Development of a Geomorphic Model to Predict Erosion of Pre-Dam

Colorado River Terraces Containing Archaeological Resources

Kate Thompson and Andre Potochnik

Principal Investigators

SWCA, Inc. Environmental Consultants

and

Gary O’Brien, Ron Ryel, and Lynn Neal

Problem

Apparent rapid gully erosion of pre-dam terraces during the past two

decades has caused loss of numerous cultural sites in Grand Canyon river

corridor

Objectives of Study

• Test hypotheses:– Has erosion increased in the post dam period?– If so, is erosion climate driven or dam-related?

• Develop a model that predicts relative vulnerability of sites to erosion

Gully Erosion

Arroyo Development

Processes Driving and Resisting Erosion

Illustrated by Gary O’Brien

Process Restoring Erosion

Cut and Fill (Cataract Canyon)

Study Locations

Comparison of Furnace Flats to Cataract Canyon

Characteristic Cataract Canyon Furnace Flats

River gradient 2.42 1.95

Reach length 11 13

Elevation 1158 807

Mean annual precipitation 220 233

Mean monsoon season precipitation 86 97

Percent of months with > 50mm precip 9.1 9.0

1957 flood stage 101,000 120,000

Cross Section of Terraces in Cataract Canyon

Part 1

Test Hypotheses

Arroyo at Paria River

Arroyo Aggradation (Paria River)

Null Hypothesis: Degree of gully erosion has remained unchanged

from the pre-dam to post-dam period

• Test 1 - Use air photos to determine the degree of channel lengthening since 1965

• Test 2 - Compare amount of gully erosion in Cataract Canyon (control section) to Furnace Flats section in Grand Canyon

Channel Lengthening Over Timen = 23

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

6/15/61 6/15/69 10/20/76 10/20/80 10/10/88 4/5/92

Calendar Year

Rel

ativ

e E

xten

t o

f D

egra

dat

ion

arroyos fully formed (%)

arroyos partly formed (%)

arroyos absent (%)

Gully Density/Depth of 3 Paired Areas

Sitescompared

Length ofArea (m)

Number ofcatchments

Gullydensity

Mean gullydepth

#1Palisades CkCross Cyn

4242

32

0.070.05

0.680.83

#2Upper UnkarRapid 12

112108

84

0.070.04

0.740.73

#3Tanner CynRapid 4

11283

43

0.040.04

2.850.20

Climatic Variation Hypothesis: High precipitation anomalies in the

post-dam period increases severity of gully erosion

• Test 1 - Evaluate previous research on variation of 20th century precipitation

• Test 2 - Investigate variation in monsoon season rainfall at equivalent time periods before and after closure of the dam

Decadal Variation in 20th Century Precipitation

Webb et al. in prep.

Monsoon Precipitation for 13 Weather Stations,

Colorado River Corridor (> 50 mm / month)

R2 = 0.89

0%

10%

20%

30%

40%

50%

60%

0% 10% 20% 30% 40% 50% 60%

PERCENT OF MONTHLY PRECIPITATION EXCEEDING 50 MM, PRE-DAM (1932-1963)

PE

RC

EN

T O

F M

ON

TH

LY P

RE

CIP

ITA

TIO

N E

XC

EE

DIN

G 5

0 M

M,

PO

ST

-DA

M (

1964

-199

5)

Lake Powell - Canyonlands

K-M Highlands

Western Grand Canyon

Lower Colorado River

Line of equal events

Monsoon Precipitation for 13 Weather Stations, Colorado River Corridor (>25 mm/day)

R2 = 0.30

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40

NUMBER OF DAYS EXCEEDING 25 MM, PRE-DAM (1932-1963)

NU

MB

ER

OF

DA

YS

EX

CE

ED

ING

25

MM

, P

OS

T-D

AM

(1

96

4-1

99

5)

Line of equal events

Base-Level Hypothesis: Reduction of sand supply and large

floods in the post-dam period increases degree of gully erosion

• Test 1 - Report on rebuilding of high-elevation sand bars in both Grand Canyon and Cataract Canyon

• Test 2 - Assess catchment and river processes at each study site in Grand Canyon

222 mile - 1923 (E.C. LaRue photo)

Granite Park1963

1000 cfs(Belnap

collection)

Granite Park1996

8000 cfs(Lisa Leap

photo)

Old Unkar Camp1963

(Belnap collections)

Old Unkar Camp1998

Cross CanyonMarch 1999

Cross CanyonAugust 1999

Rapid 12March 1999

Rapid 12August 1999

Percent of sites containing 1983 and pda deposits

0

10

20

30

40

50

60

70

80

1996 1983 1996 & 1983 pda

Sites Supporting Base-Level Hypothesis

47%

31%15%

7%

not supported

weakly supported

supported

strongly supported

n = 119

Geomorphic ProcessNo Evidence Of:

Pre-dam arroyo-cuttingCutbank retreat by riverSide-canyon erosion

Score

111

Sandy Deposit Present:

Pda and 1983Active eolian sand

Maximum ScorePossible

11

51

< 3 not supported= 3 weakly supported

= 4 supported= 5 strongly supported

Part 2

Geomorphic Model for the Small-Catchment System

Steps to Building Geomorphic Model

• Classify catchments by geomorphic setting• Construct process-based conceptual model

• Construct predictive mathematical model

• Use model to predict vulnerability of individual sites

Geomorphic Settings

Illustrated by Gary O’Brien

Mathematical Model - Step 1

• Quantify driving and resisting parameters

• Q = C*I*A (Am. Soc. Civil Engineers) Total runoff (m3 ) upper catchment

• Axt = Wt * Dt Cross-sectional area of terrace segment

Add Geomorphic Factors to Model

Mathematical Model - Step 2

• Vr = ln(Q)/ln[Axt * (1+TF)]

Vr is raw vulnerability

• FVCi = (Vr * FVCi-1)/100

FVCi is cumulative vulnerability

thus: vulnerability rating of archaeological terrace = Vr of highest terrace

and: vulnerability rating/catchment = mean FVCi

Vulnerability Plot

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Vulnerability of top terrace

Gu

lly

dep

th r

atio

Grand CanyonCataract Canyon

n = 128

Threshold line

Mean Vulnerability by Geomorphic Setting

0.00

10.00

20.00

30.00

40.00

50.00

60.00

alluvial fan tributaryplain

talus slope debris lobe dune field

Geomorphic Setting

Rel

ativ

e V

alue

s of

Cat

chm

ent A

rea

and

Vul

nera

bilit

y

mean area (1000 m2)

% mean vulnerability

Conclusions(hypotheses testing)

• Gully erosion in terraces is more severe from 1978-1999 than 1942-1977

• Gully erosion is more extensive in Grand Canyon today than in Cataract Canyon control site.

• Gully erosion is increased due to both a high precipitation anomaly and a decrease in sediment renewal.

• 78% of channels draining archaeological sites show most elements of restorative base-level process

• Eolian redistribution of fresh flood sand is on-going at about 50% of catchments

Conclusions(the predictive model)

• Process-based model works best for this small-catchment geomorphic system– it simplifies enormous variety and complexity of

small catchments

• Statistically based model does not work well– poor correlation of gully depth/width to most

measured parameters

• Highest vulnerabilities are function of large catchment area and narrow terrace width

Recommendations

• Site mitigation achieved by slowing erosion:– decrease stream power

– increase terrace diffusivity

• Data recovery suggested at sites where:– outliers occur on plot

– gully-depth ratios are close to 1.0

– there are few base-level controls

• Use vulnerability plot to:– identify high risk sites

– use as a base to track how points shift in future

Future Work

• Integrate mathematical model results with mainstem studies (Wiele, 2000).

• Refine model through application and observation.

• Quantify drainage density of uppermost terrace: could be more important than gully depth and width.

• Eolian studies: quantify redistribution of sand.• Repeat historic photography.