“THE GREAT CATARACT”

Effects of Late Holocene Debris Flows on Lava

Falls Rapid, Grand Canyon National Park,

Arizona

U.S. GEOLOGICAL SURVEY

Open-File Report 96—460

Prepared in cooperation with the

BUREAU OF RECLAMATION
“THE GREAT CATARACT”

Effects of Late Holocene Debris Flows on Lava 

Falls Rapid, Grand Canyon National Park,

Arizona

By ROBERT H. WEBB, THEODORE S. MELIS, THOMAS W. 

WISE, and JOHN G. ELLIOTT

 

U.S. GEOLOGICAL SURVEY

Open-File Report 96—460

Prepared in cooperation with the

BUREAU OF RECLAMATION

Tucson, Arizona

 1996
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

______________________________________________________

For additional information

write to:

Regional Research Hydrologist

U.S. Geological Survey, MS-472

Water Resources Division

345 Middlefield Road

Menlo Park, CA 94025

Copies of this report can be

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225
Contents I

CONTENTS

Page

Abstract ........................................................................................................................................................ 1

Introduction................................................................................................................................................... 2

Purpose and Scope.......................................................................................................................... 5

Acknowledgments .......................................................................................................................... 5

Characteristics of Debris Flows in Grand Canyon ......................................................................... 6

Debris Flows and the Colorado River ............................................................................................ 7

Methods ........................................................................................................................................................ 8

The Prospect Valley Drainage Basin ............................................................................................. 8

Historical Photographs of Lava Falls Rapid................................................................................... 10

Mapping and Age Determinations for Debris-Flow Deposits........................................................ 10

Pedogenic Calcium Carbonate........................................................................................................ 11

3He .................................................................................................................................................. 11

Desert Plant Assemblages............................................................................................................... 12

Clonal Rings of Creosote Bush....................................................................................................... 12

14C .................................................................................................................................................. 12

Climatic and Hydrologic Data........................................................................................................ 13

Characterization of Debris Flows and Debris Fans ........................................................................ 14

Discharge Estimates and Water Content ................................................................................ 14

Surface Area and Volume............................................................................................................... 15

Constrictions................................................................................................................................... 15

Surficial Geology of The Prospect Debris Fan ............................................................................................. 16

General Debris Fan Characteristics ................................................................................................ 16

Fan-Forming Debris-Flow Surfaces ............................................................................................... 18

 tfa ............................................................................................................................................... 18

 tfb............................................................................................................................................... 20

 tfc ............................................................................................................................................... 20

Inset Debris-Flow Surfaces............................................................................................................. 21

 tia ............................................................................................................................................... 21

 tib ............................................................................................................................................... 22

 tic ............................................................................................................................................... 22

 tid ............................................................................................................................................... 24

 tie ............................................................................................................................................... 24

 tif................................................................................................................................................ 24

 tig ............................................................................................................................................... 25

 tih ............................................................................................................................................... 27

 tii................................................................................................................................................ 27

 tij................................................................................................................................................ 27

Colluvium and Steep Slopes........................................................................................................... 27

 tc ................................................................................................................................................ 27

Reworked Debris-Flow Deposits.................................................................................................... 28

 rwr.............................................................................................................................................. 28

 rwc ............................................................................................................................................. 28

 rwfd............................................................................................................................................ 28

Historical Change in Lava Falls Rapid ......................................................................................................... 28

Navigation of Lava Falls Rapid in 1994......................................................................................... 28

The Wide, Stable Rapid (1872-1939)............................................................................................. 33

The Period of Frequent Debris Flows (1939-1966)........................................................................ 44

Quiescence (1966-1995) ............................................................................................................................... 53

The 1995 Debris Flow .................................................................................................................... 53

Initiation of Debris Flows in Prospect Canyon............................................................................................. 72
II Contents

CONTENTS—Continued Page

Hydroclimatology........................................................................................................................... 72

The Firehose Effect ............................................................................................................. 75

Magnitude and Frequency of Debris Flows.................................................................................................. 77

Discharge Estimates........................................................................................................................ 77

Volumes of Debris Flows ............................................................................................................... 78

Constrictions of the Colorado River............................................................................................... 79

Frequency of Debris Flows............................................................................................................. 81

Reworking of Debris Fans by the Colorado River........................................................................................ 84

Discussion and Conclusions.......................................................................................................................... 87

References Cited ........................................................................................................................................... 91

FIGURES 

1. Maps of the study area 

 A. The Prospect Valley drainage basin and Grand Canyon National Park.................................... 3

 B. The Prospect Valley drainage basin........................................................................................... 4

2. Map of Lava Falls and Lower Lava Rapids showing geomorphic features of the Colorado River . 7

3. Geologic map of Prospect Canyon. This map is modified from Billingsley and Huntoon (1983)

 and Hamblin (1994b); the original scale is 1:62,500 ...................................................................... 9

4. Graphs showing particle-size distributions for debris flows in Prospect Canyon

 A. Prehistoric and historic debris flows ......................................................................................... 17

 B. Pulses of the 1995 debris flow................................................................................................... 17

5. Graph showing source materials transported by debris flows in Prospect Canyon ......................... 19

6. Graph of cross section showing the relative elevations and positions of fan-forming and inset

 debris-flow surfaces along A - A’ (see Plate I) ............................................................................... 20

7. View of the Prospect debris fan on March 7, 1995 .......................................................................... 21

8. Stratigraphy of surface tfa on the Prospect debris fan...................................................................... 22

9. Stratigraphy of the 1939 debris-flow deposit (surface tig) .............................................................. 23

10. Stratigraphy of the 1955 debris-flow deposits (surface tih) .......................................................... 24

11. Maps of Lava Falls Rapid 

 A. Based on a 1994 aerial photograph, showing the modern configuration of Lava Falls Rapid. 25

 B. The configuration of the rapid from 1872 to 1939 ................................................................... 26

12. Replicate downstream views of Prospect Canyon and Lava Falls Rapid from Toroweap 

 Overlook ....................................................................................................................................... 29

13. Replicate downstream views of Lava Falls Rapid from the high surface on the left side ............. 32

14. Upstream view of Lava Falls Rapid from the left side................................................................... 34

15. Replicate upstream views of the head of Lava Falls Rapid from the right side............................. 36

16. Replicate downstream views of the right side of Lava Falls Rapid ............................................... 38

17. View of Lava Falls Rapid from the left side ................................................................................. 40

18. Replicate views of Lava Falls Rapid from the left side................................................................. 41

19. Replicate downstream views of Lava Falls Rapid from the left side............................................. 46

20. Replicate upstream views of Lava Falls from the left side ............................................................ 48 

21. Oblique aerial views of Lava Falls Rapid and Prospect Canyon ................................................. 50

22. Replicate downstream views of Prospect Canyon debris fan and lower Lava Falls Rapid .......... 54

23. Replicate upstream views of Lava Falls Rapid from the right side............................................... 56

24. Oblique aerial view of Lava Falls Rapid and Prospect Canyon ................................................... 59

25. Replicate views from the left side across the top of Lava Falls Rapid.......................................... 60
Contents III

FIGURES—Continued Page

26. Replicate downstream views of Lava Falls Rapid from the high surface on the left side ............ 62

27. View of the firehose effect created by streamflow falling over a 325-m fall at the head of 

 Prospect Canyon on March 6, 1995 ............................................................................................. 64

28. Replicate vertical aerial photographs of Lava Falls Rapid............................................................ 65

29. Graphs showing longitudinal profile of the water-surface fall on the right side of Lava Falls 

 Rapid 

 A. March 6-8, 1995 ..................................................................................................................... 66 

 B. Comparison of water-surface profiles in March 1994 and March 1995................................. 66

30. Replicate upstream views of Lava Falls Rapid from the lower right ........................................... 67

31. Replicate views across Lava Falls Rapid from river left .............................................................. 70

32. Graphs showing standardized seasonal precipitation for eight stations in the vicinity of western

 Grand Canyon (Appendix 2) ........................................................................................................ 73

33. Graphs showing monthly precipitation near the Prospect Valley drainage basin associated with

 Prospect Canyon debris flows and floods .................................................................................... 74

34. Graphs showing hourly precipitation at Tuweep Ranger Station and Grand Canyon during three

 historic debris flows and one flood in Prospect Canyon .............................................................. 75

35. Map showing the spatial extent of debris fans deposited by Holocene debris flows from 

 Prospect Canyon........................................................................................................................... 76

36. Graph showing the volume of sediments deposited on debris fans by historical debris flows in

 Grand Canyon (modified from Melis and others, 1994) and Holocene debris flows from 

 Prospect Canyon........................................................................................................................... 79

37. Graphs showing the constriction ratios of debris fans in Grand Canyon...................................... 80

38. Graph showing annual series of instantaneous peak discharges for the Colorado River near

 Grand Canyon, Arizona................................................................................................................ 81

39. Hydrographs of the Colorado River showing the timing of tributary floods and debris flows 

 and river discharges...................................................................................................................... 83

40. Hydrographs of the Colorado River from 1966 to 1974 showing the timing of the 1966 debris 

 flow and releases from Glen Canyon Dam................................................................................... 85

41. Graphs showing the effects of river reworking on particles on the Prospect debris fan ............... 86

42. Diagram of one cycle of debris-flow deposition and river reworking at Lava Falls Rapid .......... 88

TABLES

1. Radiocarbon dates of organic material collected from debris-flow deposits at the mouth of

 Prospect Canyon............................................................................................................................... 13

2. Preliminary ranking of the area and volume of the 15 largest debris fans of probable Holocene age

 on the Colorado River in Grand Canyon National Park.................................................................. 16

3. Mineralogy of the clay-size fraction of Prospect Canyon debris flows compared with other Grand 

 Canyon debris flows........................................................................................................................ 18

4. Dates and photographic evidence for debris flows and other floods in Prospect Canyon ............... 44

5. Peak discharge estimates from superelevation evidence for the debris flows of 1939, 1955, and 

 1963 in Prospect Canyon at cross section A - A’............................................................................ 77

6. Characteristics of debris fans deposited at the mouth of Prospect Canyon between 1939 and 1995 78 

7. Constriction percentage and constriction ratios of historic aggraded and reworked debris fans at 

 the mouth of Prospect Canyon ........................................................................................................ 82
IV Contents

CONVERSION FACTORS

For readers who prefer to use inch-pound units, conversion factors for the terms in this report are listed below:

Multiply By To obtain

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.2818 foot (ft)

square meter (m2) 10.76 square foot (ft2)

cubic meter (m3

) 35.31 cubic foot (ft3)

kilometer (km) 0.6214 mile (mi)

square kilometer (km2

) 0.3861 square mile (mi2)

cubic meter per second 35.31 cubic foot per second

 (m3

/s) (ft3/s)

gram (g) 0.03527 ounce avoirdupois 

 (oz avdp)

kilogram (kg) 2.205 pound avoirdupois

 (lb avdp)

megagram (Mg) 1.102 tons, short

(2,000 pounds)

 

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum 

derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called “Sea Level 

Datum of 1929.”
 Contents V

FRONTISPIECE. The Prospect Canyon debris fan and Lava Falls Rapid from the Tuweap Trail. 
ABSTRACT 1

“The Great Cataract”

Effects Of Late Holocene Debris Flows On Lava Falls Rapid, Grand 

Canyon National Park and Hualapai Indian Reservation, Arizona

By Robert H. Webb, Theodore S. Melis, Thomas W. Wise, and John G. Elliott

ABSTRACT

Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand

Canyon and is one of the most famous rapids in the world. Although the rapid was once thought to

be controlled by the remnants of lava dams of Pleistocene age, Lava Falls was created and is

maintained by frequent debris flows from Prospect Canyon. We used 232 historical photographs,

of which 121 were replicated, and 14C and 3He dating methods to reconstruct the ages and, in

some cases, the magnitudes of late Holocene debris flows. We quantified the interaction between

Prospect Canyon debris flows and the Colorado River using image processing of the historical

photographs. 

The highest and oldest debris-flow deposits on the debris fan yielded a 3He date of 2.9±0.6 ka

(950 BC), which indicates predominately late Holocene aggradation of one of the largest debris

fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking,

crossed the Colorado River and raised its base level by 30 m for an indeterminate, although

probably short, period. We mapped depositional surfaces of 6 debris flows that occurred after 950

BC. The most recent prehistoric debris flow occurred no more than 500 years ago (AD 1434). 

From April 1872 to July 1939, no debris flows occurred in Prospect Canyon. Debris flows in

1939, 1954, 1955, 1963, 1966, and 1995 constricted the Colorado River between 35 and 80

percent and completely changed the pattern of flow through the rapid. The debris flows had

discharges estimated between about 290 and 1,000 m3/s and transported boulders as heavy as 30

Mg. The recurrence interval of these debris flows, calculated from the volume of the aggraded

debris fan, ranged from 35 to 200 yrs. The 1939 debris flow in Prospect Canyon appears to have

been the largest debris flow in Grand Canyon during the last 125 years.

Debris flows in Prospect Canyon are initiated by streamflow pouring over a 325-m waterfall

onto unconsolidated colluvium, a process called the firehose effect. Floods in Prospect Valley

above the waterfall are generated during regional winter storms, localized summer thunderstorms,

and occasional tropical cyclones. Winter precipitation has increased in the Grand Canyon region

since the early 1960s, and the most recent debris flows have occurred during winter storms.

Summer rainfall has declined in the same period, decreasing the potential for debris flows in the

summer months.

The history of river reworking of the Prospect Canyon debris fan illustrates the interrelation

between tributary debris fans and mainstem floods in bedrock canyons. Lava Falls Rapid did not

change despite Colorado River floods of 8,500 m3

/s in 1884 and 6,230 m3/s in 1921. Floods up to

3,540 m3/s that occurred after the historical, pre-dam debris flows removed most of the deposits

within 3 years. Releases in 1965 from Glen Canyon Dam that were above powerplant capacity but

less than 1,640 m3/s removed most of the debris fan deposited in 1963, and the combination of
2 The Great Cataract 

dam releases and a 1973 flood on the Little Colorado River removed the 1966 aggradation. About

4,800 m3 of the 1995 deposit was reworked on the day of the 1995 debris flow, dam releases of

less than 570 m3/s had not reworked the remainder of the aggraded debris fan.

Lava Falls Rapid has been the most unstable reach of whitewater in Grand Canyon during the

late Holocene and particularly during the last 120 years. Rapids in bedrock canyons controlled by

tributary deposition in the main channel are aggradational features that reflect the net effect of

tributary-mainstem interactions. Boulders that form the core of rapids in Grand Canyon are

essentially immobile by both regulated and unregulated Colorado River flows. Historical

operation of Glen Canyon Dam, which was completed in 1963, has reduced the potential for

reworking of debris fans, and has accelerated the rate of net aggradation at the mouths of tributary

canyons. Because debris fans that formed after 1963 at Lava Falls have been mostly reworked by

dam releases, occasional high releases from Glen Canyon Dam could be scheduled as channel

maintenance flows to rework aggraded debris fans.

INTRODUCTION

Lava Falls on the Colorado River in Grand

Canyon (fig. 1) is one of the most difficult

navigable rapids in the continental United States

(Nash, 1989; Ghiglieri, 1992; Lindemann and

Lindemann, 1995). The rapid is located at the

mouth of Prospect Canyon at river mile 179.4, or

289 km downstream from Lees Ferry, Arizona

(Stevens, 1990). Lava Falls is the impressive finale

for approximately 20,000 river runners who run the

Colorado River every year and is the standard

against which all rapids in Grand Canyon National

Park are judged.

Misconceptions abound in the popular

literature concerning the origins, size, and

geological significance of Lava Falls Rapid. The

first explorers of the Colorado River (John Wesley

Powell and Robert Brewster Stanton) believed Lava

Falls was formed by the eroded remnants of

Pleistocene lava dams that once spanned the inner

canyon (Powell, 1875; Smith and Crampton, 1987).

Indeed, the rapid is named for the basalt flows that

poured over the nearby canyon walls (Granger,

1960; Brian, 1992), and several popular accounts

perpetuate the misconception that Lava Falls is

controlled by underwater dikes or ledges of basalt

(Fradkin,1984, p. 206; Nash 1989, p. 179). Easilyverified details about the rapid such as its fall, about

4 m (Kieffer, 1988), have been incorrectly reported

as 12 m (Stevens, 1990). 

Most of the rapids of the Colorado River,

including Lava Falls, result from the accumulation

of large boulders in the Colorado River on debris

fans at the mouths of tributary canyons (Péwé,

1968; Hamblin and Rigby, 1968; Simmons and

Gaskill, 1969; Graf, 1979; Howard and Dolan,

1981; Kieffer, 1987, 1988). Webb and others

(1988a) reported that 54 of the 57 largest rapids in

Grand Canyon are located at tributary junctures.

Most rapids in Grand Canyon are at the mouths of

tributary canyons that form along fault zones; the

faults typically cross the Colorado River, and the

presence of fracture zones downstream of rapids

may increase canyon downcutting rates (Dolan and

others, 1978).

The locations of some of the smaller rapids and

riffles are not at tributary junctures but are linked to

reworking of tributary debris fans. Leopold (1969)

observed that only a fraction of the rapids are

coincident with tributary junctures and that the

rapids and pools are evenly spaced and maintained

by quasi-equilibrium processes of flow and

sediment transport in the Colorado River. Graf

(1979) found that the distribution of rapids in Grand

Canyon is both regular and random and that 79

percent of the rapids are at tributary junctures.

Webb and others (1989) distinguish between

primary rapids at tributary junctures and secondary

rapids formed around downstream debris bars

(Howard and Dolan, 1981); the debris bars are

deposits of boulders reworked from the upstream

debris fan.
INTRODUCTION 3

Boulders accumulate in the river by the

interacting process of frequent debris-flow

deposition and reworking by river floods. Because

many rapids occur on regulated rivers, such as the

Colorado River in Grand Canyon, the pattern of

dam releases strongly affects the stability of rapids

(Graf, 1980; Kieffer, 1985; Melis and others, 1994).

Rapids have been interpreted as relics of past

periods of wetter climate (Graf, 1979) and as

landforms that formed prehistorically because of

the extreme size of streamflow floods necessary to

transport boulders out of tributary canyons

(Hamblin and Rigby, 1968). Rapids controlled by

debris flows, such as Lava Falls, may be more

unstable than other types of rapids because debris

flows transport very large boulders into the river

relatively frequently.

Using a unique collection of photography

(Appendix 1) and Quaternary dating techniques, we

demonstrate that Lava Falls is the most unstable

rapid in Grand Canyon. Six debris flows from

Prospect Canyon have substantially altered Lava

Falls Rapid in the 20th century. These debris flows

occurred in 1939, 1954, 1955, 1963, 1966, and

1995. We also identified at least six prehistoric and

late Holocene debris-flow surfaces. The largest

debris flow, which occurred in about 950 BC, is the

only plausible case of a debris flow having dammed

the Colorado River during the late Holocene.

Before closure of Glen Canyon Dam in 1963,

debris-flow deposits were removed (reworked)

within a couple of years by floods in the Colorado

River, whereas recently aggraded debris fans have

persisted longer. The reworking history of the

Figure 1. Maps of the study area. A. The Prospect Valley drainage basin and Grand Canyon National Park.
4 The Great Cataract

B. The Prospect Valley drainage basin.

Figure 1. Continued.
INTRODUCTION 5

Prospect Canyon debris fan illustrates how large

dams may be operated to simulate some of the

beneficial effects of large floods in unregulated,

bedrock-controlled rivers.

Purpose And Scope

The purpose of this report is to examine the

history of debris flows from Prospect Canyon and

to document their effect on Lava Falls Rapid. These

debris flows, which have occurred more frequently

in Prospect Canyon than in other tributaries of the

Colorado River in the 20th century, include what

probably is the largest debris flow in Grand Canyon

history. The detailed history of reworking of the

Prospect Canyon debris fan by the Colorado River

provides important information on how releases

from Glen Canyon Dam may rework aggraded

debris fans in Grand Canyon National Park. This

work was funded by the Glen Canyon

Environmental Studies Program (GCES) of the

Bureau of Reclamation. 

Acknowledgments 

A large number of people contributed to our

reconstruction of the geomorphic history of Lava

Falls Rapid. Photographs were obtained with the

help of archivists at the National Archives, the

Photographic Library of the U.S. Geological

Survey, Special Collections at the University of

Utah, the Bancroft Library of the University of

California at Berkeley, the Utah State Historical

Society, Special Collections at Northern Arizona

University, the Study Collection at Grand Canyon

National Park, the New York Public Library, and

the Huntington Library. We greatly appreciated the

loan of historical photographs taken by the original

photographers: Bryan Brown, Kent Frost, Barry

Goldwater, Don Harris, Les Jones, Martin Litton,

Gretchen Luepke, Tad Nichols, P.T. Reilly, and

Robert Sharp. John Cross II provided invaluable

information on changes to Lava Falls following the

1966 debris flow. We especially thank P.T. Reilly

of Sun City, Arizona, for allowing us to interpret his

invaluable collection of Lava Falls photography. 

We thank Nathan Carlton, Dar Crammond,

Steve Eudaley, Diane Grua, Jim Hasbargen, Don

Jensen, Shirlee Jensen, Terri Mareska, Marker

Marshall, Eric Martin, Tom Martin, Travis

McGrath, Elise Pendall, Dave Pratt, Renee

Rondeau, Rachel Sobrero, Jessica Stewart, Mike

Taylor, Judy Weiss, and Toni Yocum for their

valued field assistance. Alan Rigby, Jan Bowers,

Thure Cerling, Peter Griffiths, Richard Hereford,

Alex McCord, Mimi Murov, Bill Phillips, Betsy

Pierson, Kate Thompson, and Kelly Burke

contributed valuable technical expertise to this

project. Mia Hanson’s assistance contributed

greatly to all aspects of the project work. Betsy

Menand contributed drawings of stratigraphic

sections. We especially thank the many

photographers who helped in replicating historical

photographs of Lava Falls Rapid: Jane Bernard,

Tom Brownold, Dave Edwards, Mike Grijalva, Jim

Hasbargen, Liz Hymans, Dominic Oldershaw,

Steve Tharnstrom, and Raymond M. Turner. The

following professional river guides and support

personnel contributed their boating skills and

knowledge of the river: Jane Bernard, Steve

Bledsoe, Gary Bolton, Tom Brownold, Kelly

Burke, Michael Collier, Brian Dierker, Dave

Edwards, Chris Geneous, Jan Kempster, Kenton

Grua, Bob Grusey, Les Hibbert, Liz Hymans,

Cindy Krznarch, Archie McPhee, Sue Rhodes,

Glenn Rink, Dennis Silva, Drifter Smith, Kelly

Smith, Al Tinnes, and Meg Viera. We especially

thank Kenton Grua, Bob Grusy, and Kelly Smith

for their interpretations of changes in Lava Falls

Rapid. 

Special thanks to Frank Protiva, Chris Brode,

and Mark Gonzales of the GCES survey staff for

assistance in preparing the topographic base for

Plate I and for estimating areas, volumes, and crosssectional areas for the Prospect Canyon debris fan.

Hilary Mayes helped prepare the topographic map

of the Prospect Canyon debris fan, and Peter

Griffiths and Doug Wellington helped with the

analyses of precipitation. Steve Wiele of the U.S.

Geological Survey, Boulder, Colorado, supplied

travel times for flow in the Colorado River.

Dominic Oldershaw printed most of the repeat

photographs of Lava Falls Rapid, and Chuck

Sternberg drafted the complex figures. Finally, we

thank John Elliott, Richard Hereford, and Jack

Schmidt for critically reviewing the manuscript.

We give special thanks to Dave Wegner, the

program manager for GCES, for his unwavering

support of our research. 
6 The Great Cataract

Characteristics of Debris Flows in 

Grand Canyon

Debris flows are water-based slurries of poorlysorted sediments (Costa, 1984) that are a common

component of flash floods in Grand Canyon (Webb

and others, 1989; Melis and others, 1994). They

occur in many different environments ranging from

deserts (Blackwelder, 1928; Johnson and Rodine,

1984; Cooke and others, 1993) to montane forests

(Sharp and Nobles, 1953; Gallino and Pierson,

1985). Most debris flows occur as a result of

destabilization of landscapes by catastrophic

events, such as volcanic eruptions (Pierson, 1985;

Pierson and Scott, 1985), forest fires (Wohl and

Pearthree, 1991; Meyer and others, 1995), or poor

land-use practices (Costa, 1984; Johnson and

Rodine, 1984). Debris flows less commonly occur

in steep terrain, either as a result of rapid glacial

melting (Osterkamp and others, 1986) or severe

rainstorms (Blackwelder, 1928; Glancy, 1969;

Hammack, 1994) at frequencies usually lower than

one per decade.

Several classifications of sediment-bearing

flows have been proposed on the basis of water

content (Beverage and Culbertson, 1964; Pierson

and Costa, 1987), depositional characteristics of

sediments (Smith, 1986; Scott, 1988), and flow

dynamics (Postma, 1986). Two types of waterbased flow occur in Grand Canyon (Melis and

others, 1994): streamflow, which contains less than

40 percent sediment by volume, and debris flow,

which typically contains more than 80 percent

sediment by volume. Although hyperconcentratedflow deposits (Scott, 1988) are recognized in Grand

Canyon (Melis and others, 1994), we include this

poorly understood process with streamflow in our

discussion of flow processes from Prospect

Canyon.

In Grand Canyon, historic debris flows

typically have a recurrence interval of 10-50 yrs in

most tributaries (Melis and others, 1994; Webb,

1996). In contrast, debris flows in Warm Springs

Draw in Dinosaur National Monument, Colorado,

occur every 200-400 yrs (Hammack, 1994).

Howard and Dolan (1976) report that 10 percent of

the debris fans they studied (59 total) had

significant aggradation between 1965 and 1973. As

with any flood process, event frequency is

magnitude-dependent, and the presence of channels

affects the locus of deposition (Whipple and Dunne,

1992). Osterkamp and others (1986) found that the

frequency of “out of channel” debris flows was

lower than smaller, channelized events. Hereford

and others (1993) differentiated “fan-forming” and

“channelized” debris-flow deposits in eastern

Grand Canyon. Hereford and others (1996) grouped

fan-forming deposits into distinct periods of debrisflow activity in the late Holocene, although they do

not report recurrence intervals for specific debris

flows. Although most of the debris flows reported

by Melis and others (1994) are channelized, leaving

little depositional evidence on debris fans, others

were fan-forming debris flows that were the largest

in the recorded history of Grand Canyon. 

Grand Canyon debris flows contain 80-85

percent sediment by volume, and typical debris

flows contain from about 2-5 percent clay and 10-

30 percent sand. The chemistry and mineralogy of

silt and clay fraction is very important to sustained

flow (Hampton, 1975), and debris flows in Grand

Canyon are strongly related to the proximity to

terrestrial shale units in the bedrock (Griffiths,

1995). The clay particles in Grand Canyon debris

flows are typically illite, kaolinite, and quartz with

little smectite (Griffiths, 1995). Boulders that weigh

between 1 and 300 Mg are commonly transported to

the Colorado River (Melis and others, 1994). 

Intense rainfall and slope failures initiate debris

flows in Grand Canyon. Total rainfall associated

with debris flows has ranged from 27-355 mm

(Webb and others, 1989) with intensities up to or

exceeding 15 mm/hr (Melis and others, 1994;

Griffiths, 1995). Storms that cause debris flows

typically have recurrence intervals greater than 10

yrs, although daily totals may not be unusual

(Griffiths, 1995). Antecedent soil-moisture

conditions probably affect debris-flow initiation in

Grand Canyon because most historic debris flows

occurred during relatively wet periods. Slope

failures occur either in bedrock, typically terrestrial

shales and mudstones, or in colluvial wedges that

mantle steep shelves and ledges. The most common

initiation mechanism, called the “firehose effect”

by Johnson and Rodine (1984), consists of runoff

cascading from limestone cliffs onto colluvial

wedges, causing failure (Melis and others, 1994).

Debris flows in Grand Canyon tributaries usually
INTRODUCTION 7

travel 1 to 20 km from initiation points to the

Colorado River (Melis and others, 1994). 

Debris flows in straight, confined channels with

relatively low gradients move essentially as plug

flow (Johnson and Rodine, 1984) with little internal

shearing. In steeper channels, surging flow

propagates as waves. In both cases, considerable

turbulence and shearing occurs in bends and bedslope transitions. When unconfined, debris-flow

slurries deposit levees subparallel to the flow

direction. These levees control the location of

distributary channels and, in otherwise unconfined

areas, allow the slurry to flow over long distances.

When debris flows in Grand Canyon reach the

Colorado River, they usually decelerate quickly and

deposit a debris fan adjacent to or in the river. 

Debris-flow transport of very large boulders in

low-gradient channels is common throughout the

world (Rodine and Johnson, 1976; Johnson and

Rodine, 1984; Pierson and Costa, 1987). In Grand

Canyon tributaries, boulders with diameters greater

than 3 m are transported in typical debris flows.

Debris-flow transport of large boulders is facilitated

by three important factors in most Grand Canyon

tributaries: 1) the presence of massive limestone

and sandstone boulders that accumulate in source

areas; 2) the steep gradients of tributary channels;

and 3) the relatively short transport distances from

source areas to the Colorado River. Most of the

boulders that form rapids are transported to the river

in debris flows that consist of a single, shortduration pulse of mud and boulders followed by

alternating pulses of recessional streamflow and

smaller debris flows (Webb and others, 1988a;

Webb and others, 1989). In contrast, tributary

streamflow floods generally carry finer sediments

consisting mainly of sand with cobbles or small

boulders as the largest particles transported. 

Debris Flows and the Colorado River

Debris flows in Grand Canyon have several

important effects on the Colorado River that are

well illustrated at Lava Falls Rapid (fig. 2).

Deposition of a debris fan in the river creates a rapid

by constricting the width of the river and raising its

bed elevation (Howard and Dolan, 1981). Local

features, such as unusually large boulders or

arrangement of boulders on the bed, cause

spectacular hydraulic features that impede

navigation. Finally, boulders reworked from the

debris fan are redistributed in an orderly fashion

downstream, creating secondary rapids or riffles.

Figure 2. Map of Lava Falls and Lower Lava Rapids showing geomorphic features of the Colorado River. The

Prospect Canyon debris fan, shaped and reworked by the Colorado River, forms a rapid-pool-rapid configuration that is

typical of Grand Canyon rapids (Webb and others, 1989). The main rapid arises from the constriction and deposition of

boulders on the bed of the river. The pool downstream, which has twin eddies, is a recirculation zone (Schmidt and

Graf, 1990) that deposits sand bars at certain water levels. Lower Lava Rapid forms from cobbles and boulders that

have been eroded from the Prospect Canyon debris fan, transported through the pool, and deposited as the first of a

series of alternating debris bars below the rapid.
8 The Great Cataract

Repeated debris-flow deposition alters the

configuration of existing rapids and their

controlling debris fans (Webb and others, 1989).

This configuration typically forms recirculating

flow (eddies) downstream from debris fans that

facilitate deposition of sand bars (Howard and

Dolan, 1981; Schmidt, 1990; Schmidt and Graf,

1990; Schmidt and Rubin, 1995). The unregulated

Colorado River periodically widened constricted

rapids by eroding boulders and debris-flow matrix

from debris fans (Kieffer, 1985). Reworked debris

fans developed residual surfaces composed of

boulders lying near their original depositional sites

and sand deposited by the Colorado River (Howard

and Dolan, 1981). Because debris-fan constrictions

and individual boulders cannot be totally removed

by the regulated river, many rapids, including Lava

Falls, have become more severe during the last 30

years (Graf, 1980; Melis and others, 1994; Webb,

1996). 

During the reworking process, sediment eroded

from the debris fan, particularly cobbles, is

deposited on downstream debris bars. Debris bars

are either low-water islands or alternating bars and

have well sorted and imbricated gravel, cobbles, or

boulders. Debris bars commonly form secondary

rapids downstream of the larger, primary rapids at

tributary mouths. One such secondary rapid —

Lower Lava — is about 200 m downstream from

Lava Falls Rapid (fig. 2). Other secondary rapids

are common in Grand Canyon, particularly below

major rapids such as Hance, Granite, and Crystal (at

river miles 77, 93.5, and 98.3, respectively;

Stevens, 1990). 

METHODS

The Prospect Valley Drainage Basin

In remote northwestern Arizona, names

commonly are lacking for stream channels and

other drainage features. The drainage basin that

contributes to the Colorado River at Lava Falls

Rapid is one example. The valley leading to Lava

Falls (fig. 1b) was named Prospect Valley by

prospectors in need of a landmark in the late 19th

century (Granger, 1960; Brian, 1992); the

watercourse is unnamed. The Prospect Valley

drainage basin consists of 257 km2

 of forests,

grasslands, and desert scrub south of Grand Canyon

(fig. 1a) and is the ninth largest of 529 debris-flow

producing tributaries in Grand Canyon (Melis and

others, 1994). Although the highest point in the

drainage basin is 2,367 m, most of the upper part of

the drainage lies between 1,450 and 1,950 m. 

Prospect Valley drains northward and abruptly

changes into a small, steep lower basin informally

called Prospect Canyon (fig. 1b). The transition is a

325-m fall in a horizontal distance of approximately

250 m. The rocks exposed in Prospect Canyon

consist of Paleozoic sedimentary strata and

Quaternary basalts extended from local vents (fig.

3). The Toroweap Fault, which is downthrown to

the west, trends south across the Colorado River

and through Prospect Canyon (Billingsley and

Huntoon, 1983; Jackson, 1990a, 1990b). The

drainage of Prospect Valley and Prospect Canyon

formed along and to the west of the axis of this

fault. The upper Paleozoic strata of Grand Canyon

is exposed in the Aubrey Cliffs, which line the east

side of Prospect Valley and the Toroweap Fault

(fig. 3). 

Prospect Valley formed when Quaternary

basalt flows filled the ancestral Prospect Canyon

during the Pleistocene (Hamblin, 1994b). Basalt

flows, which occurred from 1.2 Ma to 140 ka,

formed dams across the Colorado River with lakes

that reportedly extended upstream to Moab, Utah

(Hamblin, 1990). Pleistocene lava flows and dams

were present at both Lava Falls and Whitmore

Wash (river mile 188). One lava flow that

originated near Whitmore Wash extended at least

137 km down the Colorado River (Hamblin,

1994b). Prospect Canyon was filled by a lava flow

that also produced the Prospect Dam (Hamblin,

1990, 1994b). Prospect Canyon occupies part of the

former canyon, which reached the Colorado River

upstream of the current mouth. The waterfall

separating Prospect Valley from Prospect Canyon

is the discontinuity between the two segments of the

drainage basin that causes the abrupt change in

fluvial process from streamflow in Prospect Valley

to debris flow in Prospect Canyon.

Debris flows in Prospect Canyon are generated

during floods in Prospect Valley that pour sufficient

quantities of water over the waterfall onto
METHODS 9

Figure 3. Geologic map of Prospect Canyon. This map is modified from Billingsley and Huntoon (1983) and Hamblin

(1994b); the original scale is 1:62,500.
10 The Great Cataract

unconsolidated colluvium (“the firehose effect”;

Johnson and Rodine, 1984). There is no evidence of

past debris flows on the floor of Prospect Valley;

stratigraphic evidence of debris flows begins at the

foot of the fall into Prospect Canyon. Although the

gradient of Prospect Valley (0.041) is relatively low

in comparison to other Grand Canyon tributaries,

the gradient through Prospect Canyon (0.315) is

steep enough to provide considerable potential

energy for debris flows. The largest sources of

colluvium are rockfall from a headwardly-eroding

cinder cone on the western rim and talus from the

steep slopes of Prospect Canyon (fig. 3). Both

sources provide abundant unconsolidated

colluvium that can be mobilized into debris flows.

Historical Photographs of Lava Falls 

Rapid

Repeat photography is the most important tool

we used to document 20th century debris flows in

Prospect Canyon. Numerous photographs and

movies have been made of Lava Falls Rapid,

beginning with stereoscopic views made by the

Powell Expedition in 1872 (Stephens and

Shoemaker, 1987). Every expedition on the

Colorado River had a photographer, and the

downstream view from Toroweap Overlook, which

towers 1,000 m above the river on the northwestern

rim of the canyon (fig. 1b), is frequently

photographed. We examined 232 historic views of

Lava Falls Rapid made between 1872 and 1984

(Appendix 1), which include 48 oblique aerial

views, 17 vertical aerial photographs, 129 views

taken at or near river level, and 38 views from

Toroweap Overlook. We replicated 121 of these

photographs from February 1990 to March 1995. 

The photographs of Lava Falls Rapid were not

taken uniformly in time. Photographic coverage is

sparse from 1872 through 1938, when commercial

river running began (Lavender, 1985). Photographs

are available for 18 dates from 1872 through 1939,

or 14 of 68 years; these photographs were taken

primarily by parties conducting scientific

exploration and surveys of the river. Because river

running was a luxury during World War II, only 12

photographs were available from the period 1940-

1947. Beginning in the 1950s, the photographic

record of Lava Falls expanded greatly as a new

generation of river-runners began running the

Colorado River. Photographs are available for 30

dates in the 1950s and 29 dates in the 1960s.

Although many photographs are taken every year of

Lava Falls during commercial river trips, few recent

photographs are commonly available. Vertical

aerial photography is available for several years

from 1980 through 1995, but vertical photographs

were only used to document the effects of the 1995

debris flow.

Mapping and Age Determinations for 

Debris-Flow Deposits

Between 1990 and 1994, we surveyed the

Prospect Canyon debris fan, produced a

topographic map, and mapped the surficial geology

(Plate I). Our map of the Prospect Canyon debris

fan (Plate 1) delineates geomorphic surfaces, not

deposits as depicted on other surficial geology

maps of parts of Grand Canyon (Hereford, 1996).

Some of the surfaces on the Prospect Canyon debris

fan have complex depositional histories that are

adequately expressed mappable deposits, in

particular surfaces that appear to be terraces with

snouts or push-out lobes of potentially different

ages on top. The geomorphic surface map also

allows delineation of colluvial and reworking

processes that could not be distinguished on a

deposit map.

We surveyed approximately 1,400 points

between 1991 and 1993 to develop our topographic

base map. These data were combined with 1:2,400

digital contour data calculated from 1990 aerial

photography. The 1990 data have a horizontal

accuracy of ±1.3 m and a vertical accuracy of ±0.25

m on flat slopes and ±1 m on steep slopes (F.

Protiva, GCES, written commun., 1996). Absolute

elevation and topographic control needed to

combine the two sets of data was obtained using a

global positioning system (GPS) with a horizontal

and vertical accuracy of ±0.4 m, owing to

discrepancies between the 1990 control points and

subsequent GPS measurements. Contours were

-----------------------------------------

1. Use of product names is for identification purposes only 

and does not imply endorsement by the U.S. Geological Survey 
METHODS 11

calculated using Terramodel for DOS software1.

Surficial geology was mapped on an enlarged aerial

photograph taken in October 1989 and transferred

to the base map. Differentiation of surfaces,

particularly those deposited in the 20th century, was

facilitated with historical photography (Appendix

1), which allowed differentiation and assignment of

small surfaces to specific events. 

In general, the Prospect Canyon debris fan is

composed of two distinct types of debris-flow

surfaces and several types of deposits related to

other processes. The oldest and highest deposits

form the overall shape of the debris fan, and

following Hereford and others (1993, 1996) and

Hereford (1996), we refer to these as fan-forming

debris-flow deposits. Subsequent erosion cut the

current channel through the debris fan, giving rise

to depositional sites for inset debris-flow deposits.

Inset deposits are similar to those termed

channelized debris flows by Hereford and others

(1993), although the magnitude of the Prospect

Canyon debris flows appears to be considerably

larger. Other surfaces arise from colluvial processes

and reworking by the tributary and (or) the

Colorado River.

The age of deposits on the Prospect Canyon

debris fan is inherently difficult to determine in the

absence of direct evidence such as historical

photography (Webb and others, 1989). Although

some researchers have 14C-dated abundant charcoal

in debris-flow deposits owing to forest fires (Meyer

and others, 1995) or proximity to archaeological

sites (Hereford and others, 1996), organic carbon is

rare in most debris-flow deposits in Grand Canyon.

We used age-dating techniques ranging from

correlation of soil morphology to analysis of

radiometric isotopes. Several of these techniques —

particularly 3

He and 14C analyses — yielded

absolute dates, whereas other techniques were used

to establish relative ages or to distinguish or

correlate discontinuous deposits.

Pedogenic Calcium Carbonate

Soils on the oldest surfaces are weakly

developed but contain pedogenic calcium

carbonate, which is reflective of the age of the soil

and the underlying deposit (Birkeland, 1984). The

greatest accumulation of CaCO3 in soils on the

Prospect Canyon debris fan is Stage I carbonate

morphology (see Machette, 1985, for a description

of carbonate stages) with a maximum accumulation

at about 0.50 m depth. In hot desert soils, this

amount of accumulation generally occurs in

surfaces deposited in the latest Pleistocene or

Holocene (table 2 in Machette, 1985). Hereford and

others (1996) also used soil carbonate to

differentiate the ages of debris flows in Grand

Canyon.

McFadden and Tinsley (1985) developed a

model of carbonate accumulation in desert soils.

Their model of soil-forming processes is not

completely appropriate given the semiarid and hot

conditions of the Prospect Canyon debris fan;

nonetheless, results indicate substantial carbonate

deposition in Holocene soils. In the case of a

semiarid, thermic climate, results indicate a

maximum carbonate accumulation at 0.25 to 0.70 m

depth for soils about 3,000 years old. Therefore,

debris-flow deposits on the Prospect Canyon debris

fan with Stage I carbonate accumulation are

probably of Holocene age.

3He

 The 3He age-dating procedure is reviewed in

Cerling and Craig (1994) and has been widely

applied to basalt flows with olivine phenocrysts

(Cerling and others, 1994). Use of the 3

He dating

techniques is appropriate for the Prospect Canyon

debris fan because isolated olivine phenocrysts or

xenoliths occur in two of the dam-forming basalt

flows in Prospect Canyon, particularly the Prospect

Dam flow that plugs Prospect Canyon (Hamblin,

1994b). Cosmic-ray bombardment of olivine at the

surface of basalt clasts produces 3

He, which is

retained within the dense crystalline structure of

olivine (Poreda and Cerling, 1992). 

The Prospect Canyon debris fan is an ideal

setting for estimating ages of deposits using 3

He.

Basalt clasts on the debris fan mostly are from the

Prospect Dam flows that form a near-vertical wall

at the head of the narrow Prospect Canyon (fig. 3).

Cosmogenically produced 3

He in the massive

basalt is minimal because the lava flows are

shielded by high cliffs in the narrow canyon. Slope

failures that cause debris flows produce unexposed

basalt boulders, and the short transport distance
12 The Great Cataract

between source and debris fan (1.6 km) suggests

little opportunity for long-term exposure of

boulders in the channel. Moreover, surface erosion

after deposition, which produces anomalously

young cosmogenic dates (Cerling and Craig, 1994),

is minimal on the Prospect Canyon debris fan

because of its coarse particle-size distribution.

Desert Plant Assemblages

Desert vegetation on the Prospect Canyon

debris fan, which is similar to the common

vegetation assemblages of the Mojave Desert

(Phillips and others, 1987), was used to estimate

relative ages of surfaces. Webb and others (1987,

1988b) and Bowers and others (1996) have shown

that the species composition of desert vegetation on

debris-flow deposits is related to the age of the

deposit. Long-lived species such as creosote bush

(Larrea tridentata), for example, do not become

established until several centuries after the surface

formed. The proportion of long-lived versus shortlived species increases consistently on

progressively older geomorphic surfaces (Webb

and others, 1988b; Bowers and others, 1996).

Clonal Rings of Creosote Bush

Creosote bush forms clonal rings (Vasek, 1980)

that are indicative of the age of the surface that the

plant is growing on. As a creosote bush ages, the

center of the root crown dies and the outer segment

of the root crown splits into genetically identical

clones. The ring that forms continues to expand

radially at the average rate of 0.66 mm yr-1 (Vasek,

1980). The diameter of a creosote bush clonal ring,

therefore, reflects the establishment date for the

plant and gives a minimum age for the surface on

which it is growing.

14C

We also tested and used radiocarbon (14C)

analyses to date debris flow-deposits on the

Prospect Canyon debris fan. We collected various

types of organic debris, including pieces of

driftwood and small twigs, from the top of several

debris-flow surfaces. The “best” samples appeared

to be fine-grained organic debris wrapped around or

pinned beneath cobbles and boulders in debris-flow

levees. No organic material was observed at depth

in the debris-flow deposits. The resulting

radiocarbon dates were converted to calendar ages

using computer routines (Stuiver and Becker, 1993;

Stuiver and Reimer, 1993).

14C analyses indicated a substantial residence

time for organic debris in the drainage (table 1).

Driftwood collected from the surfaces of historic

debris-flow levees was radiocarbon dated to

determine the association of organic material with

the date of the debris flow that transported it.

Driftwood on top of the 1939 deposit yielded a

calendar date range of AD 1327-1638 (table 1).

Three samples of driftwood and twigs on the 1955

deposit correspond to calendar age ranges of AD

1259-1438, 1410-1954, and 1488-1955,

respectively. Two samples of twigs from the

undifferentiated 1963/1966 deposits provided postbomb 14C activities that correspond to calendar

date ranges of AD 1963 or 1969 and 1962 or 1974,

respectively. Driftwood deposited by the 1993

flood and the 1995 debris flow had post-bomb 14C

activities that correspond to a calendar dates of AD

1959, 1961, or 1981 and 1958 or 1995, respectively

(table 1). 

The 14C analyses indicate that organic

materials are only rarely from Prospect Canyon.

This is not surprising, because Ferguson (1971)

found persistence of driftwood for as long as a

thousand years along the mainstem Colorado River.

Webb (1996) documented snags in Grand Canyon

that remained standing 400-500 yrs after the tree

died. Both the 1939 and 1955 debris flows

transported wood that was significantly older than

the known date of the transporting debris flow. 14C

ages on prehistoric debris flows may be as much as

700 yrs older than the event. Although post-bomb

radiocarbon dating has been shown to be relatively

reliable in past flood studies (Baker and others,

1985), our results confirm the unreliability of some

types of organic debris in post-bomb 14C analyses

(Ely and others, 1992). Because data on the most

recent debris flows are closer to the true age, we

speculate that several debris flows may be required

to flush most of the organic debris from the

drainage.
METHODS 13

Climatic and Hydrologic Data

No climate stations are in the Prospect Valley

drainage basin. Stations in the Grand Canyon

region (fig. 1) are from 40 to 100 km from the

center of the drainage (Appendix 2). Two of the

stations—Tuweep Ranger Station and Grand

Canyon National Park—have recording rain gages

and report hourly precipitation (e.g., U.S.

Department of Commerce, 1966). These stations

have mean annual precipitation that ranges from

216 to 646 mm; the average of the 8 stations is 334

mm. About 45 percent of precipitation in the Grand

Canyon region occurs in winter (November-March)

and 43 percent occurs in summer (July-October). 

Seasonal precipitation was standardized

following an existing technique (Hereford and

Webb, 1992) to examine the effects of antecedent

soil moisture on debris-flow initiation. We

identified the two seasons of summer (July -

September) and winter (November - March).

Values for winter were considered part of the

following year; for example, the standardized

seasonal precipitation for November 1995 through

March 1996 falls in 1996. For each climate station

(Appendix 2), we calculated the seasonal

standardized precipitation, Ps, by

Ps = ∑[(xi - μi)/σi ]/N, (1)

where xi = annual seasonal rainfall for climate

station i; μi

 = the mean and σi = the standard

deviation of seasonal rainfall for climate station i;

and N = the number of climate stations with data. 

We estimated the probability of daily

precipitation and storms for known or probable

dates of debris flows (Appendix 3). We considered

the duration of a storm to be the number of

consecutive days with rainfall irrespective of the

number of sources of precipitable moisture. We

used a modified Gringorten plotting position (U.S.

Water Resources Council, 1981):

p = ((m - 0.44)/(n + 0.12)) • d, (2)

where p = probability of the event, m = the ranking

of the event, n = the number of days in the record,

Table 1. Radiocarbon dates of organic material collected from debris-flow deposits at the mouth of

Prospect Canyon

Notes: All 14C analyses were performed by Geochron Laboratories. The raw dates are in years before 1950 (yrs BP), except for those labelled with

PMC (percent of modern carbon), which are post-1950. All raw values are ±1 standard deviation. Calendar age and 2σ range are calculated using

the calibration curves presented in Stuiver and Becker (1993) and incorporated in a computer program (Stuiver and Reimer, 1993). The range in

age for post-1950 14C ages is very small. n.a. -- not applicable.

Surface Sample number

Type of organic 

material

Radiocarbon date±

1 SD Calendar date (AD)

2σ range in date 

(AD)

tif GX-19925 wood 485±90 1434 1296-1640

(Prehistoric)

tig GX-19326 wood 460±75 1439 1327-1638

 (1939)

tih GX-19320 wood 365±90 1494, 1601, 1410-1954

(1955) 1616

GX-19324 twigs 190±95 1674, 1779, 1488-1955

1801, 1943,

1954

GX-19325 wood 635±80 1319, 1369, 1259-1438

1386

tii GX-19321 twigs 153.8±1.5 PMC 1963 or 1969 n.a.

(1963-66) GX-19322 twigs 141.1±1.1 PMC 1962 or 1974 n.a.

rwc GX-19323 twigs 127.7±1.3 PMC 1959, 1961, n.a.

 (1993) or 1981

tij GX-20788 twigs 117.5±1.0 PMC 1958, 1995 n.a.

 (1995)
14 The Great Cataract

and d = the number of days in the season. The

recurrence interval, R (yrs), 

R = 1/p. (3)

Streamflow data were obtained for two gaging

stations on the Colorado River (fig. 1): the

Colorado River near Grand Canyon, Arizona

(09402500; Garrett and Gellenbeck, 1991) and the

Colorado River above Diamond Creek near Peach

Springs, Arizona (09404200; unpublished data).

Flood frequency for the Colorado River near Grand

Canyon was estimated using the log-Pearson type

III distribution (U.S. Water Resources Council,

1981). Data from the gaging station above Diamond

Creek were used to estimate discharges in the

Colorado River immediately after the 1995 debris

flow using a travel time of 9 hrs (S. Wiele, written

commun., 1995).

Streamflow is not measured in either Prospect

Valley or Prospect Canyon. On the basis of regional

regression equations (Region 10; Thomas and

others, 1994), we estimated the streamflow flood

frequency for Prospect Valley. The estimated 2-yr

flood is only 6 m3

/s, but the estimated 50- and 100-

year floods are 500 and 800 m3

/s, respectively. The

actual long-recurrence interval discharges may be

less than the estimated discharges in Prospect

Canyon because of attenuation of flow through the

meandering, braided channel of Prospect Valley.

Also, the main channel and its tributaries are

dammed by six small stock tanks (fig. 1b), which

would reduce runoff reaching Prospect Canyon.

Characterization of Debris Flows and 

Debris Fans

Particle-size distributions for debris-flow

surfaces and fresh debris-flow deposits were

estimated using several techniques (Melis and

others, 1994). Point counts were made every 1-2 m,

depending on the size of particles, along a tape

stretched across the surface; the sizes of all particles

greater than 2 mm b-axis diameter were recorded.

In a test pit dug in the 1995 debris-flow deposit, we

recorded all particles greater than 16 mm collected

from a 1 m3

 volume. In most cases, a sample was

collected and dry sieved to determine the

distribution of smaller particles. Point count and

sieve data were combined by distributing the data

obtained by sieve analysis over the percentage of

particles less than 16 mm determined by point

counts. We use the standardized size classes for

sediment (Friedman and Sanders, 1978), except we

prefer the term gravel to pebbles for particles

between 2 and 64 mm diameter.

The source geologic units for particles was

evaluated for all point counts and simplified to the

types of basalt, sandstones from the Supai Group,

Redwall Limestone (which includes Temple Butte

Limestone), Muav Limestone, and undifferentiated

limestone, which includes Kaibab Limestone,

limestones from the Supai Group, and indistinct

particles from the Redwall, Temple Butte, and

Muav Limestones. All particles that could not be

classified by lithology are termed unknown source.

We estimated the weight of the largest boulders

on the debris fan and several debris-flow surfaces

(Appendix 4). An arbitrary number of boulders --

usually 10 -- that appeared to be the largest were

measured depending upon shape. For example, we

measured 3 dimensions for rectangular particles but

only 1 diameter for spherical particles. We then

calculated the volume of the particle and estimated

its weight using a density of 2,650 kg/m3

 for

limestone and sandstone and 2,700 kg/m3

 for basalt

(Appendix 4).

Discharge Estimates and Water Content 

We estimated the discharge of three Prospect

Canyon debris flows from depositional evidence

along or upstream of section A - A’ (Plate I). The

surface of a moving fluid typically rises on the

outside of the bend and drops on the inside of the

bend. The difference in flow elevation between the

inside and outside of bends, which is attributable to

centrifugal forces exerted on the fluid mass, is

termed “superelevation” (Apmann, 1973). The

mean velocity (Vs) is related to the difference

between the flow elevations on the outside and

inside of the bend by

Vs = (g • Rc •∆Hs/W) 0.5, (4)

where g = acceleration due to gravity (9.8 m/s2),

Rc = the radius of bend curvature along the

channel’s centerline (m), and ∆Hs = the elevation
METHODS 15

difference at the point of maximum superelevation

(m). We estimated discharges using

Q = A • Vs, (5)

where A = cross-sectional area in square meters.

Webb and others (1989) and Melis and others

(1994) report significant overestimation of

discharge using Vs and the area of the cross section

in the bend. To obtain A, the water surface

described by ∆Hs is assumed linear, although it is

likely non-linear (S-shaped). Therefore, we used

the cross-sectional flow area slightly downstream

from A - A’ where ∆Hs ≈ 0.

We estimated the water content of the less than

16 mm fraction by gradually adding water to a 5 kg

sample in a laboratory tray and observing its flow

properties (Melis and others, 1994). Samples from

debris-flow deposits typically exhibited debrisflow behavior over a 1-2 percent range in water

content, whereas samples from hyperconcentratedflow deposits did not exhibit debris-flow behavior

at any water content.

Unit stream power is estimated from

ω = γ • Q • S/T (6)

where ω = the stream power per unit width of

channel, γ = the unit weight of the flow, Q = the

discharge, S = the energy slope of the flow, and T =

the width of the flow. For a debris flow, the value of

γ is a function of water content and particle-size

distribution.

Surface Area and Volume 

The volume of sediment deposited by Holocene

and historic debris flows was estimated using a

combination of surveys, photographic rectification,

and slope projection. Some deposits, such as the

debris fan in 1993 and 1995, were surveyed and

areas and volumes were calculated from digitalelevation models. The debris fans of 1954, 1955,

1963, and 1964 are recorded in historical obliqueaerial photographs (figs. 21, 24). We used surveyed

control points and the Map and Image Processing

Software (MIPs)2

 to rectify oblique aerial and

ground photographs and measure depositional

areas. We estimated the accuracy of rectified

images to be about ±1-3 m horizontally, although

the accuracy varies according to the amount of

distortion in the photographs.

We used the topographic information in Plate I

to reconstruct the area and volume of deposits

before reworking by the Colorado River. For debris

fans with remnant deposits but no photographic

documentation, we projected the slope of the

deposit toward the Colorado River until its

projected elevation either intersected the watersurface profile of the river at a stage corresponding

to 140 m3

/s or touched the right bank. The

assumption of a linear slope is justified by the

evenness of the 1995 debris fan and other surfaces,

especially surface tfa (Plate I).

The thickness of each debris fan was difficult to

determine except for the 1995 debris flow (the

debris fan was surveyed in 1993). Thickness was

estimated using several techniques. In the case of

the 1939 deposit, its existing surface was projected

over the 1993 debris fan. Photographic evidence

was used to identify points on the 1939 debris fan

that were not eroded or buried by the debris flows

of 1954, 1955, 1963, and 1966. Also, boulders on

the 1872 debris fan that were covered by 20th

century debris flows, but not moved by subsequent

Colorado River floods, provided minimum

thicknesses for the deposits. We could not estimate

the accuracy of the estimated thicknesses.

Constrictions

Historic debris flows from Prospect Canyon

constricted the Colorado River substantially at Lava

Falls Rapid. Kieffer (1985, 1990) defined the

constriction ratio, Cr, as 

Cr = Wr / Wu (6), (7)

where Wr = the width of the rapid and Wu = the

channel width upstream of the rapid. According to

Kieffer (1990), the average Cr for Grand Canyon

rapids is 0.50. Similarly, Schmidt (1990) and

Schmidt and Graf (1990) report an expansion ratio,

which is the ratio of the downstream width to rapid

width, for 70 debris fans in eastern Grand Canyon.

----------------------------------------

2. Use of product names is for identification purposes only 

and does not imply endorsement by the U.S. Geological Survey 
16 The Great Cataract

Cr is not a totally satisfactory measure of

constrictions because Cr decreases as the amount of

constriction increases. Another measure of the

narrowing of rapids is the percent constriction, Cw,

which is

Cw = [1 - Wr(ave)• ( 1/Wu + 1/Wd )/2] • 100 (8)

where Wr(ave) = the average cross section of the

constricted channel in the rapid and Wd = the width

downstream of the rapid below the expansion zone.

For the maximum Cw, Wr(ave) is the narrowest

width of the rapid. For a rapid with equal widths

above and below the constriction, 

Cw = (1 - Cr) • 100. (9)

The widths upstream and downstream from

Lava Falls Rapid are not equal and change with

discharge of the Colorado River, in a manner

similar to other Grand Canyon debris fans

(Schmidt, 1990). 

Both Cr and Cw vary with stage through a stable

rapid. Because river banks typically are steep above

and below the rapid, and because the surface of the

debris fan typically has a relatively low slope angle,

Cw increases as the discharge decreases below the

point where a significant area of the debris fan is

exposed. In contrast, Cr decreases as the river

narrows. At Lava Falls Rapid, Cw increases

substantially below a discharge of about 150-200

m3

/s regardless of the recent depositional history.

Because of the uncertainty in image rectification

and change in constriction with stage, we rounded

Cr to the nearest 0.05 unit and Cw to the nearest 5

percent.

SURFICIAL GEOLOGY OF THE 

PROSPECT DEBRIS FAN

General Debris Fan Characteristics

The Prospect Canyon debris fan has a plan area

of 9.2 ha and a volume of 1.9 million m3

 above the

140 m3

/s stage of the Colorado River. Of debris

Table 2. Rankings of the area and volume of the largest debris fans of probable Holocene age on

the Colorado River in Grand Canyon National Park (T.S. Melis, unpublished data)

Notes: All values are for debris fans above about 140 m3/s discharge in the Colorado River and only include the area of debris-flow deposition.

Areas of sand-bar deposition, debris bars, and colluvial deposits are not included. Debris fans reported in Hereford and others (1996) in eastern

Grand Canyon are not included, with the exception of the Palisades Creek debris fan, which is of Late Holocene age. The Unkar and Comanche

debris fans, which would probably rank 2 and 3 in terms of area, may be mostly of Pleistocene age.

Debris-fan name River mile River side Area (ha) Area rank Volume (106 m3) Volume rank

Kwagunt 56.0 R 18.4 1 2.1 1

Prospect 179.3 L 9.2 2 1.9 2

Palisades 65.5 L 7.5 3 0.96 3

Basalt 69.6 R 7.3 4 0.48 6

Little Nankoweap 51.7 R 6.6 5 0.54 4

Soap Creek 11.2 R 6.1 6 0.52 5

Whitmore 188.1 R 4.9 7 0.21 10

Saddle 47.0 R 4.2 8 0.28 9

Fossil 125.0 L 3.9 9 0.31 7

Forster 122.7 L 3.5 10 0.29 8

Malgosa 57.5 R 2.9 11 0.19 11

75-Mile 75.5 L 2.5 12 0.11 18

Unnamed 189.7 L 2.4 13 0.18 13

Unnamed 49.6 L 2.4 14 0.14 15

Unnamed 49.8 R 2.4 15 0.18 12

...

Crystal 98.2 R 1.9 19 0.09 22
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 17

Figure 4. Particle-size distributions for debris flows in Prospect Canyon. A. Prehistoric and historic debris flows. 

B. Pulses of the 1995 debris flow

0

20

40

60

80

100

-12 -10 -8 -6 -4 -2 0 2 4 6

DEBRIS FLOW, 

FIRST PULSE

DEBRIS FLOW, 

MAIN PULSE

DEBRIS FLOW, 

MAIN PULSE 

SURFACE

RECESSIONAL 

DEBRIS-FLOW 

PULSE

RECESSIONAL

STREAMFLOW 

GRAVELS

Particle Size, in units φ

B.

0

20

40

60

80

100

950 BC (tfa)

AD 1434 (tif)

AD 1939 (tig)

AD 1963/1966 (tii)

AD 1995 (tij)

Cumulative Percent Finer

4,0961,024

256

64

16

4

1

0.250

0.0630.016

Particle Size, in mm

A.
18 The Great Cataract

fans of probable Holocene age in Grand Canyon,

the Prospect Canyon debris fan has the second

largest area and volume; only the Kwagunt Creek

debris fan, at river mile 56.0, is larger (table 2).

With an average thickness of 28 m, the Prospect

Canyon debris fan is the thickest debris fan of

probable Holocene age in Grand Canyon. The

second highest, the Palisades Creek debris fan

(Hereford and others, 1993, 1996), has an average

thickness of 13 m. In contrast, the Crystal Creek

debris fan, which controls the second most difficult

rapid in Grand Canyon (Nash, 1989; Stevens,

1990), ranks 19th and 22nd in area and volume

(table 2).

The surficial deposits on the Prospect Canyon

debris fan are poorly sorted mixtures of particles

that range in size from clay to boulders (fig. 4a). In

general, about 10 percent of the deposits from

Prospect Canyon debris flows are sand or finer;

typically only 1-2 percent of the particles are silt

and clay. Debris flows from Prospect Canyon

contain less fine-grained material than most debris

flows in Grand Canyon, which typically have 5-10

percent silt and clay and 20-30 percent sand (Melis

and others, 1994). The coarseness of the debrisflow deposits is caused by the basaltic source

material in Prospect Canyon. Much of the sand in

Prospect Canyon debris flows is either suspended

sediment eroded from Prospect Valley or ash from

tuffs or the cinder cone on the west side of Prospect

Canyon (fig. 3). The Hermit Shale, which

contributes clay to most debris flows in Grand

Canyon (Griffiths, 1995), is not exposed in

Prospect Canyon and only contribute sediment in

runoff from Prospect Valley. Clay particles in

Prospect Canyon debris flows are mostly illite and

kaolinite, and the mineral abundances are similar to

other Grand Canyon debris flows (table 3).

Between 40 and 63 percent of the clasts present

in debris flows from Prospect Canyon are

Pleistocene basalts (fig. 5). Typical Grand Canyon

debris-flow deposits are dominated by clasts from

the Redwall Limestone and other Paleozoic units

(Melis and others, 1994). The reason for the

abundance of basalt is the large amount of

Quaternary basalt in Prospect Canyon, especially in

the vicinity of the waterfall (fig. 3).

Fan-Forming Debris-Flow Surfaces

tfa

The highest surface on the Prospect Canyon

debris fan (figs. 6, 7) is composed of mid-Holocene,

fan-forming debris-flow deposits. The area of this

surface, the largest on the debris fan, is 4.38 ha.

Although individual lobes and snouts have surface

expressions on the surface, the surficial deposits

appear to be of uniform age. Internal drainage

channels that developed after the debris flow

sediments were deposited slightly dissect this

surface. The surface is about 15 m above the

channel of Prospect Canyon and 25 m above the

Colorado River at its maximum height (Plate I).

Three distinct strata appear in the vertical

exposures, each of which represents deposition by

an unknown number of debris flows (fig. 8). The

lower strata do not have preserved soils at their

Table 3. Mineralogy of the clay-size fraction of Prospect Canyon debris flows compared with other

Grand Canyon debris flows

Notes: Semi-quantitative clay mineralogy analyses were done by X-ray diffraction techniques and are accurate to ±20%.

* Statistics are for 12 historic debris flows in Grand Canyon (P.G. Griffiths and R.H. Webb, written commun., 1995).

Sample Location Illite

 PERCENT OF PARTICLES

 Kaolinite Smectite Quartz Carbonate Other

 Grand Canyon debris flows*

Mean 46 28 5 7 5 9

SD 15 10 12 4 5 13

 Prospect Canyon debris flows

Colluvium 36 24 2 13 21 4

AD 1939 21 28 12 8 18 13

AD 1995 34 38 4 7 15 2
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 19

tops, which suggests that deposition of the main

body of the debris fan occurred at a faster rate than

soil-forming processes.

Soils are weakly developed on tfa. Desert

pavement is not present, and cryptobiotic crusts are

common in the fine-grained soil between the

boulders. The soil has a thin and weakly developed

A horizon, stage I carbonate accumulations on

particles in the C horizons, and no cambic

development in the profile. The maximum

carbonate accumulation is at 0.50 m depth. Such

soil profile is indicative of a mid-Holocene age for

the surface. 3

He concentrations in olivine

phenocrysts in basalt on surface tfa indicate an age

of 2.9±0.6 ka (950 BC) (Cerling and others, 1995).

The Toroweap Fault crosses the Prospect Canyon

debris fan (fig. 3) without a surface rupture

(Jackson, 1990a, 1990b). The estimated age of the

most recent rupture in Prospect Valley is 3.1 ka

(Jackson, 1990a), which suggests that the 3

He age

is reasonable.

The deposit underlying surface tfa is very

poorly sorted with d50 = 0.35 m (fig. 4a) and

boulders up to 3 m in diameter; larger boulders up

to 5 m in diameter have fallen from the surface onto

lower surfaces. The larger particles, which are

subangular to rounded, consist of basalt (50

percent), limestones (25 percent), sandstones from

the Supai Group (4 percent), and other rock types

(21 percent) (fig. 5). The limestone and sandstone

clasts are highly weathered with slight to moderate

coatings of desert varnish, and the undersides of

basalt clasts have a slight orange coloration.

The desert plant assemblage on surface tfa is

dominated by creosote bush with 19 percent cover

(Bowers and others, 1996). Mormon tea (Ephedra

nevadensis) and Engelmann’s prickly pear

(Opuntia engelmannii) also contribute significant

Figure 5. Source materials transported by debris flows in Prospect Canyon.

0

10

20

30

40

50

60

70

Quaternary

Supai Group

Redwall

Muav

Undifferentiated

Unknown

950 BC Deposit

AD 1434 Deposit

AD 1939 Deposit

AD 1963/66 Deposit

AD 1995 Deposit

Percent of Particles

Basalt

Limestone

Limestone

Limestone

N=119

N=149

N=875

N=116

N=100
20 The Great Cataract

cover. Barrel cactus (Ferocactus cylindraceous) is

very prominent on this and the other fan-forming

debris-flow deposits. Creosote bush forms distinct

clonal ring-structures between 1.02 and 1.35 m in

diameter on this surface. Using Vasek’s (1980)

relation, the original plants were established

between AD 0 and 500.

tfb 

Surface tfb is a poorly defined swale of about

0.15 ha between units tfa and tfc on the

downstream side of the Prospect Canyon debris fan

(Plate I). The deposit appears to be the remnant of a

late Holocene debris flow that lapped onto and is

inset into surface tfa. Exposures of the deposit

underlying surface tfb show poorly-sorted

sediment; most of the largest particles are cobbles,

although some boulders are present. The vegetation

on surface tfb is similar to that on tfa, except that

Engelmann’s prickly pear forms dense stands at the

distal end of the surface.

tfc

This surface is inset against surfaces tfa and tfb

at about the same height (fig. 6). The deposit

consists of poorly-sorted sediments with boulders

up to 3 m in diameter. One or more beds of very

poorly sorted, subangular to rounded clasts of

basalt, limestone, and sandstone are present. Larger

clasts have Stage I calcium carbonate coatings.

Surface weathering of limestone boulders is less

intense on this surface than on tfa, possibly owing

to greater surficial erosion on the smaller tfc. The

deposit has a vertical exposure of approximately 8

Figure 6. Cross section showing the relative elevations and positions of fan-forming and inset debris-flow surfaces

along A - A’ (see Plate I).

510

515

520

525

530

535

540

545

550

0 20 40 60 80 100 120 140 160

1872 Cross Section

(approximate where buried)

1939 Deposition

1955 Deposition

Elevation (m)

Distance Along Cross Section A - A' (m)

tfc

tif

tib

A A'

tig tih

tig

tfa

rwfd

tc

rwc
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 21

m (fig. 6, Plate I). The desert plant assemblage on

this surface, which has an area of only 364 m2

, is

similar to that on surface tfa.

Inset Debris-Flow Surfaces

tia

 This triangular-shaped remnant of a debrisflow levee is the oldest of the inset debris-flow

deposits on the Prospect Canyon debris fan. The

surface, which has an area of 800 m2

, consists of

poorly-sorted deposits with occasional boulders

that have fallen from surface tfa. Unlike other inset

surfaces, large open areas with fine-grained

sediment characterize this surface; cryptobiotic

crusts are common. The larger particles are

subangular to rounded clasts of moderatelyvarnished basalt and moderately-weathered

limestone and sandstone. Most clasts of Redwall

Limestone show moderate pitting and some

sandstone clasts have disintegrated. 

The soil on surface tia has a weak A horizon

and Stage I carbonate on clasts similar to surface

tfa. Maximum carbonate development is at 0.50 m;

below a depth of 1.0 m, carbonate coatings on clasts

are very weak. Desert vegetation assemblage is

mostly creosote bush with scattered shrubs and

barrel cacti; the creosote bush clonal rings range in

diameter from 0.60 to 1.50 m, which corresponds to

an establishment age of 300 BC to AD 1100.

tib 

Figure 7. View of the Prospect Canyon debris fan on March 7, 1995 (S. Tharnstrom). Surface tfa dominates the left

center of the view. Deposition from the 1995 debris flow appears at right center adjacent to the rapid.
22 The Great Cataract

This surface, with an area of 0.28 ha, is the

largest of the inset surfaces of Prospect Canyon

(Plate I) and is inset against surface tfa (fig. 6). An

internal drainage channel bisects the surface into

upstream and downstream segments. The deposit is

inset against surface tfa; surfaces tic, tid, and tie are

push-out lobes on over surface tib. Sediment is

poorly sorted with boulders up to 3 m in diameter,

and larger boulders that fell from surface tfa lie on

the surface. Clasts are weathered and varnished

slightly less than similar clasts on surface tia and

some of these clasts appear to have faint percussion

marks. The soil has a weakly-developed A horizon

and Stage I carbonate that is similar to the soil

developed on tia. Creosote bush dominates the

desert plant assemblages, and most of the shrubs

appear to be in clonal rings that range between 0.10

and 0.60 m in diameter. These rings suggests that

the creosote bushes became established between

AD 1100 and 1800. Mormon tea and California

buckwheat (Eriogonum fasciculatum) are common

shrubs on this surface, and barrel cacti are

particularly large and numerous.

tic

 Surface tic is a large push-out lobe on surface

tib and occupies 314 m2

 near the apex of the

Prospect Canyon debris fan (Plate I). The

downslope side of the surface appears to be a relic

snout. The soil development and weathering of

clasts are similar to surfaces tia and tib. Several of

the largest boulders on this surface fell from vertical

exposures of surface tfa. Several creosote bushes

dominate the vegetation on this small surface and

form small clonal rings 0.30 to 0.50 m in diameter,

Figure 8. Stratigraphy of surface tfa on the Prospect Canyon debris fan. Dashed lines indicate the approximate contact

points between stratigraphic layers.
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 23

Figure 9. Stratigraphy of the 1939 debris-flow deposit (surface tig). A streamflow deposit of reddish

silty sand (thin hatched band) is an intermediate phase of the event, separating the initial pulse (1.5-

3.5 m) from a secondary pulse (0-1.2 m). The largest boulders at the bottom have b-axis diameters of

0.75-1.4 m.
24 The Great Cataract

which suggests they became established between

AD 1240 and 1540.

tid 

This small, arcuate surface is on a deposit that

overtops surface tic at the apex of the Prospect

Canyon debris fan. The morphology of surface tic

indicates the deposit is a push-out lobe. On its

upstream side, erosion by recent Prospect Canyon

debris flows — particularly the 1939 event — have

created a vertical exposure of about 4 m. Because of

poor sorting, lack of imbrication, and lack of soil

development, a stratigraphic break could not be

distinguished between surfaces tic and tid. Few

plants grow on this surface because it only has an

area of 190 m2

. Surfaces tic and tid may have been

deposited during the same debris flow, but we do

not have sufficient evidence to make this

conclusion.

tie

 This small (43 m2), lobate surface overlaps

surface tib. The soil properties, surface weathering,

and desert varnish on surface tie indicate an age

slightly younger than surface tib but perhaps of

similar age to tic and tid. The particle-size

distribution and source material for this surface are

similar to other inset debris-flow deposits. This

surface could be contemporaneous with surface tid,

but the two surfaces have no distinguishing

characteristics, other than their morphology as

push-out lobes, to determine their genetic link.

tif 

Debris-flow sediments form a prominent

surface along the right side of Prospect Canyon (fig.

6) and the downstream side of Lava Falls Rapid.

Driftwood collected from under cobbles on surface

tif yielded a radiocarbon age of 485±90 yrs BP,

which corresponds to a calendar date range of AD

Figure 10. Stratigraphy of the 1955 debris-flow deposits (surface tih). The 1955 debris-flow deposit (0.6-1.0 m thick,

tih) overlies the 1939 deposit (0.3-2.0 m thick). Stratigraphic breaks are faint, particularly between the 1939 deposit

(tig) and the underlying reworked channel deposits.
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 25

1296-1640 and a date of AD 1434 (table 1). The

deposit is poorly-sorted with d50 = 0.35 m (fig. 4a)

and boulders up to 2.7 m in b-axis diameter

(Appendix 4). Snouts and boulder-strewn levees are

prominent on this jumbled surface. About 49

percent of the clasts are basalt and 35 percent are

limestone (fig. 5). Boulders are lightly varnished,

and percussion marks remain prominent. 

The desert plant assemblage reflects the late

Holocene age of this surface, which has an area of

0.18 ha. Long-lived catclaw (Acacia greggii)

dominates the desert vegetation assemblage with

19.5 percent cover; Mormon tea has 11.5 percent

cover (Bowers and others, 1996) and young barrel

cacti are abundant. California buckwheat is

common around the margins of this deposit, and

creosote bush is rare.

tig

Deposits from the 1939 debris flow form

extensive surfaces that are inset against surface tfc

on channel left and surfaces tie and tif on channel

right of Prospect Canyon (fig. 6). Levee deposits on

both the left and right sides of Prospect Canyon

have a maximum thickness of about 4 m and an area

of 0.65 ha. Internal drainage between surfaces tig

and tib, tic, and tid (Plate I), which has mostly sand

and gravel at the surface, was a conduit for

recessional flow or dewatering of the 1939 deposit.

Deposits from the 1955 debris flow (surface tih)

Figure 11. Maps of Lava Falls Rapid. A. Based on a 1994 aerial photograph, taken at a discharge of 234 m3/s, this

map shows the modern configuration of Lava Falls Rapid, which was unchanged from 1973 to 1995. The Ledge Hole

evolved during the 1950s as boulders accumulated in the center of the rapid. The V Wave is initiated by the Meteor

Rock, which probably was deposited in September 1939, and by boulders near the right bank. The Big Wave forms by

flow over the Deflector Rock, which was rotated and moved downstream during the 1939 debris flow. The Right Run,

the most common route, is used at discharges less than 850 m3/s. The Left Run is only feasible at a discharge greater

than about 600 m3/s, whereas the Slot Run is possible from about 250-450 m3/s. The 1995 debris flow constricted the

rapid by 62 percent, increased the velocity of the Right Run, made the Left Run feasible at lower discharges, and eliminated the Big Wave at most water levels.
26 The Great Cataract

overlap the 1939 surface at several points along

both the right and left sides of Prospect Canyon.

Deposits on the left side of Prospect Canyon are 1-

2 m thick over an older surface, possibly tif.

Surface tig is poorly-sorted with d50 = 180 mm

(fig. 4a) and boulders with a b-axis diameter of 2.7

m (Appendix 4). Sixty-two percent of the clasts are

basalt (fig. 5), the highest amount of basalt of any of

the debris flows from Prospect Canyon. The 1939

deposit consists of at least two beds of debris-flow

strata separated by tributary streamflow deposits

(fig. 9). The stratigraphy represents several pulses

of debris flow during a single runoff event, which is

consistent with observations elsewhere in Grand

Canyon (Webb and others, 1989).

On the upstream surface on channel right,

boulders are deposited around mature catclaw trees,

and most long-lived perennial shrubs are present

downstream of boulder piles. The plants appear to

have been present when the 1939 debris flow

occurred, suggesting that the 1939 deposit is

relatively thin. Boulders on the surface show slight

weathering with prominent percussion marks.

Desert plants have colonized the surfaces; sweet

bebbia (Bebbia juncea), a short-lived species,

contributes 10.1 percent cover and 5 other shortlived species contribute significant cover (Bowers

B. The configuration of the rapid from 1872 to 1939 was reconstructed using 37 historical photographs; the water level

depicted in the map is about 280 m3/s. The Pyramid Rocks were prominent near the middle of what is now the entry to

the right run; they were exposed at discharges of less than about 500 m3/s. The Deflector Rock dominated the right

side of the rapid and forced flow to the left of the Black Rock. The left side of the rapid was a series of exposed or

shallowly submerged boulders, although a second tongue was present on the left side of the Pyramid Rocks.

Figure 11. Continued.
SURFICIAL GEOLOGY OF THE PROSPECT DEBRIS FAN 27

and others, 1996). The edges of this surface were

eroded during debris flows in 1955, 1963, and

1966.

tih

 Deposits from the 1955 debris flow are

preserved as eroded remnants that form small

surfaces along the right and left sides of Prospect

Canyon. The deposits appear superelevated along

the left side of Prospect Canyon at cross section A -

A’ (Plate I) and lap onto surface tig (fig. 6). The

deposits consist of poorly-sorted sediments

approximately 1-2 m thick inset against, and in

some cases, on top of the surface tig (fig. 10). Much

of the surface, which had an area of 800 m2

 when

we mapped it in 1993, is covered with sand and

gravel that appears to have been deposited during

the recessional flow of 1955. Desert vegetation on

surface tih consists of short-lived species such as

poreweed (Porophyllum gracile) and occasional

catclaw (Acacia greggii). Much of surface tih was

eroded during the 1995 debris flow.

tii 

Deposits of the 1963 and 1966 debris flows

were not differentiated on the Prospect Canyon

debris fan. The undifferentiated deposits form a

surface tii that has relatively fine-grained particlesize distribution, with d50 = 64 mm (fig. 4a). This

surface has poorly-sorted, subangular to rounded

clasts that are 39 percent basalt and 43 percent

Redwall Limestone (fig. 5). Desert plants

consisting of long-leaf brickellbush (Brickellia

longifolia) and other short-lived species colonized

these small surfaces, which had an area of 385 m2

when we mapped them in 1993. Surface tii was

almost totally removed during the 1995 debris flow.

tij 

The 1995 debris flow deposited a debris fan in

the Colorado River with an area of 0.56 ha. The

1995 debris flow did not create a distinct surface on

the Prospect Canyon debris fan and for that reason

is not included on Plate I. Most deposition was in or

adjacent to the channel of the Colorado River (fig.

11a), and the large volume of recessional

streamflow eroded most of the deposits left after the

initial debris-flow pulse. In addition, most of the

deposits of surfaces tih and tii were removed, and

the edge of surface tig was eroded in places by 1 m.

The aggraded debris fan (Plate I) had four distinct

areas of deposition and erosion that represent the

different phases of runoff. The first pulse of

sediment is beneath the main body of the debris fan;

its sedimentology and volume are unknown. After

this initial phase, a pulse of relatively fine-grained

(d50 = 50 mm) debris flow pushed out deposits on

the downstream side of the debris fan (fig. 4b). The

second, or main pulse of debris flow pushed

directly towards the center of Lava Falls Rapid.

These sediments, with d50 = 350 mm (test pit, fig.

4b) and boulders up to 1.6 m in b-axis diameter,

composed most of the aggraded debris fan. Particles

in the main pulse were 50 percent basalt and 21

percent Redwall Limestone (fig. 5). The recessional

pulse of debris flow was relatively fine-grained (d50

= 40 mm; fig. 4b). The recessional streamflowdeposited gravels and well-sorted sand (d50 = 10

mm; fig. 4b) in the channel and on the debris fan.

Colluvium and Steep Slopes

tc 

Colluvium and steep slopes compose 1.12 ha of

the Prospect Canyon debris fan. Unconsolidated

talus displaced from nearly vertical exposures of

surfaces tfa and tfc (fig. 6) forms steep slopes

within the incised channel of the Prospect Canyon

debris fan and along the distal edge of tfa and tfc

along the Colorado River. The deposits consists of

relatively well-sorted boulders, which are mostly

basalt clasts that have fallen from near-vertical

exposures. This deposit partially covers older, inset

deposits along the right side of Prospect Canyon; in

particular, surface tic has many boulders up to 5-6

m in diameter on its top that are part of surface tc.

Reworked Debris-Flow Deposits 
28 The Great Cataract

rwr

 This surface is mostly boulders that are the

largest clasts transported by Prospect Canyon

debris flows. These boulders were deposited in or

adjacent to the Colorado River and have not been

removed by subsequent river floods. Between 1939

and 1995, these floods had discharges up to 3,540

m3/s. This deposit has an area of 0.74 ha above the

140 m3

/s stage of the Colorado River along the edge

of the debris fan. The boulders are 90 percent

basalt, have d50 = 512 mm, and b-axis diameters up

to 3.0 m (Appendix 4). Pockets of river sand are

among the boulders. The above-water area of

surface rwr at the rapid was covered by the 1995

debris flow, except for some isolated boulders at the

downstream end of the rapid that are in the river but

not submerged at a discharge of 280 m3

/s.

rwc 

Debris-flow deposits in the channel of Prospect

Canyon are periodically reworked by tributary

floods. Surface rwc has an area of 0.9 ha, and the

underlying deposit consists of poorly-sorted

sediment ranging from sand to extremely large

boulders. Some of the boulders on surface rwc have

b-axis diameters of 7 m (Appendix 4). Boulders

larger than 3 m in diameter either remained

stationary or were rotated during the 1995 debris

flow. In places, small flood deposits of well-sorted

fine sand are present on the downstream side of

obstructions such as large boulders; deposition of

these flood deposits occurred during the 1993 flood

and during recessional flow following the 1995

debris flow.

rwfd 

Reworked debris-flow and streamflow deposits

are present in internal drainage channels that dissect

both debris fan-forming and inset deposits. The area

of this surface is 0.32 ha (Plate I). These channel

deposits are associated with infrequent runoff

generated within the Prospect Canyon debris fan.

Most of the deposit underlying this surface is sand

and gravel, but boulders are also present. Some of

these channels could have developed during

dewatering of the main debris-flow deposits and

(or) recessional flow after a debris flow.

HISTORICAL CHANGE IN LAVA 

FALLS RAPID

Navigation of Lava Falls Rapid in 1994

Traditionally, Lava Falls Rapid is scouted from

the left for discharges greater than about 700 m3

/s

and from the right for lower-water runs. On the left

side, the rapid is viewed from a point on surface tfa

called the Left Scout Point or on the debris fan (fig.

11b). On the right side, the Scout Rock commands

a full view of the rapid. The landmarks of Lava

Falls Rapid all have names that are familiar to

modern river guides. The most prominent features

of the rapid are the Ledge Hole, the V Wave, the

Big Wave, and the Black Rock (fig. 11b). 

Hydraulic features in bedrock rivers have been

classified by several researchers. Leopold (1969)

described 4 types of waves according to water

depth, cross section changes, and obstacles on the

bed. Kieffer (1985) described large waves as

hydraulic jumps, or energy conversions from

supercritical to subcritical flow. Kieffer (1987)

established a lexicon for hydraulic features that we

use to describe Lava Falls Rapid.

The Ledge Hole is in a class by itself as a

navigational hazard. The drop into the Ledge Hole

— the highest vertical fall in the rapid — is only

about 1.2 m. At typical water levels, it spans a

quarter of the entrance to the rapid, and water

appears to pour over a fall into a recirculating hole.

The linear shape of the Ledge Hole has led to

speculation that a ledge or basalt dike underlies the

hydraulic feature (Fradkin, 1984). At very low

discharge (less than 100 m3

/s), three large boulders

forming the Ledge Hole are exposed. At flood

stage, the Ledge Hole becomes a massive wave.

Before the 1995 debris flow, three runs were

possible, the Left Run, a run on the right side of the

Ledge Hole called the Slot Run, and the Right Run

(fig. 11b). Above about 600 m3

/s, the Left Run

opens adjacent to the left side of the Ledge Hole. At

lower discharges, boulders — particularly the

Domer Rock— make the left side hazardous. At

intermediate water levels (300-600 m3

/s), the Slot

Run is an option that requires a very precise entry.

In the era of dam regulation, flows are generally
HISTORICAL CHANGE IN LAVA FALLS RAPID 29

Figure 12. Downstream view of Prospect Canyon and Lava Falls Rapid from Toroweap Overlook (Stake 967). A. (April

16, 1872; J.K. Hillers, courtesy of the National Archives). Lava Falls Rapid as viewed by the Powell Expedition was

completely different from the rapid now familiar to thousands of whitewater enthusiasts. Using photogrammetric rectification, we found that the rapid began 85 m farther downstream on the left side, and the head of the rapid formed a line

perpendicular to flow. A prominent tongue of water entered the left side of the rapid, and its tail waves at the bottom

flowed directly downstream. At the discharge shown (approximately 300 m3/s), the area of the exposed debris fan is

about 0.2 ha.
30 The Great Cataract

B. (June 10, 1979; R.M. Turner). After a series of debris flows from 1939 through 1966, Lava Falls Rapid changed

substantially. At a discharge of about 340 m3/s in 1979, the head of the rapid slanted downstream to the right side, and

the main tongue was on the right side. The tail waves were much farther downstream and curved towards the left. Two

distinct levees in the mouth of Prospect Canyon were deposited between 1872 and 1966; the largest levee was

deposited by the 1939 debris flow, and the smaller, inset levee is remnant channel fill from the 1963 and 1966 debris

flows. The debris-flow deposits in the mouth of Prospect Canyon are overgrown with dense riparian vegetation.

Figure 12. Continued.
HISTORICAL CHANGE IN LAVA FALLS RAPID 31

C. (May 16, 1995; R.M. Turner). The debris flow of March 6, 1995, initially constricted the Colorado River by 62

percent, but reworking quickly widened the river and reduced the constriction, as shown here, to about 50 percent.

The constriction increased the drop through Lava Falls Rapid, accentuating its hydraulics, but some formerly

prominent waves, such as the Big Wave, were not present. The 1963 and 1966 debris-flow deposits were completely

eroded from the margins of Prospect Canyon. At this discharge of about 260 m3/s, the exposed area of the new debris

fan is 0.62 ha. 

Figure 12. Continued
32 The Great Cataract

low enough that the Ledge Hole can easily be

avoided on the Right Run. 

The Right Run at Lava Falls (fig. 11b) is used

over most of the range of dam releases but it is by

no means easy. The run consists of entering right

near the Entrance Rock, running a powerful lateral

wave, and aligning the boat for the V Wave, a

nearly symmetrical pair of reflex lateral waves that

meet in the middle of the Right Run. The V Wave

is generated from large boulders on the right shore

and a large boulder in the rapid known as the

Meteor Rock, which is submerged at most

discharges and constricts the right side of the

channel. The pair of lateral waves that form the V

Wave tend to alternatively fold one over the other,

adding an element of suspense as to which wave

will crash on a boat first. 

Once through the V Wave, boats pass through

waves that are some of the biggest in Grand

Canyon. On the bottom right side of the rapid (fig.

11b), the Black Rock, a large basalt boulder, diverts

current to the left; just upstream, a large wave

appropriately called the Big Wave typically crashes

upstream. Depending on the surges at this point, a

Figure 13. Downstream view of Lava Falls Rapid from the high surface on the left side (Stake 1510b). The channel of

Prospect Canyon, which enters the Colorado River from the left, appears in the center of the view. A. (February 27,

1890; R.B. Stanton, courtesy of the National Archives). The last vestiges of smoke from a large fire set by the Stanton

expedition are shown at left center. The left side of the rapid has flat, relatively slow-moving water adjacent to the

mouth of Prospect Canyon. The Deflector Rock is visible on the right side approximately 15 m upstream from the

Black Rock. Tail waves from the rapid are deflected away from the Black Rock and the main flow is down the center of

the rapid. The water level is 280-400 m3/s. 
HISTORICAL CHANGE IN LAVA FALLS RAPID 33

boat can run safely, be flipped by the Big Wave, or

be rafted onto the Black Rock. Downstream, the

tailwaves of the rapid dissipate into a pool.

Lower Lava Rapid is the next obstacle

downstream from Lava Falls (fig. 2). This

secondary rapid, formed from cobble and boulder

outwash from Prospect Canyon debris fan, consists

of a tight river bend against a vertical limestone

cliff. Although Lower Lava is not a noteworthy

rapid by itself, it forms a distinct hazard to

swimmers and overturned boats after an upset in

Lava Falls.

The Wide, Stable Rapid (1872-1939)

When analyzing early accounts of Lava Falls

Rapid, the level of the expedition’s whitewater

expertise, the size and type of boats used, the

context of the trip, and the discharge in the river are

important factors to consider. Advances in

technique and equipment have revolutionized river

B. (February 11, 1990; R.M. Turner). Debris flows from Prospect Canyon changed the rapid substantially in the 20th

century. The rapid now begins farther upstream. The 1939 debris flow deposited the prominent low surface at left

center. Deposition during the 1939 event forced flow through the rapid to the right; the Deflector Rock moved

downstream and was submerged at most water levels. The Black Rock deflects typical dam releases to the left.

Debris flows in 1954, 1955, 1963, and 1966 deposited sediment along the left side of the rapid, changing a relatively

quiescent area to whitewater choked with boulders.

Figure 13. Continued.
34 The Great Cataract

running, and impressions of the severity of rapids

such as Lava Falls have changed greatly during the

last three decades. Three rapids — Lava Falls,

Separation (mile 239.5) and Lava Cliff (mile 246.0)

— were serious challenges to early navigation in

western Grand Canyon; explorers typically

compared the severity of these rapids (Marston,

1976). When Hoover Dam was completed in 1935

on the lower Colorado River, the rising waters of

Lake Mead inundated Separation and Lava Cliff

rapids. 

John Wesley Powell, the first explorer of the

Colorado River in 1869, was impressed with the

lava flows over the rim and the severity of Lava

Falls Rapid. Powell considered the rapid to be an

artifact of the basalt dams that spanned the canyon.

In his diary, he wrote

Come to ... lava falls [the lava flows]. These

falls must have been very great at one time.

Lava comes down to high water mark — may

be lower — and 1500 ft [500 m] high on either

side. The canon was filled... The falls [Lava

Falls Rapid] now are among boulders some

distance below the ancient fall (Cooley, 1988,

p. 173).

Powell’s crew thought Lava Falls Rapid was

the closest thing to a waterfall that they had seen;

they portaged it on the left. Powell later commented

extensively on the difficulties his expedition

encountered downstream at Separation Rapid

(Powell, 1875), which led to the erroneous

impression that Separation Rapid was a more

formidable obstacle than Lava Falls (Marston,

1976). Powell’s photographers travelled overland

to make the first photographs of Lava Falls in April

1872 (figs. 12a and 15a; Fowler, 1972; Stephens

and Shoemaker, 1987).

In the winter of 1889-90, Robert Brewster

Stanton led the second complete expedition through

Grand Canyon (Smith and Crampton, 1987). The

Figure 14. Upstream view of Lava Falls Rapid from the left side (November 10, 1909; R. Cogswell, courtesy of the

Bancroft Library, University of California at Berkeley). Boulders that appear on the debris fan and in the river in this

view also appear in views taken by Hillers (1872) and Kolb (1912). Photographs taken in 1923 and 1927 show the

same boulders, which demonstrates the stability of the rapid in floods of at least 6,200 m3/s.
HISTORICAL CHANGE IN LAVA FALLS RAPID 35

goal of his expedition was to determine the

feasibility of a water-level railroad from Grand

Junction, Colorado, to Needles, California. He

documented the route with systematic photography

(Webb, 1996). On the afternoon of February 26,

1890, Stanton’s expedition arrived “at the head of

the great cataract formed by the lava dike” (Smith

and Crampton, 1987, p. 225). The following day,

the crew portaged the boats along the left side while

Stanton photographed the rapid (fig. 13a). In his

diary, Stanton remarked that Lava Cliff Rapid,

farther downstream and now inundated by Lake

Mead, was the most difficult whitewater he saw in

Grand Canyon, as well as the most difficult place

for constructing a railroad (Smith and Crampton,

1987). 

Following Stanton in 1896, George Flavell, a

skilled and confident outdoorsman, became the first

person to run all three of the big rapids in western

Grand Canyon (Carmony and Brown, 1987).

Regarding his historic run through Lava Falls

Rapid, Flavell briefly wrote: “A bad rapid [Lava

Falls] was run which put about eight inches of water

in the boat. It was pretty fresh, but we had to stand

it” (Carmony and Brown, 1987, p. 69). Although

Lava Falls was not very challenging, Flavell was

greatly impressed by Lava Cliff Rapid, which he

stated “was as dangerous as any on the whole river”

(Carmony and Brown, 1987, p. 70).

Two early 20th century expeditions yielded

important photographs and notes on Lava Falls. In

1909, Julius Stone led a trip through Grand Canyon

(Stone, 1932) with a photographer, Raymond

Cogswell, who took the best set of photographs of

the rapid made before the 1950s (e.g., fig. 14).

According to Cogswell’s diary,

It [Lava Falls Rapid] is impossible to run with

safety, a sheer cascade of 8 feet [2.6 m]. One

might scratch thru [sic]. We land stuff at head

and portage down about 150 yards [146 m] to

creek. Then one boat is lined and portaged

down. Heavy work ...The roar of the rapids and

the menace of it, the spice of danger, and the

thought of the comfort of a home fireside. We

are jolly around the camp fire and enjoy it. This

rapid full of rocks (R.A. Cogswell, unpublished diary, courtesy of the Huntington

Library).

Cogswell’s photographs support his description

of the length of the rapid, its fall, and the

extraordinary number of exposed rocks that made a

run implausible. 

Expeditions continued to photograph the rapid.

The Kolb brothers, famed photographers of Grand

Canyon, ran the river in the fall and winter of 1911-

1912 (Kolb, 1914); they photographed Lava Falls

after a portage in January 1912 (fig. 16a). Other

river runners photographed the rapid in 1927 and

1934 (fig. 17).

 In 1923, the U.S. Geological Survey mapped

potential dam sites along the Grand Canyon

(Birdseye, 1924; Freeman, 1924). During the

expedition, the water-surface fall of the Colorado

River was measured and “adjusted” in an

undescribed manner to a uniform discharge of 280

m3

/s. Upon arrival at Lava Falls, they photographed

the rapid at a discharge of 260 m3

/s but the

surveying crew did not measure its fall. In the

middle of the night, a flood of 3,200 m3

/s caused the

river to rise 6.4 m in 24 hrs (Claude H. Birdseye,

unpublished diary, 1923; Freeman, 1924), which

delayed the surveying for several days. The

expedition continued downstream on a discharge of

about 1,000 m3

/s, and the surveyed fall of 12 m was

measured over a 2.4-km distance that includes Lava

Falls Rapid, Lower Lava Rapid, and several riffles

downstream (Birdseye, 1924). The surveying

problem generated by the 1923 flood fueled the

current misconception of a 12-m fall through Lava

Falls (Webb, 1996).

Despite the increasing numbers of river runners

who faced the rapid, few actually ran it. In 1927,

Clyde Eddy claimed the rapid had a sheer drop of

“20 feet [6.1 m] or more” (Eddy, 1929). In

December 1928, Jack Harbin, who was searching

for two missing river runners, made the second

successful run of the rapid (Cook, 1987, p. 109).

According to an interview conducted by Otis

“Dock” Marston in March 1948, Harbin and his

passenger related troubles caused by the rocky

rapid. As his passenger watched from the right

shore, Harbin attempted to enter the rapid on the

right side but became snagged on a shallowly

submerged rock. Eventually, his boat washed off of

the rock but a wave in the rapid broke a board free
36 The Great Cataract

Figure 15. Upstream views of the head of Lava Falls Rapid from the right side (Stakes 2598 and 2662). A. (April 19,

1872; J. K. Hillers, courtesy of the National Archives). At a discharge of about 280 m3/s, the Entrance Rock appears in

the center; a very small eddy formed behind this rock in 1872. Many rocks and small pourovers are visible in the

center of the photograph. This photograph is also shown in Graf (1979).
HISTORICAL CHANGE IN LAVA FALLS RAPID 37

B. (March 8, 1993; L. Hymans). Even though flow in the Colorado River (310-340 m3/s) is only slightly higher, the

stage appears much higher than in 1890. A sizeable eddy forms behind the Entrance Rock. Now, rocks or pourovers

are not visible above 140 m3/s. Also, water flows around the right side of the large basalt boulder in the center of the

view.

Figure 15. Continued.
38 The Great Cataract

Figure 16. Downstream view of the right side of Lava Falls Rapid (Stake 2599). A. (January 1, 1912; E. Kolb,

courtesy of Northern Arizona University Special Collections). The discharge is 28-85 m3/s, and the Deflector Rock,

near the right shore, obstructs the view of the Black Rock. The sand bar at the left and rocks that are exposed in the

center of the rapid were typically covered at discharges greater than 280 m3/s. Several expeditions, including the Kolb

brothers, camped on the sand bar after portaging the rapid
HISTORICAL CHANGE IN LAVA FALLS RAPID 39

B. (March 9, 1993; L. Hymans). At a discharge of 310 m3/s, no rocks are exposed in the center of the rapid. The

Deflector Rock was rotated and moved downstream toward the center of the channel and usually is submerged.

The Big Wave, to the left of the Black Rock, forms from flow over the submerged boulder. A large part of the basalt

boulder in the right foreground has been eroded.

Figure 16. Continued.
40 The Great Cataract

Figure 17. View of Lava Falls Rapid from the left side (July 31, 1934; B. Fahrni, courtesy of the Utah State Historical

Society). The crew of the 1934 Frazier-Hatch expedition are lining boats to the head of the rapid to begin a portage.

The discharge was 60 m3/s, one of the lowest summer levels recorded in the 20th century, and the boulders that form

the rapid protrude from the water. The Pyramid Rocks are visible at the upper right of the view. Many of the boulders

shown in this view were dislodged or buried by the 1939 debris flow. After 1957, the Ledge Hole formed downstream

from the boulders visible in this 1934 view.
HISTORICAL CHANGE IN LAVA FALLS RAPID 41

Figure 18. Views of Lava Falls Rapid from the left side (Stake 2660a). A. (July 26, 1942; N. Wilson, courtesy of the

University of Utah Marriott Library). In this upstream view, the crew of the Nevills expedition is lining boats along the

left side of the rapid. The 1939 debris flow completely changed the flow pattern in the rapid. The Pyramid Rocks were

removed from the center of the rapid, and large holes punctuated the left side. The photographer stood on a debris fan

that was 3-4 m higher than the fan in 1993; the deposits of the 1939 debris flow were subsequently reworked by

Colorado River floods or Prospect Canyon debris flows. 
42 The Great Cataract

B. (March 10, 1993; S. Tharnstrom). At a discharge of 310-340 m3/s, the apparent stage is about 0.5 m lower than in

1942. Debris flows after 1942 deposited rocks in the center of the rapid that form the Ledge Hole, which does not

appear in the 1942 view. Riparian vegetation has invaded the debris fan.

Figure 18. Continued.
HISTORICAL CHANGE IN LAVA FALLS RAPID 43

from the boat, injuring Harbin’s spine. The injury

still caused Harbin problems in 1948.

The 1937 Carnegie - Cal Tech expedition chose

to portage the rapid that geologist Robert P. Sharp

described as “a short, nasty, rock rapids” (R.P.

Sharp, unpublished diary entry for November 15,

1937; courtesy of Special Collections, Cline

Library, Northern Arizona University). Buzz

Holmstrom made the third successful run in

October 1938 (Lavender, 1985). In July 1938, at a

discharge of 570 m3

/s, Norman Nevills chose to

line Lava Falls on the left, noting that a right run

was possible but risky (Cook, 1987). In July 1939,

Don Harris and Bert Loper rowed down the right

side at 232 m3

/s and “lightly touched a rock.”

Because Separation and Lava Cliff rapids were

inundated in the mid-1930s, Loper compared Lava

Falls with another rapid upstream:

...although it was a BAD rapid we did not find

it as bad as Hance Rapid ... Hance has a fall of

27 feet [8.8 m] and Lava has 23 feet [7.5 m]

fall ... in a little more than 1/2 of the distance

(Bert Loper, unpublished diary, courtesy of the

Huntington Library).

Only 5 boatmen are known to have rowed boats

through Lava Falls Rapid before 1940. For early

explorers, the decision whether to portage or run the

rapid was influenced largely by water level. The

total fall was variously described as 3 or 7 m;

photographs suggest the fall was closer to 3 m.

Large boulders, which typically were exposed or

shallowly submerged, created serious hazards for

wooden boats. At low water, no specific tongue was

present; instead, a line of drops perpendicular to the

channel occurred across the top of the rapid.

Potential runs appear in photographs on the right

and left sides, but the only documented runs were

on the right. Before 1939, tail waves in the rapid

appeared to follow a straight path to the bottom of

the rapid at all water levels. Lava Falls Rapid at that

time was short, in accord with the descriptions of

“falls” and “cascade.” At discharges below about

300 m3

/s, the rapid ended upstream from the Black

Rock (fig. 11a). 

Lava Falls Rapid had three significant features

that stand out in the pre-1939 photographs and

movies (fig. 11a). In similar views of the rapid

taken from Toroweap Point in 1872 and from an

aircraft in 1936, the rapid is wide and the lip of the

rapid slants downstream toward the left side. At

discharges less than about 500 m3

/s, a prominent

pile of boulders at the top left of the rapid marked

the beginning of the cascade. These boulders,

which collectively take on the appearance of a

pyramid at a distance, are referred to here as the

Pyramid Rocks. The boulders are not present in any

photographs taken after 1939 and are not the

boulders that form the current Ledge Hole. No

boulders are exposed at dam releases above 85

m3

/s at the former position of the Pyramid Rocks. 

Another significant feature of the pre-1939

rapid is a large basalt boulder located just upstream

of the Black Rock (fig. 11a). This boulder, which is

rectangular with exposed dimensions of 4 and 8 m,

can be distinguished in photographs taken from

Toroweap Overlook; it was submerged above a

discharge of 850 m3

/s. We named this boulder the

Deflector Rock because it forced water away from

the Black Rock and deflected the tail waves of the

rapid down the center of the channel at all water

levels. Flow immediately upstream from the Black

Rock was relatively calm because of the presence of

the Deflector Rock. Movies taken in 1938 and 1939

clearly show a mildly turbulent eddy upstream from

the Black Rock that contained driftwood. Now,

spectacular boat upsets occasionally occur near the

former position of the now-submerged Deflector

Rock.

Finally, large boulders dominated the surface of

the debris fan, particularly the part that was just

above water at 280 m3

/s. Several of these

distinctive boulders, with diameters of 3-4 m, are in

the same place in photographs taken in 1872 (fig.

15a), 1890 (fig. 13a), 1909 (fig. 14), 1912 (fig. 16a),

and 1934 (fig. 17). In addition to the lack of change

in the largest boulders between 1872 and 1909, no

smaller boulders were added to the debris fan and a

mature riparian thicket appears in the mouth of

Prospect Canyon. An 1872 view by William Bell of

the Wheeler Expedition (not shown) depicts a small

channel in Prospect Canyon where a broad debrisflow conduit appeared in 1995. A similar view,

taken by Maxon in 1936 (Appendix 1), shows the

same conditions in Prospect Canyon as in 1872.

The lack of change shown in photographs of the

debris fan indicates that no debris flows occurred

from 1872 through 1939. The small, inactive

channel in the 1872 view suggests that no debris
44 The Great Cataract

flows had occurred for a minimum of several

decades before 1872.

The wide, rocky rapid began farther

downstream on the left and was characterized by

relatively slow velocities before 1939. By

rectifying the 1872 photograph from Toroweap

Overlook (fig. 12a), we determined that the point

where whitewater began was about 85 m

downstream of the same point in 1994. Using the

movie shot during the Harris-Loper trip at a

discharge of 220 m3

/s, we estimated that the boats,

which were not rowed downstream, had a velocity

of about 2 m/s through the right run. Kieffer (1988)

estimated an average surface velocity of about 5 m/

s for the right side in 1985. 

The Period of Frequent Debris Flows 

(1939-1966)

The axiom that “debris flows beget debris

flows” (Webb, 1996) is well illustrated by what

happened in Prospect Canyon from 1939 through

1966. A major debris flow in a Grand Canyon

tributary increases the probability that more will

occur shortly thereafter. Debris flows commonly

destabilize sediment on colluvial wedges and in

channels that easily can be mobilized by later

floods. Despite the fact that severe storms required

for initiation are controlled by atmospheric

processes beyond the canyon rim, the presence of

readily mobilized sediment in the drainage basin

allows for frequent debris flows. For these reasons,

after a long hiatus, Prospect Canyon had a

succession of debris flows that altered the shape and

flow patterns of Lava Falls Rapid.

The first and largest historic debris flow in

Prospect Canyon occurred between July 15, 1939,

and August 17, 1940, as documented by a 1939

movie and 1940 photographs (Appendix 1; table 4).

A 1941 photograph by McKee (Appendix 1), from

a similar vantage point as Bell’s 1872 view of

Prospect Canyon, shows a widened channel full of

boulders at the head of the debris fan. The 1939

debris flow changed most of the features of the

Lava Falls Rapid. The Pyramid and Deflector rocks

were moved from their previous positions, the

pattern of flow through the rapid was significantly

altered, the debris fan was covered with at least 4 m

of poorly sorted sediment, and surface tig was

deposited on the left side of Prospect Canyon (figs.

6, 13). Using the historical photographs, we

identified the ten largest boulders that were

deposited by the 1939 debris flow; these boulders

were not subsequently removed from the debris fan.

The boulders, all of which are basalt, weighed

between 3 and 22 Mg (Appendix 4).

In August 1940, Norm Nevills scouted Lava

Falls Rapid as Barry Goldwater photographed the

rapid from the aggraded debris fan. The views taken

by Goldwater could not be exactly replicated in

1993 because his camera station was 4 m above the

water at the bottom of the rapid; nonetheless, his

photographs document a debris fan with a near

vertical and slightly reworked distal margin. In his

diary, Goldwater notes:

At Lava Falls most of the water flows over to

the right bank where it plunges over a fall that

Table 4. Dates and photographic evidence for debris flows and other floods in Prospect Canyon

* Dates of debris flows known accurately from eyewitness accounts or written reports.

† Probable dates of debris flows are based on rainfall records (Appendix 3)

Year Type of event

CONSTRAINING DATES OF 

PHOTOGRAPHIC EVIDENCE 

 Before event After event

Known date*

of event

Probable date† 

of event

1939 Debris flow Jul 15, 1939 Aug 17, 1940 n.d. Sep 6, 1939

1954 Debris flow Jun 14, 1954 Aug 29, 1954 July 24, 1954

1955 Debris flow Jul 20, 1955 Oct 1955 n.d. July 24, 1955

1956 Flood Apr 16, 1956 Sep 29, 1956 n.d. Jul 31, 1956

1963 Debris flow Aug 24, 1963 Sep 25, 1963 n.d. Sep 18, 1963

1966 Debris flow Jul 1966 Apr 30, 1967 Dec 6, 1966 n.d.

1993 Flood Aug 18, 1992 Mar 9, 1993 Feb 8, 1993 n.d.

1995 Debris flow Mar 5, 1995 Mar 6, 1995 Mar 6, 1995 n.d.
HISTORICAL CHANGE IN LAVA FALLS RAPID 45

seems to be twelve or fifteen feet [3.7-4.6 m]

high. That of course left only the left side to

consider as a possibility of running, and, if that

possibility didn’t show itself, we faced a portage (Goldwater, 1940, p. 93-94).

Nevills ran the rapid, but his diary does not

indicate whether he went right or left. When Nevills

returned in 1941, he decided that lining was the

“obvious” way to traverse the rapid (Heald, 1948).

From the reaction of the first boatman to face the

altered rapid, the 1939 debris flow did not change

the navigability of Lava Falls Rapid.

All sediment and boulders smaller than 1 m in

diameter deposited by the 1939 debris flow were

reworked by the Colorado River in 1941. The net

effect of the 1939 debris flow is evident in

photographs taken in 1941 and 1942 (figs. 18a and

19a). The upper left side of the rapid, which had

quiet water before 1939, became choked with

boulders. The V Wave formed (fig. 20), suggesting

the 1939 debris flow deposited the Meteor Rock in

the rapid. The Deflector Rock moved about 10 m

downstream (fig. 11) and is visible in aerial

photographs taken at a discharge of 79 m3

/s in the

1950s (e.g., fig. 21c). The Big Wave formed as

water flowed over the submerged Deflector Rock.

The tail waves, which previously flowed straight at

the bottom of the rapid, curved to the right after

1939. Owing to the increased constriction, flow on

the right side of the rapid increased and impinged

directly on the Black Rock. The rapid had fewer

boulders for river runners to avoid, but the

constriction created more powerful waves. With

only a few exceptions, rocks on the right exposed at

discharges of greater than 280 m3

/s were

unchanged, and no new boulders were added to the

right side.

We could not document the occurrence of any

debris flows in the 15-year span from 1939 to 1954.

Available photographs of Lava Falls during that

period (appendix 1) show no changes and levees

deposited in 1939 remained prominent and

unchanged.

A small debris flow occurred in Prospect

Canyon in 1954. Photographs constrain the date

between June 14 and August 29, 1954 (appendix 1;

table 4). Georgie White, a long-time river guide

(also known as Georgie Clark; Clark and

Newcomb, 1977), arrived at Lava Falls on July 24

and saw Prospect Canyon running at “full force.”

Large boulders were entering the river in a manner

White likened to a “big black lava flow” (Georgie

White, unpublished diary of 1954 river trip, Otis

Marston Collection, Huntington Library). White’s

observations are only the second eyewitness

account of a Grand Canyon debris flow, after

Robert Brewster Stanton’s account of a debris flow

in South Canyon (Smith, 1965; Smith and

Crampton, 1987; Webb, 1996). Péwé (1968)

reports a 1967 debris flow near Lees Ferry.

From low level aerial photography the 1954

debris flow appeared to aggrade the channel of

Prospect Canyon by about 1.5 m. This debris flow

again filled in the previously open left side of the

rapid with boulders, which forced the main flow

down the right side and constricted the Colorado

River to less than half its average width. The tail

waves curved farther to the left at the bottom of the

rapid. Of more significance to navigation of the

rapid, the Ledge Hole began to form. Despite this,

the 1954 debris flow decreased the overall severity

of Lava Falls (P.T. Reilly, oral commun., 1991).

Previous spaces between the large boulders were

filled in and the flow became less turbulent with

fewer holes scattered around the rapid. The volume

of sediment deposited by the 1954 event was the

smallest of the six historic debris flows in Prospect

Canyon. No depositional evidence remains of this

debris flow because of subsequent erosion.

Another debris flow, the second largest of this

century, occurred in 1955. The date of the debris

flow is constrained by historical photographs taken

on July 20 and in October 1955 (appendix 1; table

4). The 1955 debris flow removed or buried all the

1954 deposits and constricted the Colorado River

by two-thirds; the rapid was only 20 m wide in

aerial photographs taken during low water on

March 25, 1956 (fig. 21b). The deposits from this

debris flow constricted the channel of Prospect

Canyon by about two thirds and partly overtopped

the 1939 debris levee. Many large boulders were

deposited in the rapid; some of these now form the

Ledge Hole. 

The mid-1950s were low-water years on the

Colorado River. Several years of winter drought

affected much of the United States, particularly in

New Mexico, Colorado, and Texas (Thomas,

1962). Discharges in the Colorado River were low

and most of the 1955 debris-flow deposit remained

on the left side of Lava Falls (fig. 21b). Aerial
46 The Great Cataract

Figure 19. Downstream view of Lava Falls Rapid from the left side (Stake 2741). A. (July 27, 1941; W. Heald,

courtesy of the University of Utah Marriott Library). A crew member of the 1941 Nevills expedition rests on a rock

during a portage down the left shore. At a discharge of about 570 m3/s, water is piling up against the Black Rock,

shown in the upper right of the view. The pattern of the tail waves, which has changed in comparison to pre-1939

views, is the result of increased constriction of the Colorado River and displacement of the Deflector Rock during the

1939 debris flow. All foreground rocks in this photograph were deposited in 1939.
 47

B. (March 10, 1993; S. Tharnstrom). Flow in the Colorado River is about 230-280 m3/s. Although many boulders have

been transported in and out of the field of view by debris flows after 1941, several boulders in the midground remain.

The boulder that the crew member is resting on in 1941, as well as the boulders immediately upstream, are in the

same place. The foreground boulder, which has a b-axis diameter of 1.85 m, was probably deposited during the 1955

debris flow. The pattern of waves at the bottom of Lava Falls Rapid is similar in 1941 and 1993.

Figure 19. Continued.
48 The Great Cataract

Figure 20. Upstream view of Lava Falls from the left side (Stake 2046).A. (July 7, 1950; P. T. Reilly). A motor boat

passes through the V Wave, which appear in the left center of this upstream view. The large boulder at bottom center

is approximately 1.5 m in diameter. The discharge is 470 m3/s.
 49

B. (August 16, 1991; R.H. Webb). The discharge is slightly lower than in the 1950 photograph. Several boulders in the

foreground were not dislodged by debris flow and Colorado River floods after 1950. However, one foreground boulder

has been transported out of the field of view. A boulder of approximately the same dimensions was also transported

into the field of view (lower right). Both boulders probably were deposited by the 1955 debris flow.

Figure 20. Continued.
50 The Great Cataract

Figure 21. Oblique aerial views of Lava Falls Rapid and Prospect Canyon (P. T. Reilly). A. (March 21, 1955).

Although the discharge is 400 m3/s, the 1954 debris fan (at center) constricts the Colorado River by about 30 percent.

The channel of Prospect Canyon (upper left) is a meandering band of fresh debris-flow and fluvial deposits. The

tongue at the head of the rapid is on the right side, and the tail waves curve toward the left side at the base of the rapid.

B. (March 25, 1956). The 1955 debris flow, which formed a smaller debris fan than the 1939 debris flow, overtopped

the 1939 levee on the left side of Prospect Canyon in several places but did not destroy vegetation. The river was at

180 m3/s and rising when the photograph was taken; the fan extended farther into the river.

A

B.
 51

C. (September 29, 1956). The 1956 flood deposit is the fresh-looking material on the upper end of the debris fan. At a

discharge of 80 m3/s, the displaced Deflector Rock is visible upstream from the Black Rock. The view documents the

extent of reworking of the 1955 debris flow by the 1956 flood in the Colorado River, which peaked at 1,890 m3/s. The

reworked zone appears as a coarsening of particle size on the distal end of the debris fan. D. (May 4, 1957). Remnants

of the 1955 debris-flow and 1956 flood deposits were eroding under the rising waters of the Colorado River (520 m3/s).

The contact of the 1956 deposits and the 1955 debris flow deposits form a dark band penetrating the center of the fan.

Figure 21. Continued.

C.

D.
52 The Great Cataract

E. (April 20, 1958). The Colorado River peaked at 3,540 m3/s in 1957, and the flood removed most of the 1955 and

1956 debris-fan deposits. Small pockets of 1955 levee deposits were still present in April 1958. The Ledge Hole,

which formed after the 1957 flood reworked the 1955 deposit, appears in the center of the rapid, which had a

discharge of 1,870 m3/s. F. (October 4, 1958). The Colorado River peaked at 3,030 m3/s in 1958, but flow was only

190 m3/s when Reilly took his photograph. Only the largest boulders deposited in 1939 and 1955 remained on the

debris fan. The Ledge Hole is distinct at the top center of the rapid, and the Big Wave is present in front of the Black

Rock.

Figure 21. Continued.

E.

F.
QUIESCENCE (1966-1995) 53

photographs (fig. 21c) reveal that a flash flood

occurred between April 16 and September 29,

1956. With its small deposition, the 1956 flood had

little effect on Lava Falls Rapid. 

The drought ended in the winter of 1956-1957.

The 1957 Colorado River flood peaked at 3,540 m3

/

s on June 13. All evidence of the 1954 debris fan

and the 1956 flood deposit, as well as most of the

1955 debris fan, was removed (figs. 21a, 21e, and

21f). The flood entrained boulders and other

particles smaller than about 1.5 m in diameter from

the Prospect Canyon debris fan and transported

them downstream. When the flood receded, only

the largest boulders persisted on the left side of the

rapid (fig. 22a). The debris fan, which had

withstood two years of smaller floods during the

drought years, had been reworked to a

configuration similar to one that existed before

1954. The large boulders in the middle of the rapid

were rearranged to form the Ledge Hole. 

After the 1954 and 1955 debris flows, there

were no debris flows in Prospect Canyon for 9

years. With closure of Glen Canyon Dam in March

1963, spring floods in the Colorado River were

mostly eliminated and peak discharges were

reduced. Debris flows continued to occur in

Prospect Canyon, the next one between August 24

and September 25, 1963 (appendix 1; table 4). The

1963 debris flow constricted the Colorado River by

about two thirds (fig. 23a). The 1963 debris flow

had the finest-grained surface deposit of any

historic debris flow from Prospect Canyon. Despite

the presence of relatively mobile material adjacent

to the rapid, most of the deposit was unchanged by

low dam releases from 1963 to 1965 (figs. 23b, 24).

The 1963 debris fan persisted until a high dam

release in June 1965. The debris fan that appears in

1965 aerial photographs is not substantively

different from the debris fan shown in photographs

taken just before 1963, suggesting that few new

boulders were added to the rapid by the 1963 debris

flow.

Photographs taken in July 1966 and April 1967

document the occurrence of a small debris flow in

Prospect Canyon (appendix 1; table 4; figs. 25a,

26a). The deposition was first observed in March

1967 by the first river runner that year (John Cross

II, written commun., 1967). An unusually intense

winter storm in December 1966, which caused

debris flows in Crystal Creek (mile 98.3), Lava

Canyon (mile 65.5; Cooley and others, 1977; Webb

and others, 1989) and elsewhere along the river, is

the likely cause for the debris flow. An August 1967

photograph (fig. 26a) shows the extent of the

aggraded debris fan. In addition to the poorly-sorted

debris fan, small boulders were deposited on the left

side of Lava Falls Rapid, reportedly closing off a

previously used run (John Cross II, written

commun., 1967). Most of the 1966 debris fan was

removed by relatively small dam releases in 1972

and a small flood in the Little Colorado River in

1973.

QUIESCENCE (1966-1995)

Only a few small streamflow floods occurred in

Prospect Canyon between 1966 and 1995, the

period when most river runners navigated Lava

Falls. During February 1993, a streamflow flood in

Prospect Canyon deposited a small volume of

cobbles on the debris fan. Although some scouring

occurred in the bed of Prospect Canyon, Lava Falls

Rapid was unchanged.

The 1995 Debris Flow

Lava Falls Rapid changed dramatically in the

early morning hours of March 6, 1995, when a

debris flow initiated in Prospect Canyon aggraded

the debris fan and constricted the rapid (Webb and

Melis, 1995). The authors were camped at the rapid

during the debris flow and observed some of its

effects immediately afterward. The storm also

caused flooding in eastern Grand Canyon (Rihs,

1995) similar to the 1966 flood (Cooley and others,

1977). Of the six historic debris flows in Prospect

Canyon, the 1995 debris flow was unique: it entered

the Colorado River at a higher river discharge (490

m3

/s) than during the previous five.

 On March 5, steady rainfall began at midnight and

continued through the day at Lava Falls Rapid. The

storm culminated in steady heavy rainfall that

began about 6 PM and continued until after midnight on March 6. Although daily precipitation was

not excessive at Peach Springs and Seligman, precipitation was heavy at Grand Canyon and Bright

Angel Ranger Station (appendix 3); a total of 43

mm fell during the early morning hours of March 6
54 The Great Cataract

Figure 22. Downstream view of Prospect Canyon debris fan and lower Lava Falls Rapid (Stake 2002). A. (July 20,

1958; G. Staveley, courtesy of the Huntington Library). The discharge is 220 m3/s, and the boulders on the debris fan

are the net result of the 1939, 1954, and 1955 debris flows and reworking by the Colorado River. The 1957 flood in the

Colorado River reworked the deposits of the 1954 and 1955 debris flows
QUIESCENCE (1966-1995) 55

B. (February 20, 1991; D. Edwards). Although the stage was about 0.5 m higher than in the 1958 view, 6-10 boulders

with diameters of 1 m or larger remain. Most of the foreground debris fan was buried by debris flows in 1963 and 1966.

The debris fan decreased in size slightly between 1958 and 1991 although the size of boulders in the reworked zone

appears unchanged.

Figure 22. Continued.
56 The Great Cataract

 Figure 23. Upstream view of Lava Falls Rapid from the right side (Stakes 2005 and 3050). A. (September 25, 1963;

photographer unknown, courtesy of the Huntington Library). This photograph was taken a short time after the

September 1963 debris flow and shows its maximum constriction at a discharge of about 56 m3/s. Boulders on the

distal margin of the debris fan and in the center of the view created the Ledge Hole at higher discharges. The Meteor

Rock, which creates the V Wave, appears on the right side of the view. The view shows the relatively fine-grained

particle size that is characteristic of the 1963 deposits.
 QUIESCENCE (1966-1995) 57

B. (February 21, 1965; J. Visbak, courtesy of the Huntington Library). This view, which is not a match of A, shows

Lava Falls at a discharge of about 198 m3/s. Dam releases up to 550 m3/s eroded the distal edge of the debris-flow

deposit from 1963 through early 1965. Unlike the 1939 and 1955 debris flows, photographic evidence suggests that

few boulders greater than 1 m in diameter were transported in the 1963 debris flow. As a result, most boulders in the

1963 debris flow fan were entrained during high dam releases in May-June, 1965.

Figure 23. Continued.
58 The Great Cataract

C. (March 10, 1993; L. Hymans). The discharge was about 350 m3/s in this match of the 1963 view. Reworking of the

1963 and 1966 debris-flow deposits was complete by that time, leaving little evidence of the former constriction. The

boulders forming the Ledge Hole are submerged in this view but their hydraulic effect is clearly visible in the center of

the view. The Meteor Rock was also covered, but the noteworthy hole that forms downstream of the Meteor Rock is

visible.

Figure 23. Continued.
 QUIESCENCE (1966-1995) 59

at Tuweep Ranger Station. Between 1:00and 1:30

AM, we heard a roaring sound 3-5 min long from

Prospect Canyon; part of the noise sounded like

rockfall. 

Throughout the following morning, a waterfall

in two distinct cascades over a 325-m cliff jetted

what we estimated to be 10-30 m3

/s of sedimentladen streamflow into the upper part of Prospect

Canyon (fig. 27). A later inspection of the channel

above the waterfall suggested that gravel and small

cobbles were the largest particles transported over

the waterfall. Runoff from Prospect Canyon lasted

18-20 hrs and stopped after dark on March 6. 

At 6:00 AM on March 6, the new debris fan at

Lava Falls Rapid extended 32-50 m into the

Colorado River, which had a discharge of 540

m3

/s. Deposition reached the left edge of the Ledge

Hole, and the debris fan covered about 200 m of the

left side the rapid (fig. 11b). The fan sloped

continuously into the river with no cutbank on its

margin or a break in slope. As the morning

progressed, the distal margin of the debris fan was

cut laterally by 7-8 m, leaving a 2.6 m high cutbank

on the left side of the rapid. Throughout the

morning, large sections of the new fan were

undercut and fell into the rapid. Recessional flow in

Prospect Canyon cut two channels through the

debris fan, further reducing its size. Tributary

floodwater entering on the left side of the rapid

contributed to the failures of the distal margin. By

about 1:00 PM on March 6, the debris fan had

stabilized (fig. 28b).

Flow in the river was confined to a channel

about 50 percent of its previous width, which

caused distinct changes to the hydraulics of Lava

Falls Rapid. The water-surface profile (fig. 29a)

initially was greatly influenced by large waves on

Figure 24. Oblique aerial view of Lava Falls Rapid and Prospect Canyon (May 20, 1964; P. T. Reilly). At a discharge

of 37 m3/s, the 1963 debris-flow deposit constricts the river by about 60 percent; the debris fan initially extended nearly

to the middle of the Ledge Hole. This view shows the reworking by a peak discharge of 540 m3/s. The Meteor Rock is

visible in the center of the rapid, and the typically submerged Deflector Rock divided the tail waves.
60 The Great Cataract

Figure 25. View from the left side across the top of Lava Falls Rapid (Stake 2739). A. (April 30, 1967; D. Harris). The

crew of this 1967 trip stands on the 1966 debris-flow deposit while scouting routes through the rapid, which had a

discharge of about 280 m3/s. The largest visible boulder in the 1966 deposit is less than 1 m in diameter.
 QUIESCENCE (1966-1995) 61

B. (March 8, 1993; R.H. Webb). The discharge was slightly lower than that in 1967. Most of the 1966 debris-flow

deposit was removed in April 1973; the 1983 flood (2,720 m3/s) caused few changes in Lava Falls. The largest

boulders, which controlled the major waves and holes in the rapid, were unchanged.

Figure 25. Continued.
62 The Great Cataract

Figure 26. Downstream view of Lava Falls Rapid from the high surface on the left side (Stake 2964b). A. (August

1967; G. Luepke). At a discharge of about 340 m3/s, the 1966 debris flow constricts the Colorado River by about 36

percent. According to the first river runner to view the rapid, the only effect of this debris flow was to eliminate the Left

Run. A typical dam release and a small flood in the Little Colorado River on April 19, 1973, combined to form a peak

discharge of 1,080 m3/s, which removed the 1966 deposit.
 QUIESCENCE (1966-1995) 63

B. (March 6, 1995; R.H. Webb). The deposit of the March 1995 debris flow was much larger than that of the 1966

event, but the shape is remarkably similar. The amount of sediment deposited in 1995 is about half the amount

deposited during the 1955 debris flow.

Figure 26. Continued.
64 The Great Cataract

Figure 27. (March 6, 1995; S. Tharnstrom). The firehose effect created by streamflow falling over a 325-m fall at the

head of Prospect Canyon on March 6, 1995
 QUIESCENCE (1966-1995) 65

Figure 28. Vertical aerial photographs of Lava Falls Rapid. A. (June 1, 1994). Before the 1995 debris flow, the rapid

had a constriction of 28 percent at a discharge of 230 m3/s. B. (May 30, 1995). After the 1995 debris flow, Lava Falls

Rapid had a 50 percent constriction at a discharge of 260 m3/s. The debris fan did not change significantly between

March 8 and May 30.

A.

B.
66 The Great Cataract

the right side and the increase of more than 1 m in

the stage-discharge relation at the head of the rapid.

Flow velocity through the rapid, particularly in the

Right Run (fig. 11b), had increased. At a discharge

of 312 m3

/s, we measured an average surface

velocity of 4.3 m/s in the left run, whereas Kieffer

(1988) reported velocities of 3.3 m/s at a similar

discharge. The Ledge Hole, which had a new rock

lodged on its left edge, had a different shape, a

sharper drop, and a stronger wave than before. The

right lateral of the V Wave was much stronger than

the left lateral. The Big Wave initially was very

large but disappeared by the end of the day. A large,

continuously breaking wave rolled off of the Black

Rock, and large whirlpools formed to the right of

and behind the Black Rock. Cobbles and boulders

were heard rolling along the bottom above the

sound of the rapid.

Downstream from Lava Falls Rapid, eddies on

both sides of the river were replaced by fast-moving

water over a long and narrow debris bar (fig. 28b).

The tail waves, which had previously veered to the

left, now moved straight downstream (fig. 28). A

secondary rapid temporarily formed adjacent to the

Warm Springs (figs. 2, 3), but its 1-m waves

subsided to about 0.25 m as the day progressed. We

interpreted the secondary riffle as flow around and

over a new debris bar where the pool used to be; the

size of the riffle probably changed as a gravel/

cobble bar migrated downstream into Lower Lava

Rapid. 

506

508

510

512

514

0 100 200 300 400 500 600

3/12/94 340

3/08/95 400

Distance (m)

Date

Discharge 

(m3

/s)

B. Change in Water-Surface 

Elevations, 1994-1995

506

508

510

512

514

3/6/95 530

3/6/95 522

3/7/95 398

3/8/95 400

Water Surface Elevation (m)

Date

Discharge 

(m3

/s)

A. Water-Surface Elevations, March 6-8, 1995

Figure 29. Longitudinal profile of the water-surface fall on the right side of Lava Falls Rapid. A. March 6-8, 1995.

B. Comparison of water-surface profiles in March 1994 and March 1995.
 QUIESCENCE (1966-1995) 67

Figure 30. Upstream view of Lava Falls Rapid from the lower right (Stake 2962). A. (1973; M. Litton). At a discharge of

about 370 m3/s, Lava Falls appears to be continuous, frothy whitewater. For safety purposes, river runners commonly

floated in the foreground eddy until all boats were through the rapid. Dense riparian vegetation grew next to river level on

the debris fan at right. The Black Rock is prominent at left center.
68 The Great Cataract

B. (March 5, 1995; S. Tharnstrom). More rocks were exposed on the debris fan in this view, which shows about 310

m3/s in the rapid. There is no change between 1972 and 1995 except that riparian vegetation was scoured away

during Colorado River floods in 1983 and 1993.

Figure 30. Continued.
 QUIESCENCE (1966-1995) 69

C. (March 6, 1995; S. Tharnstrom). The 1995 debris flow occurred about 11 hrs before this photograph was taken. A

small flood of 500 m3/s in the Colorado River, combined with 14-28 m3/s of recessional flow in Prospect Canyon,

reworked the distal edge of the debris fan. Water pours over the Black Rock, where it previously had not until

discharges greater than 700 m3/s. Fast water replaced the eddy in the foreground because reworked cobbles and

gravel filled the pool downstream from the rapid.

Figure 30. Continued.
70 The Great Cataract

Figure 31. View across Lava Falls Rapid from river left (Stake 2969). A. (July 1952; T. Nichols). Before the advent

of rubber rafts, most river trips portaged boats around Lava Falls Rapid. Here, Mexican Hat Expeditions trip paused

during a portage with the boat completely out of the water on large boulders deposited in 1939.
 QUIESCENCE (1966-1995) 71

B. (March 7, 1995; D. Oldershaw). This match, which may be off by a meter or more, shows deposition by the 1995

debris flow. This view is probably similar to the view from the aggraded debris fans in 1955 and 1963; the 1954 and

1966 debris flows were considerably smaller. One of the boulders visible in the 1952 view is present at left center.

Figure 31. Continued.
72 The Great Cataract

The rapid changed slowly after noon on March

6. By mid-afternoon, a run developed just to the left

of the Ledge Hole. By March 8, the configuration of

the rapid and its water-surface profile stabilized

(fig. 29). At a discharge of 400 m3

/s, the

constriction from the new debris fan caused the

stage in the rapid to appear as if the discharge was

170 m3

/s higher. The drop at the head of the rapid

increased by 0.9 m from the 1994 water-surface

profile for a roughly equivalent discharge (fig.

29b), although the stage remained similar through

the rapid; in other words, the total drop through the

main part of the rapid increased from 3.1 m in 1994

to 4.0 m in 1995.

INITIATION OF DEBRIS FLOWS IN 

PROSPECT CANYON

Debris flows in Prospect Canyon are initiated

under very specific conditions. Runoff generated in

Prospect Valley must flow over the waterfall at the

head of Prospect Canyon in sufficient quantity to

erode and mobilize the colluvial wedges, channel

deposits, and debris-flow deposits below. Although

most Grand Canyon tributaries are small, and debris

flows typically are initiated during intense summer

thunderstorms, runoff in Prospect Valley is

generated by the broad array of hydroclimatic

conditions that affect other watercourses in the

southwestern United States.

Hydroclimatology 

Three general storm types are capable of

causing debris flows in Grand Canyon (Melis and

others, 1994). Intense rainfall from convective

thunderstorms, which usually occur between June

and October, commonly initiate debris flows; these

storms are part of Arizona’s summer monsoon

(Hirschboeck, 1985). Dissipating tropical cyclones,

which can cause intense and sustained precipitation

in the southwestern United States between July and

October (Hansen and Shwarz, 1981; Smith, 1986),

have caused some of the most severe flooding in the

region (Roeske and others, 1978; Aldridge and

Eychaner, 1984; Roeske and others, 1989; Webb

and Betancourt, 1992). Unusually warm winter

storms, possibly combined with rainfall on existing

snowpacks, also causes debris flows in Grand

Canyon (Cooley and others, 1977).

Monsoonal conditions result from moist air

entering the southwestern United States from the

Gulf of Mexico and (or) the eastern North Pacific

Ocean (Hansen and Shwarz, 1981). The interannual

variability of monsoonal precipitation is weakly

related to variability in global circulation patterns

(Webb and Betancourt, 1992) and affects the

number of storms with the potential to initiate

debris flows in the canyon. Hereford and Webb

(1992) documented a decline in warm-season

(June-October) rainfall on the Colorado Plateau

after about 1941. For eastern Grand Canyon,

Hereford and others (1993) reported that the wettest

period in the post-dam period (1963-1990 in their

case) was 1978-1984.

Among the 529 tributaries of Grand Canyon,

dissipating tropical cyclones are only known to

have initiated debris flows in Prospect Canyon in

only two years: 1939 and 1963. Most tropical

cyclones form in the eastern North Pacific Ocean

and travel northwestward along the coast of Mexico

before dissipating over the ocean. The residual

moisture from these storms is typically transported

into Arizona with cutoff low-pressure and frontal

systems entering Arizona from the northwest

(Smith, 1986).

Warm winter storms initiated debris flows in

Grand Canyon in 1966 and 1995. These storms,

which typically occur from December through

March, cause heavy rain and snow that mostly

affects large drainage basins like Prospect Valley.

Debris flows initiated by runoff from warm winter

storms had relatively high discharges and flow

volumes (Melis and others, 1994). These longduration storms affect large areas and may trigger

multiple hillslope failures, leading to high-volume

debris flows and sustained runoff such as the

Crystal Creek debris flow of 1966 (Cooley and

others, 1977; Webb and others, 1989). Winter

rainfall in Grand Canyon has increased, particularly

after the mid-1970s (fig. 32b).

The occurrence of debris flows in Prospect

Canyon is only weakly related to regional storms.

With the exception of the 1939 event, point rainfall

records at stations near Prospect Canyon do not

consistently show unusually high rainfall on the

dates of debris flows, although for each date, one

station may have recorded a relatively long
INITIATION OF DEBRIS FLOWS IN PROSPECT CANYON 73

recurrence-interval daily total or storm (appendix

3). For example, the 1955 debris flow appears to

have been initiated during a storm at Mount

Trumbull with a recurrence interval of 100 years,

but rainfall was not unusual in other parts of the

region. Most of the rainfall stations are at least 40

km from the Prospect Valley drainage basin

(appendix 2), and rainfall recorded at these stations

may not be representative of the storm conditions in

the drainage basin. 

Debris-flow initiation in Prospect Canyon is

not related to the amount of seasonal precipitation.

Although monthly precipitation was high when

most of the debris flows occurred (fig. 33), seasonal

precipitation was not consistently high. For

example, the 1966 and 1995 debris flows occurred

during the 24th and 18th wettest winters

(November-March), respectively, in the 92-year

record (fig. 32b). The 1939, 1954, 1955, and 1963

debris flows occurred during the 1st, 75th, 35th, and

13th wettest summers (July-September),

respectively (fig. 32a). Debris flows in Prospect

Canyon may not require season-long buildup of

antecedent soil moisture; above-average rainfall in

the preceding month may be sufficient.

The size of storm cells that spawn intense

precipitation may be too small to affect both the

Prospect Valley drainage basin and surrounding

rainfall stations. At Tuweep Ranger Station, 40 km

from Prospect Canyon, the storm that caused the

1963 debris flow was relatively brief with a

maximum hourly intensity of 4.5 mm/hr (fig. 34a;

-2.0

-1.0

0.0

1.0

2.0

3.0

Standardized Precipitation

A. July-September

1939

Debris

Flow

1954

Debris

Flow

1955

Debris

Flow

1963

Debris

Flow

-2.0

-1.0

0.0

1.0

2.0

3.0

1900191019201930194019501960197019801990

Year

1966

Debris

Flow

1995

Debris

Flow

B. November-March

Figure 32. Standardized seasonal precipitation for eight stations in the vicinity of western Grand Canyon (appendix 2).

Seasonal precipitation is converted to a time series with mean = 0 and variance = 1; positive values represent above

average precipitation, whereas negative values are below-average precipitation. A. Summer (July-September) precipitation. B. Winter (November-March) precipitation.
74 The Great Cataract

0

50

100

150

200

250

Jul

Sep

Nov

Jan

Mar

May

Jul

Monthly Precipitation, in mm

1939 1940

1939

DEBRIS

FLOW

0

50

100

150

Jun

Sep

Dec

Mar

Jun

Sep

Dec

Mar

Jun

Sep

1954 1955 1956

1954

DEBRIS

FLOW

1955

DEBRIS

FLOW

1956

FLOOD

0

20

40

60

80

100

120

140

160

Jul

Aug

Sep

SELIGMAN

MT. TRUMBULL

TUWEEP

PEACH SPRINGS

SUPAI

GRAND CANYON

1963

DEBRIS

FLOW

0

40

80

120

160

Jul

Sep

Nov

Jan

Mar

1966 1967

1966

DEBRIS

FLOW

0

100

200

300

Dec

Mar

Jun

Sep

Dec

Mar

Jun

Sep

Dec

Mar

1993 1994 1995

1993

FLOOD

1995

DEBRIS

FLOW

Figure 33. Monthly precipitation near the Prospect Valley drainage basin associated with Prospect Canyon debris

flows and floods. A. July 1939 to August 1940. B. June 1954 to October 1956. C. July to September 1963. D. July

1966 to March 1967. E. December 1992 to March 1995.
INITIATION OF DEBRIS FLOWS IN PROSPECT CANYON 75

U.S. Department of Commerce, 1963). During the

debris flow of December 6, 1966, radar maps of

cloud cover do not show unusually large storm cells

in western Grand Canyon (Butler and Mundorff,

1970). Steady, gentle rain fell at Tuweep Ranger

Station, followed by three hours of rainfall with

intensities of 10-11 mm/hr (U.S. Department of

Commerce, 1966; fig. 34b). Radar maps (not

shown) at the time of the 1995 debris flow also did

not show unusual storm cells despite significant

rainfall intensities of 11 mm/hr (fig. 34d).

Storms that initiate debris flows are different in

an important way from those that cause streamflow

floods, particularly during winter. Both the 1966

and 1995 storms ended with notably high-intensity

rainfall (fig. 34b and 34d). The runoff that initiated

both the 1966 and 1995 debris flows likely came

from microbursts of precipitation from clouds

centered over the watershed. In contrast, the rainfall

that caused the 1993 flood in Prospect Canyon had

sustained but light intensities (fig. 34c). Bursts of

rainfall at the end of the storm appear to be a

requirement for debris-flow initiation. 

The Firehose Effect 

Because of the waterfall separating Prospect

Valley and Prospect Canyon (fig. 3), the firehose

effect is the primary process of debris-flow

initiation in Prospect Canyon. This process

involves the impact of a concentrated stream of

water falling on unconsolidated colluvial wedges.

The power generated by falling water in Prospect

Canyon is extremely large; for example, a clearwater discharge of 100 m3

/s, a 10-yr flood in

0

2

4

6

8

10

12

TUWEEP RS

GRAND CANYON

Hourly Precipitation (mm)

Hour

A. SEPTEMBER 17-18, 1963

24 24 4 8 12 16 20

TUWEEP RS

Dec 3 Dec 4 Dec 5 Dec 6 Dec 7

12 24 12 24 12 24 12 24 24 12

Date and Hour

B. DECEMBER 3-7,

 1966

0 4 8 12 16 20 24

TUWEEP RS

Hour

D. March 5, 1995

0

2

4

6

8

10

12

0 4 8 12 16 20 24

TUWEEP RS

GRAND CANYON

Hour

C. FEBRUARY 8, 1993

Figure 34. Hourly precipitation at Tuweep Ranger Station and Grand Canyon during three historic debris flows and

one flood in Prospect Canyon. A. September 1963. B. December 1966. C. March 1995. D. February 1993.
76 The Great Cataract

Prospect Valley, develops a terminal velocity of

about 80 m/s under the 325-m waterfall (assuming

an uninterrupted fall and negligible friction losses).

This falling water has an impact power of

approximately 80 Mw, which is more than

sufficient to erode colluvial wedges and bedrock.

During the 1995 debris flow, streamflow

poured over the waterfall, hitting high-angle

colluvial wedges (shown in the center of fig. 27)

and the channel of Prospect Canyon. The falling

water eroded a 8-m deep, 20-m wide, and 15-m

high cylindrical section of the colluvial wedge and

a crater 13 m in diameter and 6 m deep in the

bottom of Prospect Canyon. In addition, sediment

was scoured to a depth of about 4 m from a 200-m

reach of the channel. The total volume removed

from the impact areas and channel bed was 6,000

m3

, or about 63 percent of the volume of sediment

deposited on the debris fan.

The amount of sediment transported by the

1995 debris flow increased down Prospect Canyon

when alluvial deposits, particularly channel banks,

were undercut eroded. This “bulking-up” occurs

when channel banks collapse during passage of the

debris flow initiated under the waterfall. We

assumed g = 22,000 N/m3

 for a debris flow with a

discharge of 500 m3

/s (100 m3/s of streamflow

mobilized into a debris flow). Using an energy

slope equal to the bed slope of 0.315 in Prospect

Canyon and a channel-width range of 15-20 m

yielded a range in stream power of 0.17-0.23 .

 106

W/m2. This value of stream power is more than an

order of magnitude greater than the stream power

reported for large streamflow floods (Costa and

O’Connor, 1995) because our calculation is for a

debris flow on a steep slope. However, the duration

of debris flows is short (∆t ≈ 1-3 min) and the total

energy expended is relatively small.

Figure 35. The spatial extent of debris fans deposited by Holocene debris flows from Prospect Canyon. The location

of the edge and the thickness of the debris fan were determined by projection into the river from remnant deposits.
MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS 77

Although other studies have suggested

streamflow can bulk-up into debris flow by failure

of channel banks alone (Johnson and Rodine,

1984), there is no evidence of a debris flow in

Grand Canyon that was initiated solely by

streamflow undercutting channel banks (Melis and

others, 1994). However, debris flows initiated

below the waterfall at the head of Prospect Canyon

entrain considerable amounts of additional

sediment through erosion of colluvial wedges,

channel banks, and bed sediments between the

waterfall and the Colorado River. The only

sediment exposed in the channel that was not

moved were boulders greater than 2-3 m in

diameter.

MAGNITUDE AND FREQUENCY OF 

DEBRIS FLOWS

Discharge estimates 

We identified superelevated debris-flow

deposits for the 1939, 1955, and 1963 events in

upstream from section A - A’ on the Prospect

Canyon debris fan (Plate I). With the exception of

the 1939 debris flow, we only found depositional

evidence to constrain the cross-sectional area at the

point of maximum superelevation. The site on the

Prospect Canyon debris fan was a poor one for

estimating discharge of debris flows. In the righthand bend just upstream from cross section A - A’,

a continuous line of boulders, combined with

photographic evidence, provided the elevations of

the flow surface on the inside and outside of the

bend. No bedrock is exposed in the alluvial channel,

which has changed in cross section (fig. 6) despite

the presence of large boulders that were not moved

by recent debris flows, particularly the 1995 event.

The channel slope through the bend is 0.093.

Using the depositional evidence and cross

sections, we estimated discharges for the 3 debris

flows (table 5). The 1939 debris flow had a

discharge of about 1,000 m3

/s; this discharge is

larger than the 1966 debris flow in Crystal Creek

(280 m3

/s; Webb and others, 1989), which

previously was considered the largest historic

debris flow. The 1955 and 1963 debris flows were

of a similar discharge of about 300-400 m3

/s. No

depositional evidence remains of the 1954 and 1966

debris flows, and we could not estimate a peak

discharge for the 1995 debris flow because it did

not overtop channel banks to leave depositional

evidence. Recessive streamflow after the 1995

debris flow obliterated any mudlines that might

have been deposited during the peak discharge of

the debris flow. 

To provide another perspective on magnitude

and frequency, we determined an approximate

water content of 7-14 percent for Prospect Canyon

debris flows using reconstitution techniques on less

than less than 16 mm samples. Because this size

fraction is about 20 percent of the total particle-size

distribution (fig. 4a), the actual water content may

have been less than 5 percent. As stated in the

Methods section, the estimated 100-year

streamflow flood in Prospect Valley is 800 m3

/s.

Assuming this flood would produce a debris flows

in Prospect Canyon with no attenuation of flow, and

assuming the flood water constitutes 5 percent of

the debris-flow volume, the 100-year debris flow

could be as large as 16,000 m3

/s. This number

appears to be unrealistically high and suggests that

the regression equations for flood frequency may be

inappropriate for the Prospect Valley drainage

basin.

Table 5. Peak discharge estimates from superelevation evidence for the debris flows of 1939, 1955,

and 1963 in Prospect Canyon at cross section A - A’

Year of flow Rc (m) ∆Hr (m) W (m)

Velocity

(m/s)

Area 

(m2)

Discharge

(m3/s)

1939 50 3.0 40 6.1 170 1,000

1955 50 0.7 20 4.1 70 290

1963 50 1.5 20 6.1 60 370
78 The Great Cataract

Volumes of debris fans

The entire Prospect Canyon debris fan above

the 140 m3

/s stage of the Colorado River has a total

volume of 1.9 .

 106 m3. To estimate the uneroded

volume of surface tfa (950 BC), the highest surface

on the Prospect Canyon debris fan (Plate I), we

added the volume of the entrenched channel and the

volume of the projected debris fan surface across

the Colorado River. The reconstructed debris fan

from surface tfa had an area of 16 ha and a volume

of 3.5 .

 106 m3 (table 6). 

The projected height indicates that the debris

flow crossed the Colorado River (fig. 35). The

height above the center of the river at a stage of 140

m3

/s is 19.3 m (point V), 16.9 m (point U), and 15.0

m (point T)(Plate I). Kieffer (1988) reported depths

of the Colorado River at these points; therefore, the

maximum thickness of the deposit above the bed of

the Colorado River is 30.3 m (point V), 27.7 m

(point U), and 24.2 m (point T). 

Kieffer (1988) attributes the large basalt

boulders on the right side of Lava Falls Rapid — for

example, the Black Rock, the Entrance Rock, and

the Scout Rock (fig. 11b) — to rockfall from the

basalt cliffs above the rapid. This accumulation of

boulders is unusual for the reach of channel above

and below the rapid, where relatively few large

basalt boulders appear under similar cliffs (see

Kieffer, 1988). An alternative explanation is that

these boulders were deposited by a debris flow (or

debris flows) that dammed the Colorado River,

such as the 950 BC event that formed surface tfa.

The projected area of the reconstructed tfa surface

Table 6. Characteristics of debris fans deposited at the mouth of Prospect Canyon between 1939

and 1995

 Notes: F -- streamflow flood, DF -- debris flow, nd -- no data, na -- not applicable. All areas and volumes are for sediments exposed above a

discharge of 140 m3/s. Maximum thickness were estimated during field surveys of non-eroded debris flow deposits; minimum thickness are the

thickness of debris-flow deposit that would cover immobile boulders at mouth of Prospect Creek. The constriction ratio is the river-channel width

divided by the average river channel width immediately upstream of the rapid (Kieffer, 1985, 1987); small constriction ratios indicate a highly

constricted river. The maximum constriction is the percent reduction in river width, compared with an average of upstream and downstream widths,

at the narrowest part of the rapid. 

*Areas, volumes, and constriction ratios were determined by projection of the slopes of remnant deposits (Plate I) into the Colorado River.

†Areas, volumes, and constriction ratios were determined by rectification of aerial photography (figs. 21, 24) using image processing software.

§Areas, volumes, and constriction ratios were determined by rectification of oblique ground photography (e.g., fig. 12) using image processing

software.

Year of flood 

(type)

Maximum 

debris-fan area 

(ha)

Maximum 

debris-fan 

thickness (m)

 Minimum

debris-fan 

thickness (m )

Range in

debris-fan volume 

(103 m3)

Constriction

 ratio

Maximum 

constriction

 (%)

950 BC* 15.9 22 22 3,500 0.00 100

(DF)

AD 1434* 1.07 5.0 4.0 43-54 0.50 45

(DF)

1939* 1.25 5.0 3.5 44-63 0.20 80

(DF)

1954† 0.42 2.0 1.6 3.2-8.4 0.55 40

(DF)

1955† 0.73 2.9 2.1 15-21 0.30 70

(DF)

1956† 0.24 0.8 0.5 1.2-1.9 na na

(F)

1963† 0.73 1.9 1.7 12-14 0.40 60

(DF)

1966§ 0.38 1.6 1.0 3.8-6.1 0.60 35

(DF)

1995# 0.56 1.7 1.7 9.4 0.40 60

(DF)
MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS 79

covers most of the large boulders, including the

Black Rock by 1-2 m and the Entrance Rock by 5

m, and is at about the same elevation as the Scout

Rock. Similar-sized boulders remain in the channel

of Prospect Canyon (appendix 4), supporting the

possibility of Prospect Canyon as the source for

boulders on the right side of the rapid. 

Other Holocene and historic debris flows may

not have been large enough to cross the Colorado

River. Of the other fan-forming and inset Holocene

deposits, we estimated a volume only for surface tif

(AD 1434). The remaining deposits have a

relatively steep surface slope, and projection into

the river results in only a moderate-size debris fan

(fig. 35). The 1939 debris flow, the largest historic

event, deposited a debris fan of 1.25 ha and a

volume of 44,000 - 63,000 m3

 (table 6); in

comparison, the uneroded fan of the 1966 Crystal

Creek debris flow is about 58,000 m3

 (Melis and

others, 1994), or about the same size. The 1965

debris fan at Warm Springs Rapid on the Yampa

was 40,000 m3

 (Hammack, 1994), or slightly

smaller than the Grand Canyon debris fans. Other

Prospect Canyon debris flows deposited 3,000 -

21,000 m3

 of sediment on the debris fan. The

depositional volumes of debris flows from Prospect

Canyon are comparable to the volumes of debris

flows from other tributaries in Grand Canyon,

although the range in volume is large, probably

because the Prospect Valley drainage basin is the

largest tributary from which debris flows have been

observed (fig. 36). 

Constrictions of the Colorado River

Holocene debris flows from Prospect Canyon

deposited debris fans with Cw that range from 30-

100 percent and the maximum Cw from 35-100

percent (0.00 < Cr < 0.61) (table 7). In 1872, the

average Cw (at 280 m3

/s, reconstructed by

rectifying fig. 12a) was 5 percent (Cr = 0.75), and Cr

in 1994 at 230 m3/s (fig. 28a) was 30 percent (Cr =

0.60). Therefore, historic debris flows decreased

Figure 36. The volume of sediments deposited on debris fans by historical debris flows in Grand Canyon (modified

from Melis and others, 1994) and Holocene debris flows from Prospect Canyon.

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

0.1 1 10 100 1,000

HISTORICAL DEBRIS FLOWS

IN GRAND CANYON

HOLOCENE DEBRIS FLOWS 

FROM PROSPECT CANYON

Drainage Area (km2)

Volume of Debris Flow (m3

)
80 The Great Cataract

Figure 37. Constriction ratios of debris fans in Grand Canyon. A. Prospect Canyon debris fans aggraded by

Holocene debris flows and reworked by the Colorado River (table 7). B. Major Grand Canyon rapids (Kieffer, 1987).

C. Debris fans in Marble Canyon (Schmidt, 1990).

0

1

2

3

A. Prospect debris fans

0

5

1 0

15

20

25

30

Number of Debris Fans

B. Major Grand Canyon rapids (Kieffer, 1987).

0

5

1 0

15

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Constriction Ratio, Cr

C. Marble Canyon debris fans (Schmidt, 1990)
MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS 81

the width of Lava Falls Rapid by about 25 percent

before the 1995 debris flow.

Values of Cr for Holocene debris flows at Lava

Falls Rapid span most of the previously reported

range of Cr in Grand Canyon (fig. 37). Kieffer

(1987) reported an average Cr of 0.50 for major

Grand Canyon rapids. Similarly, the average Cr

from Schmidt (1990), calculated from his

expansion data, is 0.54. Rapids affected by debris

flows, like Lava Falls, can have a wide range of

constrictions over periods of a century, rendering

the average value of Cr not very useful as an overall

measure of the stability of rapids.

In her discussion of constriction ratios, Kieffer

(1985, 1987) noted that Crystal Rapid likely had a

maximum Cr of about 0.25 before reworking. This

ratio exceeds the Cr for the 1939 debris flow from

Prospect Canyon (table 7), which indicates that the

1939 debris flow from Prospect Canyon had a

greater effect on the Colorado River. 

Frequency of debris flows

The evidence of Holocene debris flows is

sufficient to estimate recurrence intervals for this

type of flash flood in Prospect Canyon. Throughout

Grand Canyon, debris flows have an average

recurrence interval of one debris flow every 20-50

yrs (Melis and others, 1994). In a history spanning

123 years, Prospect Canyon has had six debris

flows (table 4); if these were considered

independent of one another, the historical

frequency of debris flows is one every 20 years.

Debris flows from Prospect Canyon are clustered in

time: most occurred during the middle part of the

20th century, when five debris flows occurred in a

27-year period (one every 5 years). Alternatively,

six debris flows occurred from 1939 through 1995

for a recurrence interval of about one every 10

years. Regardless of the estimation period, the

frequency of historic debris flows from Prospect

Canyon (one every 5-20 years) is greater than the

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

19201930194019501960197019801990

2

1 0

5 0

100

Discharge (m3/s)

Year

RECURRENCE 

INTERVAL (yrs)

GLEN

CANYON

DAM

Figure 38. Annual series of instantaneous peak discharges for the Colorado River near Grand Canyon, Arizona.

Recurrence intervals were calculated using a log-Pearson type III distribution for pre-dam (1921-1963) and post-dam

(1963-1995) discharges.
82 The Great Cataract

frequency of debris flows in other Colorado River

tributaries (Melis and others, 1994).

We used the estimated volumes of historic

debris flows and the AD 1434 debris flow (table 6)

to estimate a magnitude-frequency relation for

aggraded debris fans; then assumed the frequency

of production of debris fans at the mouth of

Prospect Canyon could be approximated using a

log-normal distribution. Because debris fans do not

form every year, and years with zero volume are

difficult to model using log-transformed data (Kite,

1988), the data were censored at a volume of 6,000

m3

. We then used an existing maximum-likelihood

procedure (Stedinger and Cohn, 1986; Stedinger

and others, 1988) to fit a log-normal distribution.

As a check of the distribution, we also fit a logPearson type III distribution, but the fitted skew

coefficient was close to zero, again suggesting the

log-normal distribution is the most appropriate to

use.

The recurrence intervals for the debris fans

provide a magnitude-frequency relation for historic

debris flows from Prospect Canyon. Both the AD

1434 and 1939 debris flows are 150- to 200-yr

events. The 1954 and 1966 debris flows, the

smallest ones recorded from Prospect Canyon, are

about 20-year events. Finally, the 1955, 1963, and

Table 7. Constriction percentage and constriction ratios of historic aggraded and reworked debris

fans at the mouth of Prospect Canyon

 Notes: Constriction ratio and constriction percentage is described in the notes for table 6 and in the text. Values of Cr are rounded to the nearest

0.05; values of Cw are rounded to the nearest 5%.

*Constriction data were determined by rectification of oblique ground photography (fig. 12) using image-processing software.

†Constriction data were determined by projection of the slopes of remnant deposits (Plate I) into the Colorado River.

§Constriction data were determined by rectification of aerial photography (fig. 21) using image processing software.

#Constriction data were determined by survey.

Year

Date of photograph 

or survey

Discharge on 

date (m3/s)

Maximum discharge 

between dates (m3/s)

Constriction 

ratio, Cr

Constriction 

percentage Cw (%),

1872 Apr 16 280 na 0.75* 5*

1939 Sep 6 150 8,500 0.20† 80†

1955 Jul 24 250 3,460 0.55§ 40§

1955 Mar 19 540 530 0.65§ 30§

1955 Mar 21 400 540 0.65§ 30§

1956 Mar 25 180 1,140 0.30§ 70§

1956 Apr 16 300 520 0.35§ 60§

1956 Aug 29 80 1,900 0.30§ 70§

1957 May 4 520 680 0.60§ 35§

1958 Apr 20 710 3,540 0.80§ 15§

1958 Jun 1 3,000 3,000 0.80§ 15§

1958 Oct 4 190 3,050 0.70§ 30§

1960 Oct 2 130 1,310 0.60§ 40§

1962 Nov 3 230 2,420 0.70§ 30§

1963 Aug 22 60 230 0.70† 30†

1964 Apr 20 40 550 0.40§ 60§

1965 May 18 790 1,290 0.70§ 30§

1966 May 19 500 1,650 0.65† 30†

1967 Aug 340 520 0.60† 35†

1973 Jun 19 390 1,080 0.75§ 20§

1989 Oct 8 160 2,720 0.75§ 30§

1994 Jun 1 240 970 0.60§ 30§

1995 Mar 6 530 620 0.40# 60#

1995 May 30 260 530 0.50§ 50§
MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS 83

Figure 39. Hydrographs of the Colorado River showing the timing of tributary floods and debris flows and river

discharges. A. 1939 to 1942. B. 1954 to 1958. C. 1963 to 1966.

0

1,000

2,000

3,000

4,000

1939

1940

19411942

1939 DEBRIS 

FLOW

DEPOSIT PARTIALLY 

REWORKED

MOST OF DEPOSIT

REMOVED

0

1,000

2,000

3,000

4,000

Daily Discharge, in m3/s

1954 DEBRIS FLOW

1955 DEBRIS FLOW

1956 FLOOD

1954195519561957

1958

DEPOSIT PARTLY

REWORKED

DEPOSIT MOSTLY

REWORKED

UNKNOWN EFFECT

0

500

1,000

1,500

2,000

1963196419651966

1963 DEBRIS FLOW

Year

DEPOSIT REMOVED
84 The Great Cataract

1995 debris flows are 55-, 45-, and 35-yr events,

respectively. 

REWORKING OF DEBRIS FANS BY 

THE COLORADO RIVER

Reworking of rapids by river floods was

initially described by Graf (1979, 1980) and

Howard and Dolan (1981). Kieffer (1985, 1987,

1990) presented a conceptual model for reworking

of debris fans in Grand Canyon, incorporating

elements from the previous studies. This model,

based on alteration of Crystal Rapid during the

large releases from Glen Canyon Dam in 1983

(Kieffer, 1985), consists of the stages of (1)

damming of the river by a debris flow, (2) the river

overtopping the debris fan, forming a “waterfall” on

the downstream side, and (3) headcut progression

from downstream to upstream across the debris fan,

depending on discharges in the river. The history of

reworking of the Prospect Canyon debris fan shows

the instability of rapids controlled by debris flows,

and suggests a modification of Kieffer’s conceptual

model. 

The effectiveness of unregulated flood and dam

releases in alleviating constrictions is extremely

important. Before regulation in 1963 by Glen

Canyon Dam, the 2-yr flood on the Colorado River

was approximately 2,140 m3

/s; the 100-yr flood

was 5,650 m3

/s (fig. 38). The largest historic flood

was 8,500 m3

/s in 1884; several prehistoric

Holocene floods exceeded 8,500 m3

/s and one may

have been as large as 11,000 m3

/s (Hereford and

others 1993, 1996; O’Connor and others, 1994).

Regulation by Glen Canyon Dam reduced the

apparent 2-yr flood to about 890 m3

/s. Floods on the

unregulated Colorado River were larger and of

longer duration than dam releases (figs. 39, 40).

The 1995 debris flow in Prospect Canyon

provides a good example of the interaction between

the Colorado River and a newly aggraded debris

fan. Immediately after the debris flow ceased, the

Colorado River had a maximum Cw of 60 percent,

or about 34 percent more constricted than before the

debris flow (table 7). Reworking by the Colorado

River began immediately after cessation of the

debris flow and lasted 12 hrs. Within that period, 45

percent (4,200 m3

) of the volume of the debris fan

was removed by the Colorado River; an additional

660 m3

 was eroded by recessional flow in Prospect

Canyon. Twelve hours after the deposition, 52

percent of the deposit was removed without

overtopping of the debris fan by the Colorado

River.

Reworking increased the median particle size

(d50) from 350 mm to 512 mm (fig. 41a) on the

distal margin of the aggraded fan adjacent to Lava

Falls Rapid. Reworking preferentially removed

limestone; the reworked deposit was 90 percent

basalt and 6 percent limestone (fig. 41b). The larger

reworked particles were deposited in the pool

downstream from the Black Rock in an elongated

debris bar that is submerged shallowly at 263 m3

/s

(fig. 28b). This debris bar corresponds to Kieffer’s

(1990) “rock garden.” During the reworking of

March 6, existing waves in Lava Falls Rapid

increased in size, but a hydraulic jump did not form

either in response to or because of reworking, as

Kieffer (1985, 1990) reported.

At the time of the Powell Expedition (figs. 11b,

12a), Lava Falls Rapid was wide with a constriction

ratio of 0.75 and an average constriction of 5

percent (table 7). That the rapid did not change

between 1872 and 1939 is significant because

several large floods occurred in the Colorado River

between 1921 and 1939 (fig. 38). The Pyramid and

Deflector rocks, and boulders on the debris fan,

were unaffected by floods in excess of 5,660 m3

/s

that occurred in 1884 and 1921; equivalent-sized

floods also may have occurred 1891, 1905, and

1916. Photography taken in 1909 and 1934 at low

water (figs. 14 and 17) show boulders in excess of

3 m in diameter on the bed in Lava Falls Rapid. The

boulders were wedged together in groups; water

flowing over the groups formed obstacles, large

waves, and holes that compelled most expeditions

to portage around the rapid.

The 1939 debris flow initially constricted the

Colorado River by 80 percent. If the top of the 1939

levee is projected to the right bank of the Colorado

River, the resulting width of the Colorado River

would have been only 5.2 m. The debris fan

aggraded in 1939 (table 7) and the rapid was

widened to an unknown extent by the 1941 flood

(peak discharge of 3,400 m3

/s), but no rock garden

formed below the rapid, as suggested by Kieffer’s

model. Photographs from the 1940s (figs. 18 and

19) show a slightly wider rapid than in the early
REWORKING OF DEBRIS FANS BY THE COLORADO RIVER 85

0

200

400

600

800

1,000

1,200

Maximum Daily Discharge (m3/s)

1967196819691970

1971

19721973

A. Glen Canyon Dam

DEBRIS

FLOW

REWORKING

COMPLETE

0

200

400

600

800

1,000

1,200

Annual Peak Discharge (m3/s)

1967196819691970

1971

19721973

B. Grand Canyon

Figure 40. Hydrographs of the Colorado River from 1966 to 1974 showing the timing of the 1966 debris flow and

releases from Glen Canyon Dam. A. Maximum daily discharge. B. Annual peak discharge.
86 The Great Cataract

0

2 0

40

60

80

100

-12 -10 -8 -6 -4 -2 0

DEBRIS-FLOW

DEPOSIT

TRIBUTARY

REWORKED

DEPOSIT

PARTIALLY

REWORKED

DEPOSIT

FULLY

REWORKED

DEPOSIT

Cumulative Percent Finer

4,0961,024

256

6 4

16

4

1

Particle Size, in mm

Particle Size, in units φ

A.

0

2 0

40

60

80

100

Quaternary

Supai Group

Redwall

Muav

Undifferentiated

Unknown

1995 Deposit

Reworked Deposits

Percent of Particles

Basalt

Limestone

Limestone

Limestone

N=100

N=875

B.

Figure 41. Effects of river reworking on particles on the Prospect Canyon debris fan. A. Comparison of the particlesize distribution on the reworked and intact distal margins of the debris-flow deposit. B. Comparison of rock types

transported in the main lobe of the 1995 debris flow with the composition of rocks on the reworked debris fan.
DISCUSSION AND CONCLUSIONS 87

1990s, which suggests the constriction may have

been less than 25 percent. 

Two debris flows in the 1950s again constricted

Lava Falls Rapid (table 7). The 1954 debris flow

increased the constriction to 40 percent, but

Colorado River discharges up to 540 m3

/s reduced

the constriction to about 30 percent by the

following spring. The 1955 debris flow increased

the constriction to 70 percent, which formed a

stable configuration for nearly two years despite

discharges up to 1,900 m3

/s (table 7). Aerial

photographs from 1956 and 1957 (figs. 21c and

21d) show that the debris fan was not overtopped;

instead, the river eroded the margin of the debris fan

and then transported boulders when river stage

created sufficient stream power. The first

photographs after the 1957 and 1958 floods, taken

at low discharge in October 1958, showed a

constriction of 30 percent (table 7). 

The two debris flows in the 1960s, which

occurred during the first 4 yrs of operation of Glen

Canyon Dam (table 4), were reworked by unusual

dam releases and (or) tributary floods (figs. 39 and

40). The 1963 debris flow caused a 60 percent

constriction; the 1965 releases from Glen Canyon

Dam, which peaked at 1,650 m3

/s, reduced the

constriction to 30 percent. The 1966 debris flow

created a 35 percent constriction, which was again

reduced to 30 percent by 1994 (table 7). A powerplant release of 840 m3

/s on May 26, 1972 partially

reworked the debris fan, and the combination of

low dam releases and a small flood in the Little

Colorado River on April 17, 1973, created a peak

discharge of 1,080 m3

/s, which eroded most of the

aggraded debris fan. Additional changes in Lava

Falls Rapid occurred during the 2,720 m3

/s flood of

1983, but these were relatively minor. The amount

of elapsed time from deposition to reworking

increased from 1-2 yrs to 3-7 yrs because of the

change in flow caused by operation of Glen Canyon

Dam (fig. 38).

Not including the 1995 deposit, the total

volume of sediment deposited by historic debris

flows at Lava Falls Rapid was about 110,000 m3

.

Most of this sediment was reworked by the

Colorado River, leaving a residual deposit of 2,000

m3

 that increased the river’s constriction from

about 5 percent in 1872 to 30 percent in 1994.

Including the 4,800 m3

 eroded from the 1995 debris

fan, a total of 115,000 m3

 of sediment was eroded

from the debris fan and transported downstream.

Approximately 70 percent of this volume (86,000

m3

) was boulders (greater than 256 mm, fig. 41a)

that were transported to Lower Lava Rapid or

downstream. This prodigious production of

boulders is the reason the total drop from the top of

Lava Falls Rapid to the bottom of the lowermost

secondary riffle (1.4 km downstream) is 12 m. The

five alternating debris bars that control the

downstream riffles are spread over a distance of 2

km downstream from Lava Falls (Stevens, 1990).

The history of aggraded debris fans and

reworking by the Colorado River at Lava Falls

Rapid provides the basis for a general model of

debris fan evolution in Grand Canyon (fig. 42).

Kieffer’s (1985, 1987) model does not completely

apply to reworking of debris fans at Lava Falls

Rapid because historic debris flows from Prospect

Canyon, or from other tributaries in Grand Canyon

including Crystal Creek, have not crossed and

dammed the Colorado River (Webb, 1996).

Consequently, the “waterfall” that Kieffer

hypothesized to occur on the downstream side of

the newly aggraded debris fan is actually water

flowing through the steepened, constricted rapid

and around the distal margin of the debris fan. In the

case of Lava Falls, Cw of the aggraded debris fan is

60-80 percent (fig. 42b). Relatively low river

discharges cause minor reworking (figs. 21c, 42c),

whereas large Colorado River floods remove most

of the debris-fan constriction and form secondary

riffles downstream (fig. 42d). Because dam releases

are typically much smaller than floods in the

unregulated Colorado River (fig. 38), reworking of

debris fans that formed in the last 30 yrs have

mostly followed the model shown in Figure 42c,

which is similar to Kieffer’s model.

DISCUSSION AND CONCLUSIONS 

Lava Falls Rapid is a formidable reach of

whitewater and an example of how tributaries

control the grade and hydraulics of a major river.

Lava Falls previously was assumed to be an

unchanging rapid controlled by the remnants of

Pleistocene basalt dams. Instead, we have shown

that Lava Falls is the most unstable rapid in Grand

Canyon owing to frequent debris flows from

Prospect Canyon. The highest deposits on the
88 The Great Cataract

Figure 42. Diagram of one cycle of debris-flow deposition and river reworking at Lava Falls Rapid. A. The “stable

rapid” at Lava Falls, as exemplified by the configuration present from 1872 to 1939, has a constriction ratio of about

0.75 and has an average constriction of about 3 percent. B. Debris flows seldom dam the Colorado River. Instead,

deposition generally constricts the river by 60-80 percent and creates a constriction ratio of 0.2-0.4. C. Small pre-dam

floods and typical dam releases erode only the downstream side of the debris fan and do not significantly widen the

constriction. Debris eroded from the debris fan is deposited downstream in the pool adjacent to the tail waves. D.

Large pre-dam floods, and unusual dam releases such as in June 1983, typically remove most of the constriction,

leaving a residual lag of boulders in the rapid. In the 20th century, the resultant constriction ratio and constrictions did

not return to the 1872 condition (A). Cobbles are transported through the pool and deposited on the debris bar

downstream.
DISCUSSION AND CONCLUSIONS 89

debris fan, deposited about 950 BC, indicate

predominant late Holocene aggradation of one of

the largest debris fans in Grand Canyon. Moreover,

the 950 BC debris flow raised the base level of the

Colorado River by 30 m before the deposit was

reworked. At Granite Park, 30 river miles

downstream, Lucchita and others (1995) indicate

10 m of downcutting in the last 11 ka. This

downcutting indicates that the local effects of

debris flows on the river corridor may be of the

same magnitude as downcutting of Grand Canyon

in the Holocene.

Prospect Canyon has produced the highest

frequency of debris flows of any tributary in Grand

Canyon (Melis and others, 1994). Depending on the

interval chosen, the frequency of debris flows

ranges from one every 5 years to one every 20

years. Debris flows were not random in the

historical period: from April 1872 to July 1939,

there were no debris flows in Prospect Canyon,

whereas five debris flows occurred in a 27-yr period

between 1939 and 1966. Because debris flows do

not appear to be related to regional

hydroclimatology, we attribute the nonstationarity

in debris-flow occurrence to destabilization of

colluvial wedges and channel deposits in Prospect

Canyon. As the sediment sources diminish, the

frequency of debris flows is expected to decline.

Debris flows occur more frequently in Prospect

Canyon owing to several significant differences

from other debris-flow producing tributaries in

Grand Canyon. Because of its large drainage area,

the Prospect Valley drainage basin produces runoff

in response to regional storms as well as summer

thunderstorms, which have caused most of the

historic debris flows in Grand Canyon. Debris

flows are produced in Prospect Canyon solely by

the firehose effect from a channel draining a large

area; failure of bedrock units as well as colluvium

initiated the largest debris flows from other

tributaries (Melis and others, 1994). The basalt plug

that creates the 325-m waterfall at the head of

Prospect Canyon is a lithologic control that exists

only in Prospect Canyon, the drainage from

Toroweap Valley (on the opposite side of the river

from Prospect Canyon), and Whitmore Wash. The

latter two drainages do not produce significant

debris flows.

The regional hydroclimatology strongly

influences debris-flow initiation in Prospect

Canyon. The results of Hereford and Webb (1992)

and Hereford and others (1993) are verified in our

analysis of precipitation anomalies in western

Grand Canyon, which shows a general decline in

summer precipitation, particularly after 1970 (fig.

32a). This is significant because August and

September precipitation has produced more than 90

percent of debris flows since 1939 (Melis and

others, 1994). Hereford and Webb (1992) showed

that precipitation from dissipating tropical cyclones

has declined on the Colorado Plateau except for the

years 1972 and 1983, when precipitation from this

source was well above normal. Although summer

precipitation has declined, winter precipitation has

increased (fig. 32b) and the last 2 debris flows from

Prospect Canyon have occurred during regional

winter storms.

The instability of Lava Falls Rapid is best

illustrated by the alteration of primary hydraulic

features and the historical constrictions. Debris

flows in 1939, 1954, 1955, 1963, and 1966 changed

the pattern of flow through Lava Falls Rapid,

creating the hydraulic features so well known to

river runners (fig. 11b). Before 1939, the

constriction percentage at Lava Falls was only 5

percent (table 7). In 1939, the rapid was constricted

by 80 percent, which is the largest historic

constriction known in Grand Canyon. The

interaction of debris flows and reworking by the

Colorado River increased the average constriction

from 5 percent in 1872 to 30 percent in 1994. A

debris flow in March 1995 increased the

constriction from 30 to 60 percent; half a day of

Colorado River flow reduced the constriction to 50

percent. Lighter lithologies, such as sandstone and

limestone, were preferentially removed, leaving a

lag deposit of mostly basalt (fig. 41b). 

Lava Falls Rapid represents the long-term

effect of frequent debris-flow deposition and

mainstem reworking. Between 1872 and 1939,

Lava Falls Rapid did not change despite large

Colorado River floods in 1884 and 1921. Smaller

river floods that occurred after the 1939, 1954, and

1955 debris flows reworked most of the deposits

within 2 yrs. The 1963 and 1966 debris-fan deposits

were removed by even smaller releases from Glen

Canyon Dam in 3-7 yrs. Rapids may be stable for

long periods between debris flows despite the

occurrence of long recurrence-interval floods,
90 The Great Cataract

particularly if suturing of boulders occurs on the

bed of the rapid (Webb, 1996).

Two previous estimates of the magnitude of

floods necessary to rework debris fans are

considerably higher than the discharges that have

reworked the Prospect Canyon debris fan. Kieffer

(1985) concluded that a Colorado River flood of

11,300 m3

/s is necessary to widen a severely

constricted rapid, such as Crystal Rapid, to Cr =

0.50, the average constriction ratio of rapids in

Grand Canyon (fig. 37). At Warm Springs Rapid on

the Yampa River in Utah, Hammack (1994)

estimated that a discharge of 2,750 m3

/s (a 500-

1,000 yr flood) would be required to remove most

of the debris fan aggraded in 1965. As the history of

Lava Falls Rapid shows, historic floods of only

3,400 m3

/s were sufficient to widen the more

severely constricted Lava Falls to a 30 percent

constriction (Cr = 0.70) in 1939. Kieffer’s (1985)

argument was based on the change in mean velocity

through the rapid as the constriction is removed and

the velocity required to initiate boulder transport.

Our findings indicate that the primary mechanism

that widens rapids is lateral channel erosion of the

debris fan, not entrainment of individual boulders

from the top of the debris fan. Boulders enclosed in

a poorly sorted matrix that are mobilized by lateral

erosion have initial motion, which allows for

transport by lower discharges.

The history of Lava Falls Rapid illustrates that

most rapids are not an equilibrium fluvial form

unless viewed strictly in the short intervals between

debris flows. We concur with Howard and Dolan

(1981), who concluded the position of rapids in

Grand Canyon is not controlled by equilibrium

river processes as previously reported (Leopold,

1969), but instead is controlled by point sediment

sources, typically tributaries. Secondary riffles and

rapids — such as Lower Lava Rapid — are also

controlled by tributaries because reworking of

aggraded debris fans at tributary junctures provides

the boulders and cobbles that compose the

controlling debris bar. Lava Falls Rapid also shows

that rapids are not necessarily controlled by events

that occurred in a different climatic regime (Graf,

1979); the debris flow that dammed the Colorado

River about 950 BC did not occur in a climate that

was significantly different from the 20th century

(Webb, 1996). Rapids that appear stable and of

great antiquity may be so only until the next debris

flow triggers changes by constricting the river or

adding boulders to the rapid.

Rapids in bedrock canyons controlled by

tributary alluvium are aggradational features that

reflect the net effect of tributary-mainstem

interactions (Graf, 1979). The boulders that form

the core of rapids in Grand Canyon are essentially

immobile by both dam releases and by unregulated

Colorado River floods, although boulders with baxis diameters as large as 4 m have been dislodged

and moved short distances. The presence of rapids

such as Lava Falls indicate the river’s erosional

energy is mostly expended in abrading and

removing boulders, not in eroding bedrock. 

Dam operations have decreased reworking of

debris fans at the mouths of tributary canyons. Graf

(1980), in an analysis of debris fans on the Green

River in northern Utah, concluded that 62 percent

of the rapids were stable before construction of

Flaming Gorge Dam, but that 93 percent of the

rapids were stable after the dam began operation.

Kieffer’s (1985) study of Crystal Rapid suggests

the 1966 debris fan could only be removed by a

flood larger than the largest historical discharge in

the Colorado River (6,200 m3

/s). At Lava Falls, two

debris fans that formed after closure of Glen

Canyon Dam were completely removed by flows

that were less than the pre-dam 2-yr flood.

Although previous studies suggest otherwise,

historical reworking of Lava Falls Rapid indicates

that modest dam releases — in the case of Glen

Canyon Dam, above the powerplant capacity of 890

m3

/s — could significantly rework aggraded debris

fans. Some aggraded debris fans, such as the one at

Crystal Rapid, would be unaffected, but others may

be totally removed.

A comparison of the response of Crystal and

Lava Falls rapids illustrates why it is difficult to

make generalizations about the stability of rapids.

Debris flows constricted both rapids after closure of

Glen Canyon Dam, but the interaction of the river

with the aggraded debris fans is different. The

difference is explained largely by the size of

boulders that composed the aggraded debris fans;

whereas the 1963 debris flow was relatively finegrained (d50 = 64 mm) and contained relatively

small boulders, the 1966 debris flow at Crystal

Rapid had very coarse particles (d50 = 256 mm;
REFERENCES CITED 91

Kieffer, 1987) and very large boulders. If a debris

flow similar to the 1939 event had occurred at Lava

Falls in 1963, the debris fan might have had a

history similar to that of the Crystal Rapid debris

fan.

The 1963 debris flow at Lava Falls Rapid was

largely removed by dam releases of 1,640 m3

/s in

1965 because the debris fan had not been

significantly reworked (figs. 23b, 24). Reworking

by typical powerplant releases armors the distal

margin with boulders and cobbles that may be

interlocked. This armoring greatly increases the

necessary tractive stress needed for entrainment.

The 1965 releases cut laterally into the deposit and

entrained boulders up to 1 m in diameter. The debris

fan at Crystal Rapid, enlarged in 1966, was first

reworked by about 1,080 m3

/s in 1973; major

changes occurred only when dam releases exceeded

1,410 m3

/s and peaked at 2,720 m3/s in 1983.

Although the Crystal Creek debris fan was

significantly reworked, its distal edge is armored

and should withstand most dam releases. As shown

in Figure 30c, the distal margin of the 1995 debrisflow deposit, reworked by powerplant releases less

than 560 m3

/s, is also armored. 

Historic reworking of debris fans at Lava Falls

Rapid has implications for dam operations, if

periodic aggradation of debris fans is a

management consideration. Because releases from

Glen Canyon Dam are limited by the size of its

powerplant, spillways, and jet tubes, the frequency

of large releases that could significantly rework

debris fans is low and is related to floods on the

Paria and Little Colorado Rivers. Because the Little

Colorado River has a flood of record of 3,390 m3

/s

and has a larger drainage area, the Little Colorado

River is more likely to generate a debris-fan

reworking flood than the Paria River, which has a

flood of record of 455 m3

/s (Garrett and

Gellenbeck, 1991). The magnitude of regulated

flows designed to alter the river corridor need to be

large enough to remove most recently aggraded

debris fans without significantly armoring their

distal edges. If significant armoring develops, or

boulders become sutured together, dam releases

will be ineffectual in removing aggraded debris

fans.

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292.
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID

AND THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

1872 Apr 16 Hillers 61 NA 967 T, DS 280

Apr 16 Hillers 62 NA 967 T, DS 280

Apr 16 Hillers 216 HL nm T, AC na

Apr 16 Hillers 896 NA 2689a T, AC na

Apr 19 Hillers 597 NA 2744 R, DS 280

 Apr 19 Hillers 515 NA nm R, US 280

 Apr 19 Hillers 602 NA 2613 R, US 280

 Apr 19 Hillers 616 NA 2598 R, US 280

 Apr 19 Hillers 623 NA 3005 R, AC 280

 Apr 19 Hillers 693 NA nm R, AC 280

Oct Bell 14 HL nm T, AC na

Oct Bell 243 NA 2681 T, AC na

1890 Feb 27 Stanton 621 NA 1510b L, DS 230-400

1909 Nov 10 Cogswell 935 UCB nm L, AC 280

Nov 10 Cogswell 936 UCB nm L, US 280

Nov 10 Cogswell 937 UCB nm L, US 280

Nov 10 Cogswell 938 NYPL nm L, US 280

Nov 10 Cogswell 940 UCB 1511 L, US 280

Nov 10 Cogswell 941 UCB nm L, DS 280

Nov 10 Cogswell 942 NYPL nm L, US 280

Nov 10 Cogswell 1161 NYPL 2770 L, US 280

1912 Jan 1 Kolb 631 NAU 3052 R, US 30-90

Jan 1 Kolb 632 NAU 2599 R, DS 30-90

Jan 1 Kolb 5795 NAU nm R, AC 30-90

Jan 2 Kolb 4-4 NAU 2662 L, US 30-90

1923 Sep 18 LaRue 605 USGS 1732 L, US 260

 Sep 18 LaRue 606 USGS 2368 L, US 260

Sep 18 LaRue 603 USGS 2769 L, DS 2,970

1927 Aug 2 Eddy 92 HL 1512 L, AC 510

Aug 2 Eddy 93 HL 2771 L, US 510

 Aug 2 Weatherhead 172.8 HL 2663 L, DS 510

1929 nd Scoyen 6616 NPS 967 T, DS 570

1930 Jul Fraser 307 HL 967 T, DS 280-570

GDCN 179.1

1934 July 31 Fahrni 3-254 USHS 2658b L, AC 60

 July 31 Fahrni 3-255 USHS 2658c L, AC 60

 July 31 Fahrni 3-258 USHS 2658a L, DS 60

1934 July 31 Fahrni 3-260 USHS 2743 L, US 60

1936 Feb 29 Maxon v. 83, p. 126 HL nm AR, AC nd

Feb 29 Maxon v. 83, p. 127 HL nm AR, DS 140-280

1937 Nov 16 Sharp nd OP 2045 L, AC 200

Nov 16 Maxon 275 HL nm L, US 200

1938 nd Inglesby movie #7* USHS nm R, AC >510

Jul 29 Clover 2:14:14 UU 2838 L, US 360

1939 Jul 15 Gibson movie† OP nm L, AC 220
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

1940 Aug 17 Goldwater CR 24 CCP 2657 L, AC 71

Aug 17 Goldwater CR 57 CCP 2742 L, DS 71

Aug 17 Goldwater CR 34 CCP 2659 L, UC 71

1941 May 19 McKee 1348 NAU 2689b T, AC nd

May 19 McKee 1349 NAU 2689a T, AC nd

Jul 27 Heald 3:6:6 UU 2741 L, DS 580

1942 Jul 26 Wilson 4:8:11 UU 2660a L, AC 460

Jul 26 Wilson 4:6:8 UU 2660b L, AC 460

Jul 26 Wilson 4:12:5 UU 2834 L, AC 460

1947 Jul 22 Farquhar 477 GDCN HL nm R, DS 690

179.2.2

Jul 27 Riffey 477 GDCN HL 1769 L, AC 520

179.444

 Jul 27 Marston 477 GDCN HL 1770 L, AC 520

179.2

 Jul 27 Marston 477 GDCN HL 1768 L, AC 520

179.12

Jul 27 Nevills 5:12:1 UU 2661 L, AC 520

1949 Jul 27 Anspach 497 GDCN HL 2004 L, AC 580

179.8

 Jul 27 Reilly L6-35 OP 2043 L, DS 580

1950 Jun 19 Belknap 48826 NPS 2772 L, DS 1,470

 Jun 19 Belknap 48833 NPS nm R, AC 1,470

 Jun 19 Belknap 48839 NPS nm R, AC 1,470

 Jun 19 Belknap 48841 NPS 803 R, AC 1,470

 Jul 25 Reilly R01-11 OP 2046 L, US 470

1951 Jul Litton nd OP 967 T, DS 2,260

Jul Litton nd OP 969 T, DS 2,260

Jul Litton nd OP 2959 R, DS 2,260

1951 Sep Eden 2082 NPS 969 T, DS >280

Sep Eden 2085 NPS 967 T, DS >280

 Sep Eden 2087 NPS 967 T, DS >280

1952 Jul Nichols nd OP 2935 L, DS 1,130

Jul Nichols nd OP 2957 L, DS 1,130

Jul Nichols nd OP 2969 L, AC 1,130

Jul Litton nd OP 967 T, DS 1,130

Jul Litton nd OP 2968 L, AC 1,130

Oct 22 Leding 2359 NPS 969 T, DS 164

 Oct 22 Leding 2360 NPS 969 T, DS 164

1953 Jul Beckwith nd OP 2971a L, US 708

Jul Beckwith nd OP 2971b L, DS 708

1954 Jun 14 Visbak JV VI 5 HL 2961 R, DS 360

Jul Nichols nd OP 2966 R, DS >990

Aug 29 Visbak nd HL nm R, DS 133
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

1955 Mar 19 Reilly L11-13 OP nm AR, DS 540

 Mar 19 Reilly L11-14 OP nm AL, AC 540

Mar 19 Reilly L11-15 OP nm AR, DS 540

 Mar 19 Reilly L11-16 OP nm AR, AC 540

 Mar 21 Reilly L12-13 OP nm AV 400

 Mar 21 Reilly L12-14 OP nm AR, US 400

 Mar 21 Reilly L12-15 OP nm AV 400

Apr 29 Beer nd OP 2972 L, AC 430

Apr 29 Beer nd OP 2934 L, US 430

Apr 29 Beer movie§ OP nm L, AC 430

Jul 20 Beckwith nd HL nm L, AC 209

Sep 3 Dudziak v. 83, p. 36 HL nm T, DS nd

 Oct Hamilton 8352 NPS 969 T, DS 114

 Oct Hamilton 8353 NPS 969 T, DS 114

 Oct Hamilton 5605 NPS 969 T, DS 114

1956 Mar 25 Marston 563 GDCN HL 969 T, DS 181

179.8

 Mar 25 Reilly L19-26 OP nm AR, DS 181

 Mar 25 Reilly L19-27 OP nm AL, AC 181

 Mar 25 Reilly L19-28 OP nm AL, US 181

Mar 25 Reilly L19-33 OP 967 T, DS 181

Mar 25 Reilly L19-34 OP 967 T, DS 181

1956 Mar 25 Reilly L19-35 OP 967 T, DS 181

Apr 16 Reilly L24-2 OP nm AR, US 297

Apr 16 Reilly L24-7 OP 969 T, DS 297

Apr 16 Reilly G-875 OP 969 T, DS 297

Apr 16 Reilly G-164 OP 967 T, DS 297

 Sep 29 Reilly L26-14 OP nm AV 79

 Sep 29 Reilly L26-15 OP nm AV 79

 Sep 29 Reilly L26-16 OP nm AR, US 79

1957 Apr 14 Reilly L28-29 OP nm AL, AC 249

 Apr 14 Reilly L28-30 OP nm AL, AC 249

 May 4 Reilly L29-23 OP nm AR, DS 521

 May 4 Reilly L29-24 OP nm AL, DS 521

 May 4 Reilly L29-25 OP nm AL, AC 521

 May 4 Reilly L29-26 OP nm AL, AC 521

 May 4 Reilly L29-27 OP nm AR, DS 521

 May 4 Reilly L29-28 OP nm AR, AC 521

Jul 13 Nichols nd OP 2932 L, DS 1,980

 Jul 13 Beckwith II-17 HL 1586 L, DS 1,980

Aug 29 Butchart nd HL nm L, AC 589

1958 Apr 20 Reilly L37-33 OP nm AL, AC 708

Apr 20 Reilly L37-34 OP nm AL, AC 708

May 24 McCullough 92.12.808 NAU nm L, DS 2,120
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

May 24 McCullough NAU nm L, DS 2,120

Jun 1 Reilly L40-8 OP 969 T, DS 3,000

Jun 1 Reilly L40-10 OP 969 T, DS 3,000

Jun 1 Reilly L40-13 OP nm AV 3,000

Jun 1 Reilly L40-17 OP nm AR, DS 3,000

Jun 1 Reilly L40-20 OP nm AR, DS 3,000

Jul 20 Staveley nd HL 2002 L, DS 218

 Oct 4 Reilly L41-18 OP nm AL, DS 187

 Oct 4 Reilly L41-19 OP nm AL, DS 187

 Oct 4 Reilly L41-20 OP nm AR, US 187

 Oct 4 Reilly G-407 OP nm AR, US 187

 Oct 4 Reilly G-415 OP nm AL, DS 187

 Oct 4 Reilly G-417 OP nm AR, US 187

1959 Aug 21 Dodge 8346 NPS 967 T, DS 224

1960 Jun 23 Marston 606 GDCN HL 1585 L, DS 1,140

179.2.19

 Jun 23 Marston 48832 NPS 1587b L, DS 1,140

 Jun 23 Marston 606 GDCN HL 1588 L, DS 1,140

179.18.10

Jun 23 Marston 606 GDCN HL 1587a L, DS 1,140

179.2.14

Oct 2 Reilly L48-32 OP nm AR, AC 125

 Oct 2 Reilly L48-33 OP nm AL, AC 125

Oct 2 Reilly L48-34 OP nm AR, AC 125

1961 Oct Jones 46 OP 2967 L, AC 57

1962 May 24 McCullough 217 NAU 2964b L, DS 1,440

May 24 McCullough nd NAU nm R, US 1,440

Jul 10 Reilly L58-3 OP 2003 L, AC 1,130

Jul 10 Reilly L58-4 OP 2835 L, DS 1,130

 Nov 3 Reilly L61-2 OP nm AR, DS 225

 Nov 3 Reilly L61-4 OP nm AL, AC 225

 Nov 3 Reilly L61-5 OP nm AL, US 225

 Nov 4 Reilly R74-8 OP nm AL, AC 225

1963 May Wieland 24 HL 969 T, DS 37

 Aug 22 Belknap 48822 NPS nm AL, US 57

 Aug 22 Belknap 48823 NPS nm AL, US 57

 Aug 24 Belknap 48858 NPS 2746 R, AC 57

 Aug 24 Belknap 48865 NPS 2746 R, AC 57

 Aug 24 Belknap 48824 NPS 969 R, DS 57

 Aug 25 Belknap 48866 NPS 803 R, AC 57

 Sep 25 nd 63-9-25 HL 1589 R, DS 35

GDCN 179-25
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

 Sep 25 nd 63-9-25 HL 2005 R, US 35

GDCN 179-28

1964 Apr 6 Visbak 34 HL nm AV 40

 Apr 6 Visbak 36 HL nm AR, DS 405

 May 12 Reilly L70-32 OP 2044 L, AC 348

May 20 Reilly L71-26 OP nm AL, US 37

 May 20 Reilly L71-28 OP nm AR, DS 37

 May 20 Reilly L71-29 OP nm AR, US 37

1964 May 20 Reilly L71-30 OP nm AR, US 37

May 20 Reilly L71-32 OP nm AL, US 37

 May 20 Reilly L71-33 OP nm AL, AC 37

 May 20 Reilly L71-34 OP nm AL, AC 37

 May 20 Reilly L71-36 OP nm AR, US 37

1965 Feb 21 Visbak 16 HL 1592 R, DS 200

 Feb 21 Visbak 24 HL 3050 R, US 200

 Feb 21 Visbak 14 HL 967 T, DS 200

 Feb 22 Visbak 36 HL 969 R, DS 200

May 18 USGS 83** USGS nm AV 792

May 18 USGS 84** USGS nm AV 792

May 21 Harris 2 OP 3010 L, DS 792

May 21 Harris 3 OP 2836 L, AC 792

1966 May 19 Hertzog 6525NA BOR 969 T, DS 340

Jul Belknap 83(157) NPS nm R, DS 200

1967 Apr 30 Harris P557-400- OP 2739 L, AC 280

896

Aug Luepke nd OP 2964b L, DS nd

Aug Luepke nd OP 2964b L, DS nd

1968 May 12 McCullough nd NAU 2964b L, DS 340

May 12 McCullough nd NAU nm R, US 340

Sep 8 Billingsley 195 OP nm L, US 140-280

Sep 26 Stephens 515# OP nm R, US 280

 Sep 26 Stephens 693# OP nm R, AC 280

 Sep 26 Stephens 597# OP 2744 R, DS 280

1969 Jul Harris 4 OP 2740b L, AC 570-850

 Jul Harris 1 OP 2740a L, AC 570-850

Sep 14 Marston 69-9-14 HL nm L, DS 280

GDCN 179-5

1970 Oct Mooz 16 HL nm L, US 140-370

1971 Apr 25 Billinsley nd OP nm R, DS 140

Jun 12 Billingsley nd OP nm L, US 140-280

Jul 9 Billingsley nd OP nm R, DS 140

1972 Jun 12 Billingsley nd OP nm R, US nd

Aug 5 Billingsley nd OP nm R, US 140

Sep 28 Billingsley nd OP nm R, US 140
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

nd Litton nd OP 2677 R, AC 280-424

1972 nd Litton nd OP 2962 R, US 280-424

nd Litton nd OP 2963 R, US 280-424

1973 May 18 McCullough 191 NAU 969 T, DS nd

Jun Billingsley nd OP nm R, AC nd

Jun 19 USGS 516** USGS nm AV 390

 Jun 19 USGS 517** USGS nm AV 390

1974 Jun Billingsley nd OP nm R, AC nd

Aug Billingsley nd OP nm R, AC nd

Aug Billingsley nd OP nm R, AC nd

Sep 20 Billingsley nd OP nm R, AC nd

1975 May Billingsley nd OP nm R, AC 140

Sep Billingsley nd OP nm R, AC 140

1976 Apr 29 Billingsley nd OP nm R, AC 140

Sep 26 Turner 803 OP 803 R, AC 99

1977 nd Brown nd OP 2837 R, US <85

nd Brown nd OP 2907 R, US <85

nd Brown nd OP nm R, US <85

1979 Jun 10 Turner 967 OP 967 T, DS 230

 Jun 10 Turner 969 OP 969 T, DS 230

1982 Jun 8 BOR 2-209 BOR nm AV 200

1983 Oct 31 Turner 803 OP 803 R, AC 765

1984 Oct 23 BOR 6-48** BOR nm AV 140

Oct 23 BOR 6-49** BOR nm AV 140

1994 Jun 1 BOR 102-9 BOR nm AV 234

Jun 1 BOR 102-10 BOR nm AV 234

1995 May 30 BOR 102-9 BOR nm AV 263

1995 May 30 BOR 102-10 BOR nm AV 263

 
APPENDIX 1. HISTORICAL PHOTOGRAPHS OF LAVA FALLS RAPID AND

THE DEBRIS FAN AT THE MOUTH OF PROSPECT CANYON (contd)

 

Photo- Original Original Stake Side, Discharge

Year Date grapher number source number direction (m3/s)

 

 Notes: (BOR), Bureau of Reclamation; (CCP), Center for Creative Photography; (HL), the Huntington

Library; (NA), the National Archives; (NAU), Special Collections, Northern Arizona University; (NPS), Grand

Canyon National Park; (NYPL), New York Public Library; (OP), original photographer; (UCB), Bancroft

Library, University of California at Berkeley; (USGS), the U.S. Geological Survey Photographic Library;

(USHS), Utah State Historical Society; (UU), Special Collections, University of Utah. Stake number refers

to the permanent file number in the repeat photography collection at the Desert Laboratory in Tucson,

Arizona. (AV), vertical aerial photography; (AL), photograph is an oblique aerial taken from the left side of

the river; (AR), photograph is an oblique aerial taken from the right side of the river; (T), photograph was

shot from Toroweap overlook; (L), photograph was shot from river left; (R), photograph was shot from riverright; (US), upstream view; (DS), downstream view; (AC), view across the river; (UC), view looking up the

tributary channel or away from the river; (nd), no data; (nm), the photograph was analyzed but not matched;

(na), not applicable. For photographs taken before 1921, the discharge is estimated from known stagedischarge relations. In particular, see Kieffer (1988) for general stage-discharge relations for the rapid

between 142 and 2,610 m3/s. For photographs taken between 1921 and 1963, discharge is estimated

from daily discharge records of the previous day for the Colorado River near Grand Canyon. For unknown

dates and after 1963, discharge is estimated from known stage-discharge relations. These estimates are

accurate to ±30 m3/s.

 *The movie, titled “Hiking to Lava Falls,” was shot by Dr. Inglesby in 1938 (Utah State Historical Society).

The movie shows panning shots of the rapids and closeups of the waves.

 †The movie was shot by Bill Gibson on the Harris-Loper river trip of July 1939.

 §The movie was shot by John Daggett and Bill Beer in April 1955.

 #The photographs are matches of the Hillers’ views of the same number; see Stephens and Shoemaker

(1987).

 **These vertical aerial photographs are stereo pairs.
APPENDIX 2. PRECIPITATION STATIONS NEAR THE

PROSPECT CREEK DRAINAGE

 

 

Distance†

from Mean Annual

Elevation Record Prospect Precipitation Percent Percent

Station Name* (m) Length Creek (km) (mm) Summer Winter

 

Bright Angel RS 2,726 7/48-3/95 100 646 29 60

Grand Canyon 2,204 10/04-3/95 90 403 42 46

Mount Trumbul 1,818 10/20-12/78§ 51 297 49 37

Peach Springs 1,613 7/48-3/95 56 280 45 43

Phantom Ranc 834 8/66-3/95 96 234 39 49

Seligman 1,704 12/04-3/95 75 293 49 40

Supai 1,039 6/56-2/87§ 50 216 46 41

Tuweep RS 1,551 7/48-12/86# 40 306 42 43

 

Notes:

* All stations are in Arizona.

†Distance is to the centroid of the Prospect Creek drainage basin.

§Station discontinued.

#In 1986, Tuweep Ranger Station was discontinued as a cooperative observer station, which records

rainfall in 0.01 in. accuracy and reports in increments of daily rainfall. A tipping-bucket recording rain gage,

which records rainfall in 0.10 in. increments and reports hourly as well as daily rainfall (e.g., U.S.

Department of Commerce, 1966), remains in operation.
APPENDIX 3. DAILY PRECIPITATION NEAR PROSPECT CANYON DURING KNOWN OR PROBABLE

DATES OF PROSPECT CANYON DEBRIS FLOWS AND FLOODS

 

PRECIPITATION (mm)

 

RECURRENCE

DATE IN SEPTEMBER 1939* INTERVAL (yrs)

Climate Station 3 4 5 6 Total 1-Day† Storm§

Seligman 10 39 38 25 112 4 53

Mount Trumbull 0 33 9 52 94 15 20

Grand Canyon 0 24 42 32 98 7 158

Bright Angel RS# 0 41 69 39 149 18 81

Climate Station 10 11 12 13 Total 1-Day† Storm§

Seligman 0 0 43 12 55 5 4

Mount Trumbull 9 42 47 0 89 7 33

Grand Canyon 23 25 13 12 73 <1 9

Bright Angel RS# 21 21 33 26 101 2 18

 

DATE IN JULY 1954

Climate Station 24 25 Total 1-Day† Storm§

Seligman 0 2 2 <1 na

Peach Springs 0 3 3 <1 na

Tuweep RS 25 0 25 1 na

Mount Trumbull 25 6 31 <1 <1

Grand Canyon 27 0 27 1 na

Bright Angel RS 0 27 27 <1 na

 

DATE IN JULY 1955

Climate Station 24 25 Total 1-Day† Storm§

 

Seligman 3 6 9 <1 <1

Peach Springs 26 0 26 1 na

Tuweep RS 10 11 21 <1 1

Mount Trumbull 111 1 112 100 100

Grand Canyon 7 15 22 <1 <1

Bright Angel RS 15 8 23 <1 <1
APPENDIX 3. DAILY PRECIPITATION NEAR PROSPECT CANYON DURING KNOWN OR PROBABLE

DATES OF PROSPECT CANYON DEBRIS FLOWS AND FLOODS (continued)

 

PRECIPITATION (mm)

 

RECURRENCE

DATE IN JULY 1956 INTERVAL (yrs)

Climate Station 29 30 31 Total 1-Day† Storm§

Seligman 0 0 37 37 3 na

Peach Springs 0 4 0 4 4 na

Tuweep RS 13 1 0 14 <1 <1

Mount Trumbull 10 8 0 18 <1 <1

Supai 0 0 12 12 <1 na

Grand Canyon 0 9 6 15 <1 <1

Bright Angel RS 0 0 0 0 na na

 

DATE IN SEPTEMBER 1963**

Climate Station 16-17 18 19 20 Total 1-Day† Storm§

Seligman 0 19 0 0 19 <1 na

Peach Springs 0 21 0 0 21 <1 na

Tuweep RS 0 0 0 19 19 <1 na

Mount Trumbull 23 0 0 0 23 <1 <1

Supai 0 12 0 0 12 <1 na

Grand Canyon 0 9 0 0 9 <1 na

Bright Angel RS 0 22 0 0 22 <1 na

 

DATE IN DECEMBER 1966

Climate Station 3-4 5 6 7 Total 1-Day† Storm§

Seligman nd 27 8 7 42 2 5

Peach Springs 9 16 11 14 50 <1 14

Tuweep RS†† 25 35 91 2 154 63 63

Mount Trumbull 17 nd 38 19 74 34 21

Grand Canyon 37 43 26 13 111 16 160

Phantom Ranch 27 9 8 53 97 4 11

Bright Angel RS -- -- -- -- 280-305 §§ na 57
APPENDIX 3. DAILY PRECIPITATION NEAR PROSPECT CANYON DURING KNOWN OR PROBABLE

DATES OF PROSPECT CANYON DEBRIS FLOWS AND FLOODS (continued)

 

PRECIPITATION (mm)

 

RECURRENCE

DATE IN FEBRUARY 1993 INTERVAL (yrs)

Climate Station 8 9 10 Total 1-Day† Storm§

 

Seligman 24 24 6 54 2 10

Peach Springs 14 42 0 58 32 32

Grand Canyon 12 10 1 23 <1 <1

Phantom Ranch 7 4 0 11 <1 <1

Bright Angel RS nd 24 23 47 <1 <1

Tuweep RS 25 0 0 25 2 na

 

DATE IN MARCH 1995

Climate Station 4 5 6 Total 1-Day† Storm§

 

Seligman 0 0 13 13 <1 na

Peach Springs 0 19 0 19 1 na

Grand Canyon 5 24 37 66 7 3

Tuweep RS 0 0 43 43 5 na

Bright Angel RS 18 48 78 144 4 4

 

Notes: Data on the climate stations are given in Appendix 2.

na -- not applicable. 

*The precipitation is associated with an unnamed tropical cyclone (Smith, 1986).

†Recurrence interval of 1-day storms is for summer (Jul-Oct) or winter (Nov-March) months.

§Recurrence interval for storms is for total rainfall on consecutive days for summer (Jul-Oct) or winter (NovMarch) months.

#From Gatewood and others (1946).

**The precipitation was probably related to Tropical Storm Katherine (Smith, 1986).

††From U.S. Department of Commerce (1966).

§§Cooley and others (1977) estimated this value from total precipitation at another station. Using multiple

regression on 9 independent December storms recorded at this station and Grand Canyon and Tuweep

RS, we estimated a total storm precipitation of 234 mm, which has the same recurrence interval.
APPENDIX 4. SOURCE AND SIZE OF BOULDERS TRANSPORTED

BY DEBRIS FLOWS IN PROSPECT CANYON.

 

 

Source DIAMETER (m)

Type of of Volume Weight

deposit boulder a-axis b-axis c-axis Shape (m3) (Mg)

 

AD 1434 B 3.7 2.7 2.4 E 13 34

debris B 2.3 2.0 1.7 E 4.1 11

flow B 3.3 1.6 1.2 E 3.3 9

(tif) B 2.6 1.5 1.3 E 2.7 7.2

B 1.9 1.4 1.3 E 1.8 4.9

SU 2.2 1.5 1.0 E 1.7 4.6

B 2.0 1.0 0.8 E 0.8 2.3

B 1.5 1.0 1.0 E 0.8 2.1

B 1.0 1.0 1.0 E 0.5 1.4

RW 1.0 0.9 0.8 E 0.4 1.0

AD 1939 B 3.3 2.6 2.5 E 11.2 30

debris B 3.2 2.7 1.9 E 8.6 23

flow W 2.9 2.3 1.8 E 6.3 16

(tig) W 3.6 1.5 1.4 E 3.9 10

RW 2.3 2.0 1.6 E 3.9 10

B 2.4 2.0 1.6 E 3.0 8.1

B 1.8 1.8 1.6 E 2.7 7.3

RW 1.8 1.3 1.0 E 1.2 3.3

B 1.6 1.2 1.2 E 1.2 3.3

B 1.7 1.3 1.0 E 1.2 3.2

AD 1995 B 1.7 1.1 0.9 EC 1.4 3.7

debris B 1.3 1.1 0.9 R 1.3 3.5

flow B 1.6 1.6 0.6 EC 1.2 3.2

(tij) W 2.4 1.1 0.8 E 1.1 2.8

B 1.8 1.1 0.8 E 0.8 2.3

M 1.4 0.8 0.7 R 0.8 2.0

B 1.5 0.8 0.5 R 0.6 1.6

B 1.3 1.1 0.8 T 0.6 1.5

B 1.3 0.9 0.9 E 0.6 1.5

B 1.0 0.9 0.7 R 0.6 1.5
APPENDIX 4. SOURCE AND SIZE OF BOULDERS TRANSPORTED

BY DEBRIS FLOWS IN PROSPECT CANYON.

 

 

Source DIAMETER (m)

Type of of Volume Weight

deposit boulder a-axis b-axis c-axis Shape (m3) (Mg)

 

Reworked B 3.2 3.0 1.6 E 8.3 22

debris B 2.9 1.8 1.4 E 7.3 20

fan B 3.9 2.2 1.6 E 7.2 19

(rwr) B 3.9 1.8 1.7 E 6.2 17

B 2.8 1.7 1.6 T 3.8 10

B 2.4 1.7 1.6 E 3.5 9.5

B 3.7 1.9 0.9 T 3.4 9.1

B 2.5 1.4 0.9 E 3.3 8.6

B 1.8 1.7 1.4 H 2.5 6.7

B 2.4 1.2 0.8 T 1.1 3.0

Channel W 8 7 4.4 R 250 640

of B 6.7 4.8 4.3 E 72 195

Prospect B 10 7 3.8 SE 70 190

Canyon B 4.8 4.5 4.3 E 49 131

(rwr) B 4.7 3.9 2.1 EC 30 82

B 5.0 3.4 3.1 E 28 74

B 6.5 4.9 2.8 SE 23 63

B 3.6 2.7 2.1 R 20 55

B 4.0 3.4 2.9 T 20 53

B 4.5 3.2 2.5 E 19 51

 

 

 Notes: Only the ten largest boulders transported during the indicated debris flow are listed in this table. For

descriptions of the source rocks, see Huntoon and others (1986). W -- welded tuff (2.60 Mg/m3), B -- basalt

(2.70 Mg/m3), S -- Supai sandstone (2.65 Mg/m3), SU -- Surprise Canyon Formation (2.65 Mg/m3), RW --

Redwall Limestone (2.65 Mg/m3), M -- Muav Limestone (2.65 Mg/m3). Boulder axes are based on idealized

geometric shapes: E -- ellipsoid, EC -- elliptical cylinder, SE -- semi-ellipsoid, T -- triangular solid, R --

rectangular solid. a-axis, the longest dimension of the boulder; b-axis, the intermediate dimension of the

boulder; and c-axis, the shortest dimension of the boulder.