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![Page 1: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/1.jpg)
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
![Page 2: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/2.jpg)
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
![Page 3: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/3.jpg)
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
![Page 4: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/4.jpg)
Gully Erosion
![Page 5: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/5.jpg)
Arroyo Development
![Page 6: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/6.jpg)
Processes Driving and Resisting Erosion
Illustrated by Gary O’Brien
![Page 7: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/7.jpg)
Process Restoring Erosion
![Page 8: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/8.jpg)
Cut and Fill (Cataract Canyon)
![Page 9: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/9.jpg)
Study Locations
![Page 10: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/10.jpg)
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
![Page 11: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/11.jpg)
Cross Section of Terraces in Cataract Canyon
![Page 12: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/12.jpg)
Part 1
Test Hypotheses
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Arroyo at Paria River
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Arroyo Aggradation (Paria River)
![Page 15: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/15.jpg)
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
![Page 16: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/16.jpg)
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 (%)
![Page 17: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/17.jpg)
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
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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
![Page 19: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/19.jpg)
Decadal Variation in 20th Century Precipitation
Webb et al. in prep.
![Page 20: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/20.jpg)
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
![Page 21: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/21.jpg)
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
![Page 22: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/22.jpg)
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
![Page 23: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/23.jpg)
222 mile - 1923 (E.C. LaRue photo)
![Page 24: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/24.jpg)
Granite Park1963
1000 cfs(Belnap
collection)
Granite Park1996
8000 cfs(Lisa Leap
photo)
![Page 25: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/25.jpg)
Old Unkar Camp1963
(Belnap collections)
Old Unkar Camp1998
![Page 26: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/26.jpg)
Cross CanyonMarch 1999
Cross CanyonAugust 1999
![Page 27: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/27.jpg)
Rapid 12March 1999
Rapid 12August 1999
![Page 28: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/28.jpg)
Percent of sites containing 1983 and pda deposits
0
10
20
30
40
50
60
70
80
1996 1983 1996 & 1983 pda
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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
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Part 2
Geomorphic Model for the Small-Catchment System
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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
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Geomorphic Settings
Illustrated by Gary O’Brien
![Page 33: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/33.jpg)
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
![Page 34: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/34.jpg)
Add Geomorphic Factors to Model
![Page 35: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/35.jpg)
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
![Page 36: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/36.jpg)
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
![Page 37: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/37.jpg)
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
![Page 38: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/38.jpg)
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
![Page 39: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/39.jpg)
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
![Page 40: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/40.jpg)
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
![Page 41: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649ef05503460f94c01207/html5/thumbnails/41.jpg)
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.