, 1955, chap. 5). Tractive force, r=ydS, the "boundary shear" of fluid mechanics, is the product of the 

specific weight of the transporting medium, y, water 

depth, d, and the slope of the hydraulic energy gradient, 

$, and represents the drag force per unit area of bed 

surface. Because the relation applies to uniform, steady 

flow and does not consider solid grain stress, it must be 

regarded as yielding an approximation of competence.

The slope of the energy gradient may be approximated by the slope of the channel surface which was 

measured on an enlarged part of a Ti^-minute topographic map with 40-foot contours. Depth of flow was 

measured as the distance between thalweg elevation and 

high-water marks by level at previously defined crossprofile locations and by a hand level, mounted on a

Jacob's staff, at other localities. Specific weight of the 

transporting medium, although doubtless variable, could 

only be treated as a constant.

A plot of changes in tractive force in a downstream 

direction (fig. 29) shows that variation was extreme. 

Flow was probably sufficiently competent throughout 

the course of the surge to transport boulder-size material 

in terms of the tractive force-particle size relations summarized by Fahnestock (1963, fig. 31). However, extrapolation of Fahnestock's data to the much coarser 

detritus of this study is certainly not warranted.

In spite of high competence, the fact that most large 

boulders were transported only short distances is shown 

by the striking predominance of locally derived rock 

types in boulder deposits along the lower unglaciated 

parts of the Kubicon and American Kivers. The small 

amount of longitudinal movement was due to the short 

duration of the surge, the extreme macroturbulence that 

rapidly moved debris to lateral areas of lowered 

velocity, and the fact that much erosion and supply of 

coarse material to the flood occurred after the flood crest. 

Study of the depositional forms clearly indicates that 

local sources of supply were a major factor in controlling particle diameter at a point.

In view of the large transport capability of the surge, 

an attempt was made to determine competency in terms 

of actual size of particle moved and to extend the rela200

PARSLEY BAR

100

I I [ T I \\ \ I I I I

S) 50

<£.

10

0.1 0.5 10 50 100

DISTANCE FROM DAMSITE, IN MILES 

FIGURE 29. Relation of indirectly measured tractive force to distance downstream from damsite.
M32 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

tion between shear and size of particle moved. Particle 

size was measured on the assumption that all boulders on 

or only moderately buried in the bed had been moved 

by the flood. The following criteria were employed to 

indicate specific boulder movement: (1) Buried vegetation, (2) fresh abrasion on angular corners of downstream sides of blocks, and (3) presence in or on a depositional form clearly related to the flood wave, as indicated by buried or protruding vegetation and comparison of preflood and postflood aerial photographs.

Such an indirect approach is obviously uncertain, in 

part because we are dealing just with the exposed depositional remains of the flood detritus, which may be only 

a crude approximation of the actual maximum-particle 

transport past the sampling point. Furthermore, the 

velocity or tractive force permitting deposition of a 

particle should be less than that required to initiate or 

maintain movement. However, Kramer (1935, p. 824) 

found that under some circumstances deposition occurred at a higher value of tractive force than that at 

which movement began, because of roughness differences. In addition, rolling or a settling lag (Postma, 

1961) may cause movement to continue into areas of 

substantially reduced tractive force.

The average intermediate diameter of the 10 largest 

particles at each of nine localities was measured to reduce the possibility that an unmoved boulder was included in the data. Measurements were restricted to the 

lower nonglaciated part of the channel, and only particles showing effects of abrasion were included. Angular blocks without abrasion and located near channel 

sides were attributed to purely lateral movement and 

were excluded. Data are summarized in table 4.

A plot of the mean intermediate diameter of the 10 

largest particles against tractive force at each of the 

nine localities yielded a direct relation (fig. 30), similar to what would be expected in projecting the direct 

measurements of competence-size relations collected and 

summarized by Fahnestock (1963, fig. 31). Fahnestock's 

White River data are based on measurements of particles being transported and on shear calculated from 

mean depths; other data compiled by Fahnestock are 

based largely on particle erosion. The indirect method 

used in the present study, which utilizes depositional 

remains of a flow and maximum shear at the location of 

each particle, yields compatible results. Such an indirect 

determination of competence assumes that a spectrum 

of particle sizes up to and above the size which the flood 

is capable of moving is available.

Field relations are considerably more complicated 

than suggested by figure 30. There is great longitudinal 

variation in particle size, only in part related to the 

pool 'and rifle pattern, along a reach in which tractive

. 1-

CC. 7 

LJ ^

S Z

LJ .

5 LJ 

Q d ,r,

INTERME MEAN

ARGEST PARTIC 

1

CID 

O

1

1

1 1

.

 *

1 1 1 1

1 1

1 1

1 1 II

*

MM

9

1024 N

512

5 10 50 100 

TRACTIVE FORCE, IN POUNDS PER SQUARE FOOT

150

FIGURE 30. Relation of maximum particle size to indirectly measured 

tractive force.

force must have been fairly constant. Measurements of 

maximum particle size and associated stage were made 

at localities that contained the coarsest particles in a 

given reach.

The extremely coarse material in the Rubicon gorge 

was clearly moved only very short distances. Additional 

evidence for small increments of transport of coarse 

material are particle-size changes at the point the flow 

left the plutonic terrain of the upper Rubicon and 

crossed the contact with metamorphic rocks of the Calaveras Formation. In the channel, just upstream from 

the contact, the average 5-axis (intermediate axis) of 

the 10 largest granitic boulders measures 10.8 feet. In 

the metamorphic terrain 50-60 yards downstream, the 

10 largest boulders of granitic composition average only 

4.7 feet in diameter. The contract is below the maximum 

extent of glaciation.

SOUNDNESS

Downstream changes in particle shape were determined by application of Krumbein's visual comparison 

charts (1941 a, pi. 1) to photographs of the bed material, 

as well as to individual particles observed in the field. 

Roundness is expressed as the ratio of the average radius

TABLE 4. Summary of maximum particle-size data

Distance downLocation stream 

from dam 

(miles)

Rubicon gorge; change in canyon configuration, plutontcRubicon gorge; base of largest 

slide.. 

Unnamed bar 3 miles below 

Middle Fork American 

River at Foresthill gaging

Philadelphia Bar. ... 

12.1 

15.6 

18.2

18.5

38.5 

47.2 

50.7 

60.2 

61.0

Mean 6-axls 

particle 

diameter

mm

3,290 

2,380 

1,450

1,830

497 

899 

479 

457 

625

Ft

10.80 

7. SO 

4.74

6.00

1.63 

2.95 

1.57 

1.50 

2.05

Stage 

(ft)

71.6 

63.9 

64.1

61.1

56.2 

53.1 

45.0 

36.2 

25.8

Slope Tractive 

(ft per force 

ft) (Ib per 

sqft)

0. 0226 

.0290 

.0179

.0169

.0039 

.0036 

.0026 

.0030 

.0034

101.0 

115.7 

71.6

64.5

13.7 

11.9 

7.3 

6.8 

5.5
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA. M33

0.6

0)0.5

Z 0.4 

Q

0.1

1 1 1

EXPLANATION

Mainly diorite

O

stream alluvium *

Value at damsite = 0.12

1 1 1

1 1 1 1

c

«

t

1 1 1 1

1 1 1

0 r, 0

0 uo 0 oo

 .*

1 1 1

0.5 1.0 2.0 

DISTANCE FROM DAMSITE, IN MILES

5.0 <

FIGURE 31. Relation of particle roundness to distance downstream 

from damsite. Each value represents the average of at least 20 determinations. The range of values is much greater in the normal 

stream alluvium relative to the rockfilll diorite.

of curvaturei of particle edges to the radius of curvature 

of the maximum inscribed sphere. Thus, roundness is 

theoretically independent of sphericity the degree of 

approximation of the particle to a spherical shape and 

is less sensitive to the internal structure of particles than 

sphericity. Changes in roundness are illustrated in figure 31, in which the average roundness of at least 20 

particles is plotted for each locality.

Eoundness of the rockfill particles increases rapidly 

downstream. Studies of roundness in streams and abrasions mills indicate that the rate of rounding decreases 

with distance and that roundness changes after a first 

small increment of transportation may be marked 

(Krumbein, 1941b, 1942, p. 1385; Plumley, 1948, p. 

559). Experimental data indicate that rate of rounding 

may vary considerably with grain size.

The downstream grain-size decrease accompanying 

the rounding increase probably causes the apparent rate 

of rounding interpreted from figure 31 to be too low. 

Projection of the rate of rounding indicates that the 

dam fill would have obtained an average roundness corresponding to the roundness of the normal alluvial 

material in less than approximately 5 miles of downstream movement.

Eoundness wa's also determined for the boulders containing shotholes (fig. 32) and the values are similar to 

those at the sampling localities, additional evidence that 

the bulk of diorite within the channel below the dam is

Z 0.4 

Q

0.2

1 1 I 

EXPLANATION

o 

Boulders in or near

thalweg

Boulders on lateral bars 

or terraces o

° °°

1 1 1

1 1 1 1

0 °°

o o

<

1 1 1 1

1 1 1

o

I 

1 1 1

Q

0

Z

o

cc

-<

<

D

0.1 0.5 1.0 2.0 

DISTANCE FROM DAMSITE, IN MILES

3.0

FIGURE 32. Relation of roundness of cored boulders to distance 

downstream from damsite.

redistributed fill. Several cored boulders were discovered 

on and in the reworked alluvial and terrace gravel 

occurring as berms or bars along the main channel and as 

terrace accretions (fig. 11). Such boulders are perceptibly more angular than cored boulders within the central 

part of the channel. Distribution of the depositional 

forms and contacts between units indicates that the 

lateral bars, berms, and terrace accretions were formed 

earlier in the flood than the thalweg deposits. A lesser 

degree of rounding of boulders transpored during an 

earlier interval of the flood passage suggests that these 

boulders may have been carried in suspension by macroturbulence, or in part caught up and rafted by uprooted 

forest debris.

Roundness of the cored boulders indicates that transition from angular (0-0.15) to subangular (0.15-0.25) 

occurred almost immediately downstream from the damsite, using the roundness class limits of Pettijohn (1957, 

p. 59). Transition from subrounded (0.25-0.40) to 

rounded (above 0.40) in both cored boulders and bedmaterial samples occurred approximately 1.5 miles 

downstream.

In comparison with other geologic and experimental 

studies of rounding as related to time and distance, for 

comparable rock types, the greatest rounding per interval of distance transported was attained in this example. 

The greater particle size in this example is, of course, 

the major cause. Downstream dilution of the samples of 

dam fill by indigenous, previously rounded particles 

was assessed and did not influence the findings. The unusually rapid rounding occurred, however, in spite of 

the noteworthy uniformity and apparent durability of 

the freshly quarried diorite (fig. 6). An additional 

factor adding to the high rate of rounding probably is 

the intense turbulence and the resulting rigor of the 

transport environment. Schoklitsch (1933), for example, 

has shown that abrasion is intensified as velocity increases. Another factor is the possibility of bedload 

movement as subaqueous rock flows in which attrition 

by grinding could have been intense. As noted above, 

the deposits of rockfill from the damsite are openwork 

gravel in which the particles are dominantly boulders. 

The cushioning effect of fines was decidedly less than 

in normal bedload transport.

CAUSE OF DOWNSTREAM DECLINE IN PARTICLE 

SIZE

Most studies have emphasized abrasion as the fundamental cause of the often reported decline in size of 

fluvial sediment particles downstream. Experimental 

abrasion studies (Krumbein, 1941b; Kuenen, 1956) have 

shown that clasts become rapidly rounded under condi-
M34 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

tions simulating stream transport. The rate of rounding 

is, among other parameters, highly sensitive to particle 

size and composition, sorting, and position of a given 

particle within the distribution.

A second factor, progressive sorting, is difficult to 

assess, but is currently believed to be of at least moderate importance. Progressive sorting involves two aspects : (1) the lagging of larger particles, caused by the 

differences between particle velocity and flow velocity 

which are related to particle size under conditions of 

constant or increasing competency, and (2) the progressive deposition of the coarser grades during reduction 

in competency.

As previously shown, the bed material in the channel 

to a point 1.6 miles downstream from the dam originated at the damsite and has a known degree of dilution. Therefore, the effects of abrasion and progressive 

sorting can be evaluated. Over an interval of 1.2 miles, 

beginning at a point 0.4 mile downstream where rock 

recovery operations have not disturbed continuity of 

the deposits, the graphic mean size, in terms of intermediate diameter, changes from <j> 8.0 (256mm) to 

<£ 6.5 (91mm), a reduction of approximately 65 percent. Over the same interval, average rounding increases 

form 0.25 to 0.40. Krumbein's experimental results 

(1941b, fig. 3) show that limestone pebbles undergoing 

the same roundness change would be decreased in 

weight by only about 3 percent. Composition is not critical in extending these determinations to the diorite rockfill. In general, a fragment may become moderately 

well rounded without any appreciable effect on size and 

shape. Translation of weight decrease to the percentage 

decrease in particle diameter would be highly variable, 

depending on the individual particle configuration, but 

the intermediate-diameter change can be conservatively 

estimated as less than 5 percent. Therefore, more than 

90 percent of the size decline is the result of progressive 

sorting. Plumley (1948, p. 570), using a different line 

of reasoning, concluded that 75 percent of the size decline in Black Hill terrace gravel was due to progressive sorting.

This determination pointedly neglects inclusion of 

a factor for size reduction by breakage. Thorough study 

of the rockfill deposits over their entire length revealed 

few broken rounds or fresh fracture surfaces. Consequently, breakage was not significant in the overall size 

decline, because of the tough, massive character of the 

fresh diorite. Smaller particles produced by flaking and 

chipping of coarser detritus were either carried past 

the area of fill deposition or infiltrated the openwork 

boulder deposits and did not appreciably influence the 

mean size of the downstream surficial samples.

Fracturing may be considerably more important in

the big-picture view of fluvial sediments, however. According to Bretz (in Petti John, 1957, p. 537) broken 

rounds do not normally exceed 15 percent of a gravel 

deposit, but they may dominate, as in glacial-outwash 

gravel of the Snake River (Bretz, 1929). In view of the 

rarity of splitting in the rockfill boulders and its prevalence in older gravels, particle breakage may be chiefly 

due to in-place weathering along planes of weakness. 

The internal lithologic fabric of the particles is naturally an important consideration.

Figure 29 shows that size decline in the reach below 

the dam occurred under conditions of nearly continuously decreasing competency. However, when graphic 

mean size at each locality is plotted against tractive 

force (fig. 33), at best only a very crude association is 

evident. Such a relation emphasizes the importance of 

local channel configuration in controlling deposition 

and the longitudinal as well as lateral variation in particle size within the stream channel.

Although the downstream decrease in particle size of 

the bed material derived at the damsite clearly relates 

to progressive sorting and, in part, to a concomitant decrease in competence, the lagging of the larger particles 

because of fluctuations in flow competency is probably 

a significant cause of size decline on the Kubicon and 

Middle Fork American Rivers. Large fluctuations in 

maximum competency shown in figure 29 were present 

along the flood course, and similar changes in competency would occur for any flow in the canyon.

MACBOTUBBULENCE

The dynamics of the surge were such that turbulence 

was extreme, particularly in the reach below the dam 

where virtually no fines had yet been incorporated in 

the flow to dampen turbulence effects. Table 5 illustrates the effects of the turbulence as indicated by the 

diameter and height above the streambed of boulders 

deposited by the surge. Only boulders which were

-9.0

~

5

£ -8-0

LJ «

5 t

< z

a D

z E ~ 7-°

< CL

Ld

5 Z

O

E -6.0 

a.

<

(£

O

_K n

1 1 1

0

o

0° 

0 0

0

8

0

1 1 1

1 1 1 1

00

1 1 1 1

£> O

512

256

128 N 

to

LLJ

_l 

O

64 a 

(L

Z 

< 

LU * > s

10 50 100 200 

TRACTIVE FORCE, IN POUNDS PER SQUARE FOOT

FIGURE 33. Relation of mean particle size to indirectly measured 

tractive force.
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M35

freshly deposited were measured. There is a general 

decrease in magnitude of turbulent effects throughout 

the mile of channel below the dam. Below Parsley Bar, 

which was a source of a great quantity of fines to the 

flood, boulder movement did not occur much above the 

level of the channel bottom.

Matthes (1947) described forms of macroturbulence 

in streams, and emphasized the efficacy of spasmodic 

vortex actions which cause upward suction. Such phenomena, designated as kolks by Matthes, act in swift 

and deep water in a manner analogous to tornados in 

air, develop cavitation, and can lift bedload materials 

and pluck fragments from a jointed rock surface. 

Probably, such action was responsible for deposition of 

boulders well above the level of the channel and was 

effective in removal of some bedrock, particularly in 

reaches underlain by jointed granitic rocks.

EFFECTS OF THE SURGE RELATIVE TO NORMAL 

STREAM PROCESSES

Steep incised bedrock canyons of mountain rivers like 

the Rubicon may be blocked occasionally by massive 

landslips, the breaching of which could yield abrupt 

surges similar to that resulting from the embankment 

failure. The Eubicon Eiver surge was more abrupt than 

most natural flood waves, and its effects would not 

expectedly duplicate those of ordinary floods. However, 

abrupt surges with unusually high discharges may be 

significant in rock-walled canyons along which damming by landslides or ice dams is a frequent geologic 

occurrence. The collapse of unstable ice dams probably 

produced many such surges in mountainous regions during the Pleistocene. Catastrophic floods, the jokulhlaups 

of Iceland (Thorarinsson, 1939), are also produced by 

the periodic glacier-damming of lakes, which are rapidly 

emptied when the impounded water buoys up the ice 

barrier.

Regardless of duration, catastrophically high flood

TABLE 5. Size and height above present thalweg and distance 

from point of origin of boulders deposited by macroturbulence

6-axis particle-size

mm

1,550

396

518

1,158

344

396

213

183

683

274

518

213

366

683

213

305

Ft

5.1

1.3

1.7

3.8

.8

1.3

.7

.6

.6

.9

1.7

.7

1.2

.6

.7

1.0

Height (ft)

28.0

35.0

30.0

34.0

41.0

36.0

28.0

22.0

20.0

17.0

33.0

40.0

34.0

17.0

18.0

15.0

Distance from damsite

(miles)

0.07

.08

.17

.18

.19

.20

.31

.37

.41

.42

.50

.51

.51

.57

.85

.85

peaks must greatly influence the morphology and sediment regimen in streams having extremely coarse bed 

material. The coarse detritus may be derived from glacial deposits, from gravity movements on valley sides, 

or by hydraulic plucking of joint blocks. The surge 

probably had a continuous maximum competency sufficient to transport large boulders for considerable distances ; yet it did not, as indicated by tracing of material 

from the point source of sediment at the damsite, predominance of locally derived rock types, and size 

changes in a given rock type across a geologic contact. 

Difference between surge velocity and the slower velocity of tractionally moved particles caused entrained 

boulders to be soon stranded as they were bypassed by 

the surge peak. Lateral as well as longitudinal variations in competency were pronounced.

The usual annual floods are probably competent to 

move most materials in the nonglaciated parts of the 

canyon. Flows competent to move the coarsest material 

in the glaciated part of the canyon and the huge blocks 

supplied to the channel in the Rubicon gorge are probably so infrequent that decomposition, disintegration, 

and abrasion to a size transportable by unusual yet 

expectable floods outranks movement by catastrophic 

discharges in dispersal of the coarsest fraction. Many 

large blocks included in the till and terrace deposits of 

the upper reaches show deep weathering rinds.

The surge wave was probably at least the equal of the 

highest discharge attained in any part of the Rubicon 

River since glacial recession (within aproximately the 

last 10,000 years), as indicated by the erosional effects 

on glaciofluvial deposits in even the lower part of the 

river. The peak discharge of at least 300,000 cf s attributable to the surge compares with an estimated natural 

peak of about 40,000 cfs in the Hell Hole area.

Although not as important as a much greater number 

of annual floods in terms of volume of sediment transported and in denudation rates (Wolman and Miller, 

1960), such catastrophic floods may control the morphology and character of depositions! forms in mountain streams because of the extreme coarseness of the 

bed material. The surge passage totally modified and 

reestablished most depositional forms along the Rubicon River. This situation contrasts with alluvial channels in which both form and patterns are attributed to 

events of moderate frequency (Wolman and Miller, 

1960, p. 67). The amount of actual landscape sculpture 

produced by catastrophic floods relative to normal 

stream processes, in both mountain and alluvial streams, 

is probably small.

One possibly unique aspect of such catastrophic 

events is the mass and wave movement of coarse bed 

material, such as occurred in the Parsley Bar reach.
M36 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

Another effect of catastrophic floods may be the triggering of periods of increased mass movement by oversteepening of slope profiles. Tricart (1961) noted that a large 

flood in the French Alps activated a period of intensified soil creep and slumping. Such a cycle may have been 

initiated in the steep-walled yet thickly mantled Rubicon gorge. Observation of the flood route in April 1966 

indicated that 38 new landslides, in addition to the 32 

formed at the time of the surge, had occurred during 

the 1965-66 wet season, one of subnormal precipitation. 

The slides were generally smaller in size, having an aggregate volume probably less than that of the original 

slides. Soil creep not present the preceding year was 

evidenced by generally small areas of tilted trees 

adjacent to the flood channel. One large area of new 

slippage, 1,500 feet in length by 900 feet in height, was 

formed on the north side of Cock Robin Point, approximately 5 miles downstream from the gaging station, 

Middle Fork American River near Foresthill.

Another possibly unique effect of catastrophic surges 

is the supply of coarse sediment to the channel by lateral 

erosion of colluvium. However, the effects of erosion 

and apparent transportation of this single event indicate that the bulk of even coarse-sediment disposal 

may be accomplished by more frequent but less spectacular events in combination with in-place particle 

weathering.

SUMMARY AND CONCLUSIONS

1. The partly completed Hell Hole Dam failed on December 23,1964, in response to a torrential 5-day 

rainfall of approximately 22 inches in the basin 

upstream from the damsite. Natural peak discharges in the region were generally equivalent to 

the maximum discharges attained in previous 

floods for which there is record, but the surge released at the damsite was greatly in excess of previously recorded flows along its entire route 61 

miles of the Rubicon, Middle Fork American, and 

North Fork American Rivers. The flood surge 

was released over a period of 1 hour, during which 

the mean dischage was 260,000 cf s. Probable continuous attenuation in discharge occurred, and 

with the exception of the narrowing section of 

channel between Parsley Bar and the Rubicon 

gorge, reduction in height of the flood wave also 

occurred. Wave velocity averaged 22 feet per 

second.

2. Natural sources of sediment contributed to the flood 

included, in relative order of magnitude, stream 

alluvium, colluvium, glacial-outwash terrace 

gravel, landslides, glacial deposits, soil, and bedrock. More than 30 discrete landslides were triggered as a result of disturbance of slope equilibrium by scour along channel sides. Most of the 

mass movement occurred after the crest of the 

wave had passed, so large slides were not a significant source of sediment to the surge. Substantially more important as a source of sediment were 

the nearly continuous removal of colluvium by 

scour and flood-induced mass movements on a 

small scale.

3. A pronounced change in channel morphology occurred throughout the bedrock-controlled flood 

course, from a V-shaped channnel to one more 

broadly U-shaped. Erosion of channel sides was 

concomitant with thalweg aggradation, which 

was measured at five cross-profile sites and probably was nearly continuous along the flood course. 

Extensive aggradation in the 1.6 miles of channel 

below the damsite is related to deposition of rockfill from the breached dam embankment.

4. Depositional forms in the channel were strongly influenced by local sources of sediment. Terracelike 

berms were formed along channel walls in the 

reach below the dam, in which flow was highly 

macroturbulent, turbulence was not damped by 

included fines, and boulders were transported in 

suspension. As much as 5 feet of boulder gravel 

was added to the surface of a glacial-outwash terrace 28 feet above the presurge thalweg at a peak 

stage of 45 feet relative to the presurge thalweg.

5. Bonlder fronts as much as 7 feet high and 250 feet 

long were formed transverse to flow direction in 

an expanding reach with a declining gradient. 

Such forms are interpreted as the snouts of discrete flows of bed material rather than waveforms, 

but wave movement also occurred. The evidence 

for movement of bed material as viscous subaqueous rockflows includes: (a) The presence of boulder fronts in the channel, (b) the presence in the 

deposits associated with the boulder fronts of extremely coarse particles with diameter equal to the 

thickness of the deposit, (c) the unusually poor 

sorting of the deposits, (d) the occurrence of the 

boulder-front deposits in reaches where the maximum tractive force applied by the surge did not 

approach the critical tractive force necessary to 

move the coarser individual particles. The terracelike boulder berms are believed to represent backwater deposition of bedload moved in this manner 

or by wave movement.

6. Some change in the pool-riffle pattern resulted from 

the surge passage, but because of the scale of all 

preflood aerial photographs, change could be 

documented at only one locality. New riffles were
FLOOD SURGE ON THE RUBICON RIVER, CALIFORNIA M37

formed by lateral supply of coarse material by 

slides and by cutting of boulder fronts, and some 

presurge riffles were removed.

7. The rockfill from the damsite moved as mass flows 

or as a series of waves without mixing in the channel thalweg, as far as 1.6 miles from the damsite, 

and a single boulder traveled 2.1 miles. Deposits 

above and lateral to the channel in this interval 

are attributed to an early phase of the surge, probably near the crest, and consist in large part of 

normal stream gravel and reworked terrace 

gravel. Compositional contrast between the two 

episodes of deposition from a single flood wave is 

pronounced.

8. Movement of the rockfill from the damsite demonstrated a pronounced downstream decline in mean 

diameter as well as an imperfect increase in degree of sorting. No regular change in skewness 

was detected.

9. The downstream decline in particle size occurred 

under conditions of continuously decreasing competency, as indicated by indirect measurements of 

tractive force. Sporadic measurements of tractive 

force along the rest of the flood route show that 

longitudinal variation in maximum competency 

was substantial. The competency of the flood was 

also determined by measuring the size of boulders 

moved. Maximum tractive force attained at a locality is related to the maximum size of boulders 

whose movement could be attributed to the flood, 

and the relation was compatible with previous 

studies of tractive force and particle size. With 

the exception of coarse material supplied laterally 

to the channel at a late stage in the surge when 

flow was not sufficiently competent to cause movement, virtually all detritus in the channel was 

moved. Distances traveled by individual boulders 

were small, however.

10. Bounding of the rockfill occurred more rapidly over 

a much smaller increment of transport than was 

anticipated from previous field and experimental 

studies; this unusual rate of rounding is explained 

by the coarseness of the material, the unusual 

rigor of the transport conditions, and the possibility of transport in subaqueous rock flow. Boulders carried in part in suspension at or near the 

surge peak and deposited on lateral bars are not 

as well rounded as those deposited in the thalweg. 

The massive diorite boulders were rounded after 

only 1.5 miles of downstream movement.

11. Consideration of the relative importance of abrasion 

and fracturing versus sorting effects indicates that 

progressive sorting is the dominant cause of the 

downstream decline in particle size of the rockfill.

12. If the effects of the surge passage are analogous to 

natural catastrophic floods, jokulhlaups, and those 

caused by the breaching of landslide dams, such 

floods, in mountain streams with bed material of 

boulder size, may set and control pool-riffle patterns, initiate periods of increased mass movement, and give rise to subaqueous mass flow in the 

channel. Another striking result of the surge was 

the introduction into the channel of large quantities of valley side material, a major source of 

coarse material. Greater dispersal of the coarse 

sediment fraction is accomplished by normal 

floods in combination with size reduction by 

weathering than by natural catastrophe floods, 

however.

REFERENCES CITED

Bretz, J H., 1929, Valley deposits immediately east of the channeled scabland of Washington: Jour. Geology, v. 37, p. 

505-541.

Brush, L. M., Jr., 1961, Drainage basins, channels, and flow 

characteristics of selected streams in central Pennsylvania : 

U.S. Geol. Survey Prof. Paper 282-F, p. 145-181.

Emery, K. O., 1955, Grain size of marine beach gravels: Jour. 

Geology, v. 63, p. 39-49.

Fahnestock, R. K., 1963, Morphology and hydrology of a glacial 

stream White River, Mount Rainier, Washington: U.S. 

Geol. Survey Prof. Paper 422-A, 70 p. [1964].

Folk, R. L., 1964, Petrology of sedimentary rocks: Austin, Tex., 

Hemphill's, 154 p.

Gwinn, V. E., 1964, Deduction of flow regime from bedding character in conglomerates and sandstones: Jour. Sed. Petrology, 

v. 34, p. 656-658.

Inman, D. I., 1952, Measures for describing the size distribution 

of sediments: Jour. Sed. Petrology, v. 22, p. 125-145.

Kramer, H., 1935, Sand mixtures and sand movement in fluvial 

models: Am. Soc. Civil Engineers Trans., v. 100, p. 798-838.

Krumbein, W. C., I941a, Measurement and geological significance 

of shape and roundness of sedimentary particles: Jour. Sed. 

Petrology, v. 11, p. 64-72.

 I94lb, The effects of abrasion on the size, shape, and 

roundness of rock fragments: Jour. Geology, v. 49, p. 

482-^520. 

-1942, Flood deposits of Arroyo Seco, Los Angeles County,

California : Geol. Soc. America Bull., v. 53, p. 1355-1402.

Krumbein, W. C., and Tisdel, F. W., 1940, Size distributions of 

source rocks of sediments: Am. Jour. Sci., v. 238, p. 296-305.

Kuenen, Ph. H., 1956, Rolling by current, pt. 2 of Experimental 

abrasion of pebbles: Jour Geology, v. 64, p. 336-368.

Leliavsky, S., 1955, An introduction to fluvial hydraulics: London, Constable and Co., Ltd., 257 p.

Leopold, L. B., Wolman, M. G., and Miller, J. P., 1964, Fluvial 

processess in geomorphology: San Francisco, W. H. Freeman and Co., 522 p.

Matthes, F. E., 1930, Geologic history of the Yosemite Valley: 

U.S. Geol. Survey Prof. Paper 160,137 p.

Matthes, G. H., 1947, Macroturbulence in natural streamfiow: 

Am. Geophys Union Trans., v. 28, p. 255-262.

Pettijohn, F. J., 1957, Sedimentary rocks [2d ed.] : New York, 

Harper and Bros., 718 p.
M38 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVEES

Pettijohn, F. J., and Potter, P. EM 1964, Atlas and glossary of 

primary sedimentary structures: New York, Springer-Verlag, 370 p.

Philbrick, S. S., and Cleaves, A. B., 1958, Field and laboratory 

investigations, in Landslides and engineering practices: 

Highway Research Board Spec. Kept. 29, Nat. Resources 

Council Pub. 544, p. 93-111.

Plumley, W. J., 1948, Black Hills terrace gravels; a study in 

sediment transport: Jour. Geology, v. 58, p. 526-577.

Postma, H., 1961, Transport and accumulation of suspended 

matter in the Dutch Wadden Sea: Netherlands Jour. Sea 

Research, v. 1 (2v.), p. 148-190.

Schoklitseh, A., 1933, Ueber die Verkleinerung der Geschiebe in 

Flusslaufen: Akad. Wiss., Wien, v. 142, p. 343-366.

Sharp, B. P., and Noble, L. H., 1953, Mudflow of 1941 at Wrightwood, southern California: Geol. Soc. America Bull., v. 64, p. 

547-560.

Simons, D. B., and Richardson, E. V., 1961, Forms of bed roughness in alluvial channels: Am. Soc. Civil Engineers Hydraulics Jour., v. 87, no. 3, p. 87-105.

Thiel, G. S., 1932, Giant current ripples in coarse fluvial gravel: 

Jour. Geology, v. 40, p. 452-458.

Thorarinsson, Sigurdur, 1939, The ice-dammed lakes of Iceland

with particular reference to their values as indicators of 

glacier oscillations: Geog. Annaler, v. 21, p. 216-242.

Trask, P. D., 1932, Origin and environment of source sediments 

of petroleum: Houston, Tex., Gulf Publishing Co., 323 p.

Tricarte, J., et collaborateurs, 1961, M6canismes normaux et 

phgnomenes catastrophiques dans 1' Evolution des versants 

du bassin du Guil (Htes-Alpes, France): Zeitschr. Geomorphologie, v. 5, p. 277-301.

Varnes, D. J., 1958, Landslide types and processes, in Landslides and engineering practice: Highway Research Board 

Spec. Rept 29, Nat Resources Council Pub. 544, p. 20-47.

Walker, R. G., 1963, Distinctive types of ripple-drift cross-lamination : Sedimentology, v. 2, p. 173-188.

Wolman, M. G., 1954. A method of sampling coarse river-bed 

material: Am. Geophys. Union Trans., v. 35, p. 951-956.

Wolman, M. G., and Miller, J. P., 1960, Magnitude and frequency 

of forces in geomorphic processes: Jour. Geology, v. 68, p. 

54-74.

Young, L. E., and Gruff, R. W., 1966, Magnitude and frequency 

of floods in the United States; pt. 11, Pacific slope basins in 

California Coastal basins south of the Klamath River 

basin and Central Valley drainage from the west: U.S. Geol. 

Survey open-file rept, 70 p.
INDEX

[Italic numbers indicate major references]

	Page 

Alluvial deposits, erosion ___ ........ ______________ ......... _ M12

Amador Group ... ___ . _______ ______ . _________ . _ 5

American River, drainage area. __ ..... _______ . ___ . ________ 2

Amphibolite... _ ........ _ . _ _ .... _ ...... __ _ _ ................ 5

Arid Transition life zone __ _ .... _ . _ ................... _ ........ _ 3

Auburn gaging station, discharge ____________ . _____ .. . ____ 10,12

Bars, downstream changes In patterns ____ . _________ ______ 27

Bed material, transport and deposition.. __ . ______ _____ . ___ 87

Bed-material forms ___ ... ___________________________ 86

Bedrock, erosion __ .... _ _ _____ _ ___ ____ .. _ _ _ . _ 12

Berms, deposition. __ _____________ ________________ 18

downstream changes in patterns.. ____ . ______________ . _ 27

Biotite quartz diorite.. _ _ . ______ _____________ . ___ . _ 4

Boulder bars.. . ____ .. ___________________________ . 19

Boulder fronts.. __________________________________ 20

Boulder jams ___________________________ ________ 21

Boulder movement, criteria indicating.. __ _____ . _________ . _ 32

episodic ... __ ........ ___ _____________________ 8S

Bridges, destruction _________ . ___ . _ . _____________ .... 6

Calaveras Formation. _ __ _ ....... _ .......... _ _ ............ _ 4,14

Channel morphology, effects of flood surge _____________ _ __ .. IS

Cherokee Bar, erosion _______________ ______ _ ... _ ..... 13

Chlorite schist _______________________________ ___ 5

Climate __ ...... -. .... ....................-. - ............ --. S

Cock Robin Point, slippage -..-........................................ 36

Colluvium, erosion.. _ _______________ .. ____ _ .. __ 12,13,36

Competence, water.. ___ ____ . _________ .. ______ .... _ . SI

Crossbedding. ........................................................ 21,23,26

Current-ripple lamination... _______ .. ___ . _ .... _ . _ ........ _ 26

Delta movement ____________ ...... __ .. _________ ....... 21

Deposition, bed material ___________ _____ . ___________ . 27

Depositional features ________ . ____ _________________ 17

Diabase _____________________________ _ ______ . 4

Dikes......... ....... ................................... ................... 5

Diorite..... . .. .. . . ....... 4,18,27,33

Discharge, downstream from Hell Hole Dam.. _ _ . ____ .......... _ .. 18

flood surge ___________ . _______________ . __ 6,10

Rubicon River.......... _ ............ __ ............................ 35

Dune movement.. .... _ _ _ ___ ...... ______ _ _ . _ . _ .. 21

Embankment failure. ... _________________________ ..... 6

Erosional effects of the surge.. .. __ ... __________________ .... . 18

Floods, 1956-1963..... ....

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

Folsom Lake, floodsurge

precipitation ranges

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

.............. ......... .............. ......... 8

	1 

. ______ . ____ _________ . 3

Foresthill gaging station ___________________________ .. .. 9,12

French Alps, flood.. _____________________________ . _ 36

Qabbro. -...-. ...................... ... ... ... ........ _ ...... _ ........ 4

Geology.-.-.. ........ ........ ..... ...... ...................... ........ 4

Qeomorphic history, Rubicon Canyon.... _ _ . _ _ ...... _ _ ...... 5

Georgetown, storm runoff. ____ . ___________________ 10

Georgetown gaging station. ___ . ______________________ . 9,10

Glaciation ............ __ ........... ........... _ ........... ......... 5

Grading, berms _ .. ________ . __________________ 18

Granodiorite....... ______ ..... _ _ _ .. _ .. __________ 4

Greenstone ____________________________________ . 5

Page

Hell Hole, precipitation ranges..____. . . M3

Hell Hole Dam, construction.. .. ..... 4

failure..................................................... ...... 6

inflow and outflow____ ___ . - 10 

Hydrographs____.... 10

Inflow, Hell Hole Dam. 

Joints-_______...

Kolks.

10

5

35

Landslides--......... - -. . . . .-.- /4,36

Lateral berms. ______ _ 18

Location of area. *

McCreary-Koretsky Engineers....._.... ...... 6,10

Macroturbulence_ - 19,54

Manning equation...._......__ . 12

Mariposa Slate.. .. - 5

Mass movements.. - -- 14

Meteorological conditions, time of flood. 8

Middle Fork American River, berms.- - ----- 19

floodsurge . ..... 1

gaging stations..... . 10

peak discharge..__ . 12

pool and rime pattern... __.. . .- 25

slope........ ..................-... . 2

Middle Fork American River Project.....-.....---... ................... 6

Mother Lode belt... ............ .. ............. .... 5

Mountain-valley stage, Sierra Nevada development .. 5

North Fork American River, berms ---- 19

flood surge___.............. .......... ..... ... 1

pool and riffle pattern.______.. . 25

slope _____.. - 2

Oregon Bar, erosion_.. . - 13

Outflow, Hell Hole Dam.................................... 10

Parsley Bar, bed material.._..__ .... .--.. 27

boulder bars.__... - 19 

boulder front.. ........ .............................. 20,23,24

geology-_ - 4

loss of timber_.... 13

pool and riffle pattern.-..-._ - 25

stratigraphy..____.............. .. ....... 5

Particle size, cause of downstream decline. .- 33

flood material-_.................. ............ ............ ..... 27

Photography, channel forms _.. . .................. 17

Physical setting..... -- 8

Pleistocene ice terminus, Rubicon River canyon.. 2

Pleistocene uplift______.......... -- 5

Pliocene uplift ... .......-........--------~ ............. .---.- 5

Pool and riffle pattern. .......................................-. ...-- 85

Portland-Pacific Cement Co. bridge ...................... ....... 6

Poverty Bar, erosion........ .. 13

Precipitation, annual.._. .-.-.... ........ ........... 3

December 19-23..... - .......... ........ .... ..... 8

Pyramid Peak..._.... ........ . .. . . --. - 2

M39
M40 INDEX

Page

Rosin's law...-._._---------_-....___.._________.___._ M30

Roundness, flood material..-__......................_-....-_.--..-.---. 32

Rubicon River, discharge.------__.............._.___--.._--------- 10

drainage area..._....................................................... 2

flood surge..........................__................................. 1

gaging stations.. ....................................................... 9

pool and riflle pattern.--..-....__..........................._........ 25

Rubicon River canyon, geomorphic history and stratigraphy.-..--._-_--_.--- 5

landslides.._...................._....----...------..-.----.....-.--.. 14

slope.-...__----........-....___........____________._.. 2

Rubicon Springs, discharge__. 

Rubicon Springs gaging station. 

Runoff, floods...................

..-. .. ..... _.. 10

.. ... ......- ... 9

 ....... .- .-. 9

 . -... ----.. 2

............................... 21

 .- .. .. 26

.. .. -. ... 17

 .. ..- .. ...... 1

............................... 8

............--- ............ 10

............................... 30

Slope-conveyance method, discharge..._..________.________.... 12

Snow, effect on flood ._---.-.-.-----.-..-._._______________ 8

Soil, erosion....____.........._._._.___......___.______ 12

Sacramento_--..--..-....-----....-..-

Sand wave-.--......-.-.........,-..._

Sedimentary structures, internal__....

Seismic measurements, landslides_.--- 

Sierra Nevada...........................

Sierra Nevada floods, effect of snowpack. 

Silver Creek, storm runoff-.. --------

Skewness, flood material__............

Page

SoUcreep . . . - . ------ M36

Sonoran Transition life zone............... ...-....-....---------------- 4

Sorting, flood particles... ..... .. ---- 34

Stage, surge wave .-----.--...-- ....---...----.----.-.---. -------- 12

Storm, December 1964.------.-......-....---.-...------.------..----------- 8

Stratigraphy, Rubicon Canyon_.--., ..-----.-.--......------------- 5

Streambed material, composition.....---_.____.____---_----.--.--.--------- 27

Structure, internal....____...-----.--. ------- _-.-..--__---- 26

major trends__ -....--....-.-----...---------------------. ------- 5

Temperatures, December 21-23...-._____.__.- ----- ... ------ 8

Terrace accretion._---__.-----_------..--._---------------------- -... 19

Terrace levels, Rubicon River canyon..... . . ... 5

Timber, effects of flood.__.._..--.-----..-.--..-- ----- . - 13

Tractive force---.....---.......-...........- ...... ..... ------------- 31

Transport, bed material----.---.-.-----.-..-.---- . . - ---- 27

Trask sorting coefficient-........--.---__ -..... ... ----- - 29

Twin Lakes, snowpack_..-.......--....---..--...----..--. ...-. -.... 8

Union Valley Dam, storm runoff....-._... . - - - 10

Vegetation..-...---..-.--...........- -...--.... ............. .. 3

Wave height, flood surge......__........ .. . . . 12

Wave velocity, flood surge.-.--._.-.-.... ...-...--...-- ............. 8,12
Physiographic and 

Hydraulic Studies 

of Rivers, 1961

GEOLOGICAL SURVEY PROFESSIONAL PAPER 422

This volume was published 

as separate chapters A-M
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director
CONTENTS

[Letters designate the separately published chapters]

(A) Morphology and hydrology of a glacial stream White River, Mount Rainier, Washington, by Robert K. Fahnestock.

(B) Hydraulic geometry of a small tidal estuary, by Robert M. Myrlck and Luna B. Leopold.

(G) Drainage density and streamflow, by Charles W. Carlston.

(D) Channel patterns and terraces of the Loup Rivers in Nebraska, by James O. Brice.

(B) Channel geometry of Piedmont streams as related to frequency of floods, by F. H. Kilpatrick and H. H. Barnes, Jr.

(F) Sediment yield of the Castaic watershed, Western Los Angeles County, California a quantitative geomorphic approach, by

Lawrence K. Lustig.

(G) The distribution of branches in river networks, by Ennio V. Giusti and William J. Schneider. 

(H) River meanders theory of minimum variance, by Walter B. Langbein and Luna B. Leopold. 

(I) An approach to the sediment transport problem from general physics, by R. A. Bagnold. 

(J) Resistance to flow in alluvial channels, by D. B. Simons and E. V. Richardson. 

(K) Erosion and deposition produced by the flood of December 1964 on Coffee Creek, Trinity County, California, by John H.

Stewart and Valmore C. LaMarche, Jr.

(L) River channel bars and dunes theory of kinematic waves, by Walter B. Langbein. 

(M) Flood surge on the Rubicon River, California hydrology, hydraulics, and boulder transport, by Kevin M. Scott and George

C. Gravlee, Jr.

O