U.S. DEPARTMENT OF INTERIOR 

U.S. GEOLOGICAL SURVEY

Dating Prehistoric Tributary Debris Fans, 

Colorado River, Grand Canyon National Park, 

Arizona, with Implications for Channel Evolution 

and River Navigability

OPEN-FILE REPORT 97-167

Prepared in cooperation with the 

BUREAU OF RECLAMATION

Flagstaff, Arizona 

1997

This report is preliminary and has not been edited for conformity with U.S. Geological Survey editorial standards or the 

North American Code of Stratigraphic Nomenclature. Any use of trade, product, or firm names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government.
Frontispiece

View upstream of the Colorado River in eastern Grand Canyon. Distant 

skyline in upper right is Palisades of the Desert. Vegetated area in lower 

right is the prehistoric debris fan of Comanche Creek and Espejo Creek.

Copies of this report can be purchased from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, Colorado 80225
CONTENTS

Abstract 1 

Introduction 1

Previous studies of Grand Canyon debris flows and debris fans 4 

Methods 4

Summary of Depth Measurements 6 

Rate of Pit Deepening 7

Calculation of pit deepening rate 9 

Age and Correlation of Prehistoric Fan Surfaces 11 

Discussion and Conclusions 13

Implications for channel evolution and river navigability 14 

Acknowledgments 15 

References 15

Figures

1. Study area 2

2. Photograph of Palisades Creek debris fan 3

3. Box-plot statistical summary 7

4. Standard deviation of depth measurements 7

5. Calibration of pit deepening rate 8

6. Calculated pitting rates 8

7. Age and correlation chart 12

8. Density trace of average pit depth 13

Tables

1. Results of ANOVA 6

Open-File Report 97-167
Dating Prehistoric Tributary Debris Fans, Colorado River, 

Grand Canyon National Park, Arizona, with 

Implications for Channel Evolution and River Navigability

by Richard Hereford, Kathryn S. Thompson, and Kelly J. Burke

Abstract

The ages of prehistoric debris-fan surfaces and 

related debris fans are relevant to long-term channel 

management of the Colorado River in Grand Canyon 

National Park, Arizona. The debris fans formed by deposition of coarse-grained sediment in the river channel 

by fan-forming debris flows, which were substantially 

larger than most recent debris flows. These large debris 

flows were capable of severely constricting the mainstem channel, resulting in rapids that would probably 

have been unnavigable or waterfalls in extreme cases. 

There is no reason to believe that debris flows of this 

size will not occur in the future. At any particular tributary, fan-forming debris flows tend to happen every 850 

years on average. The last canyon-wide episode of 

fan-forming debris flows was around 790 cal yr B.P. 

Thus, it is possible that another episode of increased 

debris-flow activity will ensue within 60 years. The age 

of debris-fan surfaces in Grand Canyon can be determined only rarely by conventional methods. For this reason, we developed a technique for dating surfaces based 

on the time-calibrated dissolution of carbonate boulders, 

which are abundant in debris-flow sediment. Weakly 

acidified rainfall and the metabolic activity of 

blue-green algae form small, roughly hemispheric dissolution pits on the initially smooth, exposed surfaces of 

carbonate boulders. The average depth of dissolution 

pits increases systematically with relative age of fan surfaces. The deepening rate, as measured by the depth of 

pitting, averages 2.54 ± 0.40 mm/ka (n = 7; ka = 1,000 

years), which was calculated from several radiometrically dated surfaces and an archeologic structure. The 

calculated rate is a linear function of time over at least 

the past 3 ka, consistent with the process of carbonate 

dissolution and with dissolution rates determined from 

carbonate tombstones and other archeologic structures. 

Measurements (n = 6,973) of pit depth were made on 

617 boulders on 71 fan surfaces at the 26 largest debris 

fans in Grand Canyon. Average pit depth for all surfaces 

ranges from 1.2 to 17.42 mm, which indicates that surfaces in Grand Canyon range in age from 500 to 7,000 

cal yr B.P. Most fan surfaces (75 percent) are younger 

than 2.8 ka, probably reflecting removal of older debris 

fans by the Colorado River. The distribution of calculated ages throughout Grand Canyon has five clusters: 

790, 1460, 2360, 2950, and 3820 cal yr B.P. These clusters may coincide with periods of increased debris-flow 

activity.

Introduction

This report addresses the ages of prehistoric (or late Holocene) debris fans along the 

Colorado River in Grand Canyon (fig. 1). The 

results of this study have implications for longterm channel management and navigability of 

the river. Composed of poorly sorted, bouldery 

sediment, segmented debris fans with multiple, 

mostly prehistoric, surfaces (Hereford and others, 1996a) are ubiquitous in Grand Canyon at 

the mouths of tributary streams (Hamblin and 

Rigby, 1968). We estimate the ages of these 

prehistoric surfaces and correlate debris-flow 

deposits using a quantitative dating technique 

based on time-calibrated dissolution of carbonate boulders.

Correlation of debris-fan surfaces along 

the Colorado River in Grand Canyon is important for identifying periods of increased debrisflow activity. Increased activity may correspond with unusual climate conditions, particularly increased precipitation. In addition, the 

age distribution of fans is important for understanding erosional removal of debris fans by 

the Colorado River in the natural discharge 

regimen. The age distribution of prehistoric 

fans in Grand Canyon partly reflects the longterm interaction between the Colorado River 

and the sediment output of tributary streams. 

The long-term effect of the river is to erode, 

reshape, and remove debris fans from the 

channel. This erosion was severely reduced by 

virtual elimination of natural floods with closure of Glen Canyon Dam in 1963. Thus, 

while geomorphically significant mainstem 

floods have been eliminated tributary debris 

flows continue unabated.

Open-File Report 97-167
LAKE MEAD

NATIONAL

RECREATION

AREA

-36°

113 112°

Explanation

O Tri butary debris fan study sites

1 24.5 Left

2 Little Nankoweap Creek

3 Nankoweap Creek 

4 53 1 Left

5 Kwagunt Creek 

6 Palisades Creek

7 Espejo Creek

8 Comanche Creek

9 Tanner Creek

10 69.6 Left

11 Basalt Canyon 

12 Cardenas Creek

13 Unkar Creek

[Too] River mile, starting at Lees Ferry, Arizona

14 Monument Creek

15 Crystal Creek

16 Forster Canyon

17 124.2 Left 

18 Fossil Canyon

19 Prospect Canyon

20 195 Left

21 195.5 Left

22 196 Right 

23 208.7 Left

24 209 Mile Canyon 

25 Granite Park Canyon

26 220 Mile Canyon

Figure 1. Study area, Grand Canyon National Park, Arizona.

Fans are the largest and perhaps most 

important depositional feature along the floor 

of the Grand Canyon. The size and shape of 

the fans control the course of the river between 

bedrock walls and the location and severity of 

whitewater rapids (fig. 2; Stevens, 1990). In 

addition, important sites of fluvial deposition 

develop in zones of low current velocity 

upstream, downstream, and along the margins 

of debris fans (Howard and Dolan, 1981; 

Schmidt, 1990; Schmidt and Graf, 1990). The 

fluvial deposits form terraces that were farmed 

and occupied prehistorically by the Anasazi,

Open-File Report 97-167

ancestors of the Hopi Indians of northern Arizona. The debris fans were also used by the 

Anasazi (Fairley and others, 1994), and the 

largest archeologic site in Grand Canyon is on 

the Unkar Creek debris fan (fig. 1; Schwartz 

and others, 1980).

Although widely recognized as important 

geomorphic elements of the river system, the 

age and correlation of prehistoric debris fans 

have not been studied on a canyon-wide basis. 

Fan surfaces and deposits are difficult to date 

because organic material is rare in the coarsegrained deposits. Dated archeologic sites are
Figure 2. Photograph of Palisades Creek debris fan, a typical late Holocene fan in eastern Grand Canyon. 

Fan is bisected by entrenched, historically active channel with channelized debris-flow deposits; relatively 

dark area is the inactive, primary fan surface consisting of younger, intermediate-age, and older fan-forming surfaces discussed in text. Position of Colorado River against right side of canyon and location of Lava 

Canyon Rapids are controlled by bouldery sediment of debris fan.

present on fan surfaces locally, but sites provide only minimum ages. The relation of 

debris fans to dated fluvial deposits, however, 

constrains the age of the fans along the river to 

the late Holocene (Hereford and others, 

1996a).

In this report, the ages of debris-fan surfaces are estimated using a simple calibration 

function that relates surface age to the average 

depth of dissolution pits on carbonate boulders; the estimated ages are mostly less than 

2.8 ka. Dissolution pits result from weakly 

acidified rainfall and the metabolic activity of 

endolithic cyanobacteria. Although some controversy surrounds the significance of biogenic 

weathering (Cooke and others, 1993, p. 44), a 

major cause of dissolution pits may be the 

release of excess CO2 following rainfallinduced photosynthesis (Danin, 1983; Danin 

and Garty, 1983). Regardless, atmospheric or

metabolic CO2 combines with water to form 

carbonic acid (H2CO3 ) that slowly etches the 

surfaces of carbonate boulders.

The depth of dissolution pits was time-calibrated using several dated fan surfaces. A 

deepening rate of 2.54 ± 0.40 mm/ka was calculated from the average depth of dissolution 

pits on carbonate boulders on the dated fan 

surfaces. Using this calibration, the age of any 

weathered fan surface in Grand Canyon can be 

estimated. In addition, the age of walls and 

other structures built of carbonate stones by 

prehistoric people can be estimated in the 

absence of temporally diagnostic potsherds. 

Generally, the surface exposure age of any 

mid- to late Holocene deposit with unmetamorphosed carbonate boulders can be estimated by this method if the local carbonate 

dissolution rate is known.

Open-File Report 97-167
Previous studies of Grand Canyon debris 

flows and debris fans

Studies of modern depositional activity 

demonstrate that debris flow is the main process of sediment transport in tributaries of the 

Colorado River in Grand Canyon (Webb and 

others, 1989; Melis and Webb, 1993; Melis 

and others, 1995). Debris flows originate in the 

steep headwaters of tributaries following prolonged or intense rainfall. Failure of partly saturated, talus covered slopes initiates debris 

flows. The poorly rounded, bouldery sediment 

is moved downstream in a gravity-driven 

slurry where it is deposited on a debris fan in 

the channel of the Colorado River.

Prehistoric debris fans in eastern Grand 

Canyon were studied by Hereford (1996) and 

Hereford and others (1996a). The typical 

debris fan has two geomorphic elements: the 

relatively large surface of the fan itself which 

is inactive and mostly of prehistoric age, and 

the historically (post-A.D. 1890) active debrisflow channel which is substantially smaller 

and entrenched into the fan surface. The inactive surface of any particular fan is about 6 

times larger than the active channel. Median 

area of the inactive fan and active channels is 

4.6 and 1.2 ha, respectively. The median total 

area of the two elements for all fans in eastern 

Grand Canyon is 5.4 ha; individual fans range 

from 1 to 22.1 ha. This range of fan sizes is 

typical of the debris fans studied in this report, 

although the lack of large-scale topographic 

maps precludes area measurements canyonwide.

The typical fan in eastern Grand Canyon 

(fig. 2) and elsewhere is segmented into several surfaces of different ages distinguished by 

relative topographic position and degree of 

weathering (see fig. 3 in Hereford and others, 

1996a; and figs. 2-5 in Hereford and others, 

1996b). The surfaces parallel the underlying 

deposits and are contemporaneous with deposition; we informally refer to these as the older, 

intermediate, and younger fan-forming surfaces or deposits. The relative age of fan surOpen-File Report 97-167

faces corresponds to topographic position with 

the most elevated surface being the oldest. The 

younger fan-forming surfaces occupy the lowest position on the fans and are only slightly 

weathered in most cases. These surfaces were 

not differentiated from intermediate-age surfaces for this study. Only the oldest of the 

younger surfaces have carbonate boulders with 

shallow dissolution pits. Extremely young 

(between 75-500 cal yr B.P.) fan-forming surfaces without dissolution pits are uncommon, 

but we have mapped them at Nankoweap Rapids, Palisades Creek (fig. 1; Hereford and others 1996b; Hereford, 1996), and Granite Park. 

The intermediate-age and older surfaces are 

distinctly weathered with carbonate boulders 

having well-developed dissolution pits.

Clasts in the debris-flow deposits are 

derived from the more competent units of the 

Paleozoic and Proterozoic rocks exposed in the 

walls of Grand Canyon (Hereford and others, 

1996a, b). For the most part, the clasts are 

composed of limestone, dolomitic limestone, 

and sandstone. These rocks are unmetamorphosed and lack penetrating fractures or 

shears. For estimating the age of fan surfaces, 

we used carbonate boulders derived from the 

Kaibab Formation (Permian) and Redwall 

Limestone (Mississippian). These boulders are 

abundant in the debris-flow deposits, and their 

relative abundance is a function of drainage 

basin size and relief (Melis and others, 1995). 

Redwall Limestone is typically the most abundant lithology because the unit is thicker and 

closer to the river than the Kaibab Formation, 

which crops out on the rim of Grand Canyon 

(Huntoon and others, 1986).

Methods

Twenty-six tributary debris fans were 

studied along the Colorado River from River 

mile 24.5 to 220, almost the entire length of 

the river in Grand Canyon (fig. 1). The studied 

fans are only a small sample of the 529 tributaries (Melis and others, 1995) in Grand Can-
yon, but those studied are among the largest in 

terms of contributing basin area and area of the 

debris fan. Median basin area of the 26 tribu-

-^

taries is 35 km ; interquartile range is 3.7 to

-^

37.3 km ; and range of basin size is 0.4 to 257

km2 according to the morphometric data in 

Melis and others (1995, appendices 2, 3).

Low-altitude color aerial photographs 

(approximate scale 1:4,800) of the river corridor flown in 1993 were used to select debris 

fans for study. The fans typically have one or 

more elevated segments with relatively low 

albedo indicating rock varnish. The spatial distribution of the studied fans is not random. 

These large fans tend to occur where the canyon at river level is relatively wide. The sampled fans are probably representative of debris 

fans in the size range 1 to 22 ha, which is 

approximately the same size as those in eastern 

Grand Canyon.

At each fan, the number of fan segments 

was determined by examining the degree of 

weathering and subsurface soil development. 

Surfaces were mapped on the aerial photographs to delineate their spatial distribution 

and to locate measurement transects. The 

youngest of the fan-forming surfaces (mostly 

younger than about 500 cal yr B.P.) were not 

studied, as surface weathering is virtually 

absent and carbonate clasts are unpitted. intermediate-age and older fan-forming surfaces 

were identified on the presence of Stage I soilcarbonate morphology (Machette, 1985), 

which is slightly deeper and more fully developed on the older surfaces (see fig. 2 in Hereford and others, 1996b). In addition, the 

relative abundance and degree of disintegration 

of sandstone and siltstone clasts increases 

between the intermediate-age and older surfaces. Darkness of rock varnish on sandstone 

boulders also increases with age, and darkness 

correlates well with depth of dissolution pits 

(Hereford and others, 1996a). One or more 

transects were made on each fan surface.

Surfaces were studied above the height of

mainstem floods to eliminate erosional alteration of the surface. The preferred sample 

locality is usually upstream of the medial portion of the fan. A few sites, however, are 

debris-flow levees upstream of the fan head in 

the mouth of the tributary canyon. As discussed in a following section, surfaces in this 

setting are the oldest known to us. Small- to 

medium-size boulders (256-1012 mm) were 

measured. Boulders of this size are probably 

large enough to have been at the surface of the 

debris flow at the time of deposition. Weathering of the boulders began immediately after 

deposition rather than during subsequent erosional lowering of the surface.

The depth of dissolution pits on carbonate 

boulders was measured with a machinists 

depth micrometer with resolution of ±2.5 fim 

(±0.001 in). Ideally, the pit is a half-sphere, 

and the depth of the pit is essentially the radius 

of the sphere. The rock area or septa between 

the pits is assumed to be the initial, unweathered level of the surface of the boulder. The 

depth of individual pits on a single boulder 

varies considerably, particularly on the older 

surfaces. For this reason, the entire upper surface of the boulder was sampled with the number of measurements per boulder averaging 11.

Analysis of variance (ANOVA) was used 

to test homogeneity of pit depth among boulders in each transect. If the analysis showed 

that boulders of a particular transect were samples from the same population, then measurements from individual boulders were 

combined. The average depth and standard 

deviation of the combined data were assumed 

to be representative of the surface.

A number of transects, however, had one 

or more boulders with average depth that was 

significantly shallower or deeper than other 

boulders in the transect. Mixed populations of 

pit depths on a surface are possible and are 

expected in certain situations related to local 

contamination of the surface and from lithologic variation of carbonate boulders. A surface will locally have boulders of two ages

Open-File Report 97-167
Table 1. Statistics of pit-depth data showing changes after analysis of variance (ANOVA)

Transects 

Boulders

Depth 

Average depth (mm)

Standard deviation of 

depth (mm)

Total n 

before ANOVA

dfy & dfi dfo

53

299

3,322

3.11

1.41

61

318

3,651

6.54

2.70

Total n retained 

after ANOVA

dfi (%) dfo (%)

53(100) 61(100)

258 (86) 248 (78)

2,730 (82) 2,670 (73)

2.96 6.30

1 .28 2.33

where a younger debris flow overtops an older 

surface adjacent to the debris-flow channel. 

Large boulders of the older debris flow remain 

above the surface of the younger flow. These 

older boulders have been exposed longer and 

have slightly deeper average pit depth than the 

boulders of the younger debris flow.

In addition, the lithology of carbonate 

clasts may control weathering rate to some 

extent. We were not able to identify subtle differences in carbonate lithology because examination of fresh surfaces was not practical or 

appropriate in a National Park given the fragile 

character of debris-fan surfaces. For this reason, sampled boulders were not broken, overturned, or removed. This practice minimized 

damage to the fan surfaces, which are essentially pristine. Thus, boulders of dolomitic 

limestone, dolomite, and sandy dolomitic 

limestone were sampled inadvertently. These 

rocks probably have slightly different weathering rates than relatively pure limestone. Presence of such boulders on the fans is related to 

the source of the debris-flow sediment, which 

in turn is related to diagenetic and facies 

changes in the bedrock unit.

If ANOVA revealed mixed populations of 

measurements within a transect for reasons 

discussed above, then boulders with anomalous measurements were eliminated one-at-a 

time by repeated application of multiple-comOpen-File Report 97-167

parison ANOVA until a homogeneous group of 

boulders with statistically indistinguishable 

mean and variance was attained. Measurements from the remaining boulders were combined to form a single sample of the surface. 

This procedure simplifies further statistical 

analysis and comparison with other surfaces as 

the resulting data are normally distributed. In 

practical application, this data manipulation 

has little influence on conclusions regarding 

age and correlation of debris fans. The affect 

of dropping anomalous boulders from 

transects is summarized in Table 1. Overall, 

the average was mostly unaffected, but the 

standard deviation decreased.

Summary of Depth Measurements

The pit-depth data are summarized by surface age in figure 3. All individual measurements from the combined younger and 

intermediate-age category and from the older 

surfaces are shown in figure 3a. Pit depths on 

younger and intermediate-age surfaces range 

from 0.36 to 11.94 mm with average depth of 

2.96 ± 1.28 mm. On the older surfaces, depths 

range from 0.71 to 24.13 mm and average 

depth is 6.3 ± 2.33 mm. The substantial overlap of pit depth between the age categories 

shows that the two surfaces cannot be separated with only a few measurements. Individ-
25

20

1 15 

c

n = 2,671

- n = 2,730

10

T

n = 37

25

20

15

10

A.

I | Younger and 

' ' intermediate-age

B.

Older

Figure 3. Box-plot statistical summary of 

pit-depth measurements by age category. A) 

Full range of all individual measurements 

grouped by surface age, and B) Average pit 

depth of mapped surfaces. Box is interquartile 

range, horizontal line is median depth, capped 

vertical line is range of 5 to 95 percentiles, and 

solid circles are measurements and averages 

below and above the 5 and 95 percentiles, 

respectively.

05 '&

 a

 a

|

CO

s = 0.22(d) 

r2 = 0.86

0 4 8 12 16 20 

Average pit depth (d), in mm

Figure 4. Standard deviation of depth measurements as a function of average depth.

ual measurements overlap widely on surfaces 

known to be of different age based on degree of 

weathering of sandstone boulders, darkness of 

rock varnish, soil-carbonate morphology, and 

relative elevation.

Figure 3b shows the distribution of average 

depths of all sampled intermediate-age and older 

surfaces. Average pit depth of intermediate-age 

surfaces, which includes the oldest of the 

younger age category, ranges from 1.2 to 4.67 

mm; average pit depth of older surfaces ranges 

from 4.62 to 17.42 mm. This plot (fig. 3b) shows 

that intermediate-age and older surfaces are 

clearly differentiated on the basis of average pit 

depth. Variability of pit depth increases with age 

of the surface as shown in figure 4, which shows 

standard deviation of pit depth as a function of 

average depth without classification into intermediate-age and older categories. On average, 

the variability of depth increases by about 0.2 

times the average depth.

Rate of Pit Deepening

This section of the report derives the pitdeepening rate of carbonate boulders on debris 

fans in Grand Canyon from the depth of dissolution pits on independently dated surfaces. Three 

dated debris fans and a closely dated archeologic 

feature were used to estimate deepening rate. 

Figure 5 shows the mean value of the ages and 

pit depths, the variability of this data, and the 

estimated deepening rate, which is a linear function of time as discussed in a following section 

of the report.

Samples one and three are radiocarbon ages 

of debris-flow deposits. The location and stratigraphic context of these deposits and surfaces 

are described in Hereford (1996, table 1, localities 2, 3 and 4) and Hereford and others (1996a, 

fig. 10, table 3). The radiocarbon ages were calibrated to calendar years and plotted as calibrated 

years B.P. (cal yr B.P.). Measurements were 

made on the fan surface of the same deposits 

near the medial portion of the debris fan. Sample 

one is a radiocarbon-dated hearth penetrating the

Open-File Report 97-167
Figure 5 (opposite). Age of fan surfaces (nos. 1, 3, and 4) and an archeologic structure (no. 2) with 

corresponding average depth of dissolution pits (solid circles). Vertical bars 

are the ±2 standard deviation range of 

pit depth. Horizontal bars of numbers 

1 and 3 are the ±2 standard deviation 

range of calibrated radiometric dates. 

Number 2 horizontal bar is age range 

of archeologic structure made of carbonate boulders; number 4 horizontal

*j

bar is calculated uncertainty of He 

data (Thure E. Cerling, written commun., 1996).

10

E 

E

£ 8

0)

Q. 4 

0) 

O) 

CO

I 2

d = 0.00254(1)

x (3)n = 222 

y EXPLANATION

' n = no. of depth measure- 

(2} n = 144 merits on an average of 

11 boulders per surface 

= 180

1,000 2,000 3,000 4,000 5,000 

Exposure time (t), in cal yr B.P.

JC

^

E

_c

of

0)

1

Q.

Q

4

3

2

1

Ejg

EXPLANATION

* Measured rate

 Derived rate

a Derived-rate outlier

Average rate, 2.54

+/- 0.40 mm/ka

'

^ "

*

I . I , I

1,000 2,000 

Time, in cal yr

3,000

Figure 6 (opposite). Calculated pitting rates. 

Derived rates (solid squares) are the absolute differences of all paired combinations of ages and 

average pit depths.

surface of a younger intermediate-age deposit 

at the Palisades Creek debris fan. The age (840 

cal yr B.P., fig. 5) provides a minimum age of 

the debris-flow deposit. This age, however, is 

almost identical to another from just below the 

base of the deposit where the distal, fine 

grained facies of the debris flow is interbedded 

with alluvium of the Colorado River. This alluvium is older than about 750 cal yr B.P. based 

on archeologic remains (Hereford and others,

Open-File Report 97-167

1996a). The debris-flow deposit is near the top 

of the alluvium. The deposit, therefore, is 

somewhat older than 750 cal yr B.P, which is 

consistent with the radiocarbon ages.

Sample three is the median of two radiocarbon ages collected from just below and 

immediately above the distal facies of an older 

intermediate-age debris flow interbedded with 

alluvium of the Colorado River. The charcoal 

samples were collected in the subsurface
where the debris-flow deposits are interbedded 

with alluvium. The age of 1310 cal yr B.P. is 

consistent with the age of the underlying surface whose age is between 1250-1650 cal yr 

B.P. (Hereford and others, 1996a).

Sample two is from an archeologic feature 

near the head of Nankoweap Canyon (Hereford and others, 1996a). This archeologic 

structure is a small check dam built of Redwall 

Limestone boulders by the Kayenta Anasazi. 

Potsherds indicate clearly that the structure 

was built in Pueblo II time between 800-950 

cal yr B.P. (the error bar in fig. 5); construction 

age is assumed to be 875 cal yr B.P. The Anasazi collected the limestone boulders used in 

construction from the bed of a nearby wash, 

therefore, the boulders were not pitted at the 

time of construction.

Sample four is a cosmogenic He surface 

exposure age from basalt boulders (Robert H. 

Webb, 1995, oral commun.; Cerling and others, 1995) on the topographically highest and 

oldest surface on the Prospect Canyon debris 

fan in western Grand Canyon (fig. 1). The age 

of 2,900 cal yr B.P. is the oldest Holocene surface yet dated in Grand Canyon. Pitting of 

limestone boulders on this surface is well 

developed, although older fan surfaces with 

greater pit depths are not unusual elsewhere in 

Grand Canyon.

Calculation of pit deepening rate

Measured deepening rates and rates 

derived from the absolute differences among 

the measured data (fig. 5) are shown in figure 

6. The average deepening rate was calculated 

from the four measured and three derived 

rates, excluding the three derived-rate outliers. 

These rates are normally distributed with average of 2.54 ± 0.40 mm/ka (n = 7); the probability that the rates are not normally distributed is 

0.42. Rate of pit deepening is independent of 

time as indicated by linear regression and the 

lack of correlation between time and average 

depth. Slope of the regression line is 0; probability that slope is not 0 is only 0.18. Time and

pit deepening rate are unrelated, as the correlation coefficient is not significantly different 

from 0 with probability > 0.05. These results 

suggest that pit deepening rate of the carbonate 

rocks is constant and has not changed during 

the late Holocene or at least the last 3 ka.

Three of the derived rates are outliers (fig. 

6). The anomalous rates are 4.81, 4.79, and 4.7 

mm/ka. These rates are too large for the known 

age of intermediate-age fan surfaces (750- 

1,290 cal yr B.P., Hereford and others, 1996a) 

in eastern Grand Canyon. For example, if the 

rate was actually 5 mm/ka, these dated intermediate-age surfaces should have average 

depths ranging from 3.75 to 6.45 mm. About 

75 percent of intermediate-age surfaces, however, have average pit depth < 4 mm, and even 

the oldest intermediate-age surfaces have 

depths < 6.45 mm (fig. 3).

A variety of mathematical functions fit the 

data shown in figure 6, however, a linear function is conservative and is expected from physical processes of limestone weathering. A 

linear relation is also supported by weathering 

rates estimated from dated tombstones and 

archeologic structures made of carbonate 

stone. Weathering of limestone proceeds at a 

constant rate that varies only with climate and 

solubility of limestone (Colman, 1981), 

whereas weathering of silicate rocks decreases 

with time. Weathering of limestone is a congruent dissolution process which does not produce surface residues that eventually slow 

weathering. The carbonate rocks in debris fans 

of Grand Canyon are not pure limestone. Nevertheless, weathering of these rocks as measured by surface roughness and pitting appears 

to approximate a constant rate during at least 

the late Holocene (fig. 6).

Numerous studies address surface weathering and pitting of limestone and marble 

tombstones and archeologic structures as old 

as 2,600 cal yr B.P. (Danin, 1983; Meierding, 

1981; 1993; Klein, 1984; Dragovich, 1986; 

Neil, 1989; Cooke and others, 1995). The 

depth of dissolution pits is a linear function of

Open-File Report 97-167
time on well-dated limestone walls and monolithic tombs in Jerusalem up to 2,600 years old 

(Danin, 1983), a period comparable to the age 

of debris-fan surfaces in Grand Canyon. On a 

shorter time-scale, pitting of carbonate tombstones reveals a constant pitting rate, although 

rates vary widely within an area and from 

region-to-region. These studies, described 

below, report constant pitting rates, with the 

exception of Klein (1984) and Cooke and others (1995).

Klein (1984) questioned the application of 

linear regression for estimation of pitting rate 

and suggested an exponential function was 

more appropriate. Klein (1984) also suggested 

that changing weathering rates are related to 

the additional time needed to attack the polished surface of the tombstone. Cooke and others (1995) inferred that short-term pitting rates 

could increase with time, remain constant, 

decrease with time, or follow some combination of the three. In urban areas, for example, 

gradually increasing atmospheric pollution 

would increase weathering. Constant rate 

implies that the stone is in equilibrium with the 

environment. Decreasing rate suggests that the 

weathering process is self limiting, as shown 

by Colman (1981) for silicate rocks. It seems 

unlikely, however, that atmospheric pollution 

has affected pitting of limestone boulders in 

Grand Canyon and we see no evidence that 

carbonate weathering is self limiting on a time 

scale of several thousand years.

The main non-anthropogenic factor controlling weathering rate of carbonate tombstones and presumably naturally occurring 

carbonate stone is average annual rainfall 

(Meierding, 1981; 1993; Neil, 1989; Reddy, 

1989). In a study of aqueous geochemistry of 

rainfall runoff from carbonate stone, Reddy 

(1989) found that surface recession was 

directly proportional to the rainfall amount and 

that the rate of recession is the same for limestone and marble. Likewise, in a study of 

marble tombstones in areas of widely different 

climate, Meierding (1981) found that pitting

Open-File Report 97-167

rate is directly proportional to the amount of 

rainfall; pitting rates range from of 1.7 to 15 

mm/ka with mean annual rainfall of 200 to 

1,000 mm/yr, respectively. The influence of 

mean annual temperature on pitting rate is 

ambiguous and studies have not specifically 

analyzed this factor (Neil, 1989), although one 

could argue that temperature is equally as 

important as precipitation.

Reported rates of limestone weathering 

(Cooke and others, 1995, table 1, summarize 

recent studies) are difficult if not impossible to 

compare with pitting of carbonate boulders in 

Grand Canyon because of different methods 

used to estimate weathering, aspect of the 

stone, and climate. The various methods used 

to measure surface recession of tombstones 

differ considerably from measurement of pit 

depth used in this study. Typically, legibility of 

the tombstone inscription, relief of lead lettering, or relief of siliceous veins (which are relatively unaltered by weathering) are used to 

measure surface recession due to weathering. 

We cannot compare measured surface recession from these earlier studies with the depth 

of dissolution pits on carbonate boulders in 

Grand Canyon. Studies of tombstones control 

for aspect of the stone, but aspect is nearly 

impossible to control with irregularly shaped 

and randomly oriented boulders on debris-fan 

surfaces. Finally, climate of Grand Canyon is 

substantially drier than temperate regions 

where most carbonate weathering rates have 

been determined.

Of the several investigations cited above, 

Danin's (1983) study of archeologic sites in 

Jerusalem most closely resembles the present 

study in terms of method and time scale; nevertheless results are not directly comparable 

with Grand Canyon. Danin (1983) measured 

the maximum depth of dissolution pits on 

west-facing limestone walls of dated archeologic structures. The average pitting rate on 

these surfaces over 2,600 years is 5 mm/ka, 

which is nearly twice the rate of 2.54 mm/ka 

estimated for carbonate boulders in Grand

10
Canyon. Comparison of pitting rates is difficult 

because maximum pit depth rather than average depth was used, west-facing vertical surfaces rather than mostly sub-horizontal 

boulder surfaces were sampled, and the climate of Jerusalem is substantially wetter and 

cooler than the Grand Canyon (Hereford and 

others, 1996a).

In summary, average pit depths of carbonate boulders on four dated surfaces in Grand 

Canyon show that pit depth increases on average by 2.54 mm/ka. This rate is a linear function of time over the past 3 ka, which is 

consistent with physical process of limestone 

weathering. For purposes of dating surfaces in 

Grand Canyon, we assume that the rate is 

essentially constant over at least the mid- to 

late Holocene and that the 2.54 mm/ka rate 

applies throughout Grand Canyon along the 

Colorado River, a region that is broadly similar 

in terms of elevation and precipitation. Finally, 

dissolution of carbonate stone is closely linked 

to annual precipitation. Because the process is 

cumulative, increases and decreases of precipitation tend to average out, resulting in an 

essentially constant long-term average pitting 

rate.

Age and Correlation of Prehistoric 

Fan Surfaces

This section of the report deals with the 

ages and spatial and temporal correlation of 

debris-fan surfaces. Ages were estimated from 

the 2.54 mm/ka average deepening rate; estimated ages range from 500 to 7,000 cal yr B.P. 

The dates reported here are subject to revision 

because additional radiometric dating may further refine the pitting rate.

Average depth of dissolution pits and estimated ages of 71 fan surfaces at the 26 tributary fans are shown in figure 7. The five shaded 

areas (fig. 7) are clusters with relatively high 

number of correlative surfaces that include 66 

percent of the surfaces. The mid-point of the 

clusters (horizontal lines) correspond with the

peaks in figure 8. Overall, 75 percent of the 

surfaces are younger than 2.8 ka. The oldest 

surface preserved on a fan is at River mile 

124.2L (fig. 1) with dissolution pits averaging 

12.22 mm deep. This corresponds to about 

4,800 cal yr B.P. Even older surfaces are 

located at the mouths of Forster and Fossil 

Canyons and in the tributary canyons at River 

mile 220R. The three surfaces have dissolution 

pits averaging 17.29, 17.41, and 17.42 mm, 

respectively; this is about 6,850 cal yr B.P. 

These debris-flow levees are preserved in the 

mouths of the canyon away from the erosional 

effects of the Colorado River.

The youngest surfaces dateable by this 

method are at 209 Mile Canyon, Granite Park 

Wash, Palisades Creek, and Nankoweap Canyon (figs. 1 and 7). Dissolution pits on the four 

surfaces average 1.2 to 1.47 mm deep, which is 

probably between 500 to 600 cal yr B.P. An 

average depth of around 1.2 mm is probably 

the minimum amount of surface pitting detectable by this method; carbonate boulders on 

fan-forming surfaces younger than this do not 

have measurable dissolution pits.

Many widely spaced tributaries have 

debris-fan surfaces with essentially identical 

exposure times, as shown by average depth of 

dissolution pits on carbonate boulders (figs. 1, 

7). The degree of time-stratigraphic correlation 

of fan surfaces (or the between fan surface-age 

variation) among the 26 debris fans is quantified by considering the nonparametric statistics 

of the differences (n = 70) of the sorted list of 

average depths. The median difference in pit 

depth among the surfaces is only 0.1 mm, and 

the interquartile range is 0.05 to 0.16 mm. In 

many cases, this median difference and range 

are smaller than the measurement errors. Nevertheless, fan surfaces with carbonate boulders 

having nearly identical average pit depth are 

typical of widely spaced tributaries, which 

suggests the surfaces are time correlative 

within the limits of this method. This result 

suggests that the conditions leading to formation of debris-fan surfaces influence the entire

Open-File Report 97-167

11
River mile

00 CM CO Is* Oi

4 -

E

_c

i.

-O

'a. 

0

8-

12

16

20 -I

i 

T

EXPLANATION 

n = 71 surfaces

at 26 tributaries 

- Average

95 percent confidence 

interval

Pit depths < 1 mm, 

age from stratigraphic 

relations

High number of correlative surfaces, see 

figure 8

Not debris fans

0

o.

CD

4,000 |

0"

- 6,000

8,000

i i r \ \ \ \ \ ii i i f \ \ i i i i i i i i i i i 

1 23456789 1011121314151617181920212223242526

Debris fan (see figure 1) 

Figure 7. Age and correlation chart of debris-fan surfaces inferred from average depth of dissolution pits.

Grand Canyon, although these conditions do 

not affect every tributary.

The typical time between formation of fan 

surfaces (or the within fan variation of surface 

ages) is estimated from the differences in pit 

depth calculated for each tributary fan with 

more than one surface (23 of the studied fans). 

The median difference (n = 45) in average 

depth of dissolution pits on the 23 fans is 2.13 

mm and the interquartile range is 1.32 to 3.22 

mm. This suggests that surfaces of the typical 

fan in the period 500 to 7,000 cal yr B.P. (fig. 

7) differ in age by about 850 years within a 

range of 500 to 1,250 years. This 850 year 

recurrence interval is much longer than the 1- 

to-100 year average recurrence interval (Melis 

and others, 1993) of channelized debris flows 

during historic time.

Open-File Report 97-167

A smoothed density trace of average pit 

depth (fig. 8; Chambers and others, 1983, p. 

32-41) of the 71 studied surfaces suggests that 

ages of the surfaces cluster in time. A striking 

feature of the density trace is the progressive 

increase in the number of older surfaces beginning about 4,700 cal yr E.P. The clustering of 

fan surfaces in the late Holocene contrasts with 

the lack of early and middle Holocene surfaces, but the clustering probably did not result 

entirely from increased debris-flow activity. 

Older fan surfaces are under-represented 

because of progressive erosion and removal by 

the Colorado River. The dominance of relatively young fan surfaces, therefore, is largely 

a function of preservation.

The density trace has five peaks or clusters 

(fig. 8). The youngest and least prominent is

12
Time, in cal yr B.P. 

2,000 4,000 6,000

0.20

0

0 5 10 15 

Average pit depth, in mm

Figure 8. Density trace of average pit depth.

around 2.0 mm average depth, corresponding 

to an age of 790 cal years B.P. Two prominent 

peaks around 3.7 and 6.0 mm average depth 

account for 41 percent of the surfaces. These 

two periods, around 1,460 and 2,360 cal yr 

B.P., have a large number of preserved fan surfaces, suggesting that debris-flow frequency 

was relatively high at these times. Two other 

peaks at 7.5 and 9.7 mm average depth include 

about 13 percent of the studied surfaces. These 

are also episodes of increased fan preservation 

around 2,950 and 3,820 cal yr B.P.

Briefly, fan surfaces at widely separated 

tributaries are correlative as they have similar 

exposure times. Increased debris-flow activity 

is clustered into at least two and possibly five 

distinct episodes in the late Holocene. In most 

cases, the surfaces of any particular fan differ 

in age by about 850 years, which is probably 

the recurrence interval of fan-forming debris 

flows in the Grand Canyon. Early and midHolocene fan surfaces are not preserved at the 

river.

Discussion and Conclusions

Preservation of prehistoric debris fans is 

linked to interaction between the frequency of 

fan-forming debris flows and removal of the 

deposits by the Colorado River. This interaction has probably been controlled by climate of 

the Grand Canyon region through its affect on 

local runoff and by the climate of the Colorado 

River drainage basin which controlled the size 

of mainstem floods. Only fans deposited in the 

last 3 to 4 ka have been preserved (fig. 7), suggesting that gradual, flood-related shifting of 

the main channel eventually removed the earlier deposits.

Erosional modification of debris fans in 

the past several thousand years by relatively 

large floods was suggested by Kieffer (1985; 

1990). Large-scale topographic maps show 

that debris fans are extensively modified. For 

example, the distal margins of debris fans in 

eastern Grand Canyon, Nankoweap rapids 

area, Prospect Canyon at Lava Falls Rapid, and

Open-File Report 97-167

13
Granite Park (fig. 1) are truncated along the 

river (Hereford and Thompson, 1994a, b; 

Hereford and others, 1996a, b; Webb and others, 1996). Truncated debris fans were eroded 

by the river in the past 1-3 ka, most likely by 

relatively rare and large floods such as those 

documented by O'Connor and others (1994). In 

the future and without the reduction of floods 

caused by Glen Canyon Dam, existing debris 

fans would probably be removed entirely or 

reduced to high-level remnants near the margins of the channel. This gradual removal of 

the fans is presently interrupted or greatly curtailed by regulated streamflow, but the possibility of a fan-forming debris flow at any of the 

large fans studied in this report is unchanged.

Mainstem floods are clearly implicated in 

erosion of debris fans. The frequency of large 

floods of the Colorado River in Marble Canyon (O'Connor and others, 1994) and in nearby 

tributaries outside of Grand Canyon (Ely and 

others, 1993) is broadly correlated with the 

number of preserved debris fans. Large floods, 

both studies agree, were relatively frequent 

from 4,800 to 3,100 cal yr B.P. and from 1,300 

cal yr B.P. up to the present. In Marble Canyon, these floods were estimated to be larger

>j

than 5,700 m /s; floods of this size have annual 

exceedence probability of 1 percent or less 

(O'Connor and others, 1994). In addition, 

slackwater flood deposits at the mouth of Lava 

Creek in eastern Grand Canyon have nine beds 

deposited by the Colorado River from 550 to 

110 cal yr B.P. (Hereford, 1996). Although the 

maximum size of the floods is not determined, 

the deposits are above the historic flood level

of 8,500 m /s. Large floods, therefore, were 

relatively numerous during 550 to 110 cal yr 

B.P; these floods probably reworked the toes 

of the debris fans forming the present configuration of the channel. Likewise, the early 

period (4,800 to 3,100 cal yr B.P.) of frequent 

floods probably removed older debris fans or 

continued an erosional episode that began earlier in the Holocene.

Open-File Report 97-167

An alternative explanation for the lack of 

old debris fans is that early and mid-Holocene 

deposits are present and buried by net aggradation of the fans, but this seems unlikely given 

that older surfaces have the highest elevation 

relative to younger surfaces on any particular 

fan. Early Holocene debris-flow deposits are 

present (fig. 7) in protected sites in the mouths 

of tributary canyons. However, the surfaces of 

the early Holocene deposits project downstream to the debris fan where they are topographically above younger surfaces.

Temporal clusters in the ages of fan surfaces (figs. 7, 8) probably result from increased 

debris-flow activity. The late Holocene climate 

of the Grand Canyon region is not well understood, so it is not possible to compare 

increased debris-flow activity with episodes of 

increased precipitation. Increased debris-flow 

activity around 3,820 and 1,460 cal yr B.P., 

however, is roughly contemporaneous with 

high stands of Mono Lake in eastern California 

(Stine, 1990). The level of Mono Lake fluctuates directly with the amount of snowmelt runoff from the Sierra Nevada. In addition, the 

period 3,000 to 4,000 cal yr B.P., known as the 

early Neoglacial climate episode, was probably cooler and wetter than at present throughout the Southwest (Enzel and others, 1992). 

These fluctuations and the Neoglacial episode 

suggest that winter precipitation in the Grand 

Canyon region may also have been high, leading to a relatively large number of debris flows 

around 3,000 to 4,000 cal yr B.P. and 1,460 cal 

yr B.P.

Implications for channel evolution and river 

navigability

With Glen Canyon Dam in place, future 

development of the river channel and navigability of the river are controlled by tributary 

debris flows rather than the large mainstem 

floods of the pre-dam era. Constriction of the 

channel by deposition of debris-flow sediment 

and subsequent reworking by floods are well 

understood for channelized debris flows of the

14
past hundred years (Kieffer, 1985; Webb and 

others, 1989; 1996). However, these recent 

debris flows are small relative to the prehistoric, fan-forming debris flows discussed in 

this report. In addition, the prehistoric floods 

that reworked the deposits were probably 

larger than any historic-age flood, controlled 

or otherwise. Sediment from fan-forming 

debris flows probably extended across the 

channel to a depth of several meters (Hereford 

and others, 1996a), unlike recent deposits that 

typically did not cross the channel (Webb and 

others, 1996). The large volume of sediment 

deposited in the channel by prehistoric debris 

flows quite likely increased the severity of any 

particular rapid, possibly forming a waterfall 

(Kieffer, 1985). A striking example of an 

ancient waterfall is the Prospect Canyon debris 

fan in western Grand Canyon (fig. 1). Reconstruction of the 2,900 cal yr B.P. deposits 

shows they must have extended across the 

channel to a depth of 5-10 m (Webb and others, 1996, fig. 35). Although not as dramatic, 

debris-flow deposits extended across the channel several times in the past 1-3 ka at most tributary fans in eastern Grand Canyon and 

Marble Canyon (Hereford and others, 1996a, 

b; Hereford, 1996).

Any attempt to predict the occurrence of 

debris flows in Grand Canyon is difficult at 

best and impossible at worst. Nevertheless, 

fan-forming debris flows will occur during 

the projected life span of Glen Canyon Dam. 

Moreover, if the prehistoric patterns of 

debris-flow activity (figs. 7, 8) are repeated in 

the future, the channel will likely become 

severely constricted at many of the major tributaries (fig. 1). The cluster of prehistoric 

debris-flow activity about 790 cal yr B.P. (fig. 

7) suggests that another episode might begin in 

only 60 years, assuming the average time 

between events canyon-wide is 850 yrs. The 

youngest fan-forming deposits known to us are 

in Marble Canyon and eastern and western 

Grand Canyon; these deposits might be the 

precursors of fan forming debris-flow activity.

Acknowledgments

This work was done in cooperation with 

the Bureau of Reclamation, Glen Canyon 

Environmental Studies. We thank program 

director David L. Wegner for continued support of the work. National Park Service archeologists Janet R. Balsom and Helen C. Fairley 

introduced us to the archeology of Grand Canyon. Logistical support for river trips through 

Grand Canyon was provided by Brian Dierker, 

Chris Geanious, Doree Stonebraker, and Greg 

Williams. Kenneth L. Pierce and John W. 

Whitney provided numerous critical comments 

that improved the manuscript.

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16
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17