The Degraded Rea

The Degraded Rea

Rate and Pattern of B

Rate and Pattern of B

Bank Adjustment in th

Bank Adjustment in th

25 km Downstream fro

25 km Downstream fro

Canyon Dam Canyon Dam

Paul Grams, David Topping, Jack S

Paul Grams, David Topping, Jack S

and Sara Goeking and Sara Goeking
Why Study Glen Can

•

Has bed degradation continued s

•

What has been the spatial distrib

bed degradation within Glen Ca

•

How have channel-side alluvial 

changed since the pre-dam perio
Components of Stu

• Changes in bed elevation

–

Reclamation monitoring cross-sec

–

USGS gage station records of bed 

• Channel-side deposits

–

Mapping from historical aerial pho

–

Historical oblique photographs

–

Comparison with cross-section dat
The Reclamation monitoring 
Lees Ferry Gaging Stat

Lower Cableway1929-1965

Upper Cableway1921-1966

Modern Cableway1966-present and gage
Glen Canyon - 1956
Glen Canyon 1889 - 19

1889 
Glen Canyon 1956 - 20

July 1956 Sept
Glen Canyon 1954 - 19

19561998
Glen Canyon Location M
935

940945950955960

1/1/21 1/1/31 1/1/41 1/1/51 1/1/61 1/1/71 1/1/81 

cable-gage ht. measuredcable-gage ht. calculated

Elevation, inmeters

Date

water surface elevations (discharge measurements)

water surface (Range 1 sur

minimum b(Range 1 s

minimum bed elevations (discharge measurements)

1965 scourin

Lees Ferry Upper Cable

Minimum Bed Elevati

0

500

1000150020002500

3/1/65 4/1/65 5/2/65 6/2/65 7/3/65

Discharge, in cubic meters persecond

Date

cross-section measurement

annual peak discharge

1965 scourin
“Channel Cleaning” Flo
Cross section showing bed de

1956-2000

looking downstream

945955965975

-100 -50

0 50 100 

Ra

n

g

e 1

6, 4.3 k

m do

w

nstrea

m fro

m Glen Ca

n

y

o

n 

Elevation, in meters

Distance from

b

aseline, in m

eters

195

6

20

0

0
Cross section showing bed de

1956-2000

looking 

94

5

95

5

96

5

97

5

-300 -250 -200 -15

0 -10

0 

Ra

n

ge 11a, 1

0.3

k

m downstream from 

Gle

n Canyo

Z-11A-195

6

Z-11A-195

9

Z-11A-196

5

Z-11A-197

5

Z-11

A-19

8

3

Z-11

A-19

9

0

Z-11

A-20

0

0

Z-11A-grav

el

Elevation, in meters

Distance from baseline, in meters

19

5

6

20

0

0
Minimum bed elevation from 195

for cross-sections located in riffle

02468

10

195

0 1960 19

7

0 198

0 1990

2

0

0

0 201

0

R2

0

R1

5

R11AR5

Bed degradation, in meters

Ye

ar

1.0 k

m

10.3 k

m

5.8 k

m

18.4 k

m

Riffle

s

a)

02468

10

1950 1960 19

R18R16R14

Bed degradation, in meters

b)

7.5 k

m
Adjustment of Stage-Disc

Relations: Greatest at upstrea

Reach

950951952953954955956957

0

5

0

0 100

0 15

0

0

200

0

R-1

0 Sta

geDischarg

e Relation

12.8 k

m do

w

nstre

a

m from da

m

19

5

6

195

9

1960-19

6

3

1965-

2

0

0

0

Elevation, in meters above sea level

Dis

c

harge, in cubic

m

eters p

er seco

n

d

94

8

94

9

95

0

95195

2

95

3

0

500 

R-1 Stag

e-Disc

h

arg

23.9 k

m from

d

a

m

Elevation, in meters above sea level

Discharg

e, i

n 
Drop in Stage-Discharge R

Decreases with Distance Dow

0

0.

5

1

1.

5

2

2.

5

0 5 10 15

2

0

Decrease in stage for indicated discharge, in meters

Distance downstr

e

am from Glen Canyon 

D

am, in kil
Eroded bank and “Perched” te

km downstream from Glen Ca
Longitudinal Profile of Bed De

1956 - 2000

93

5

94

0

94

5

95

0

95

5

96

0

0

5 10 15 

Elevation, in meters

Dist

a

n

c

e

d

o

w

nstr

e

a

m from Glen C

any

o

n Da

m, i

n

kilo

m

195

6 W

ater Surf

a

c

e 200

0 W

ater Surf

a

c

e

T

op of Bur

Erode

d Gra

vel 1956-2000
Accumulated sediment evacua

Glen Canyon

-2 106

0 1002 1064 1066 1068 1061 107

1.2 107

0

5 10 15 

195919651983

19

7

5

19

9

0

20

0

0

Accumulated volume of degradation, in cubic meters

Dist

a

nce

d

o

w

n

stre

a

m from Glen C

anyon Da

m, i

n

From 1956 to indicated 

y

ear

8.3 million m3 sediment evacuation predicted in 1957

9.9 million m3 measured in 1975

10.7 million m3 measured in 2000 –

exceeds predicted by ~ 30%
Changes in Channel-

Deposits

•

Measurements at Lees Ferry Lower C

provide long pre- and post-dam time location

•

Mapping from aerial photographs pro

comprehensive coverage from the daFerry
Progressive decrease in channe

Lower Cableway

18

5

19

0

19

5

20

0

20

5

21

0

95

0 950.5 95

1

951.5 95

2

952.5 95

3

953.5 

5/12/19

29 t

o 6/19/193

5

6/2

0/19

35 t

o 5/11/193

6

5/12/19

36 t

o 6/9/1938

6/10/19

38 t

o 5/23/194

8

5/2

5/19

48 t

o 1

0/21/19

6

8

Channel width, in meters

Elev

atio

n of w

ater surfa

ce at lo

w

e

r c

a

ble

way, in 

19

3

5 t

o 1

936 May 1948

June 1938
Bed Scour and bank deposition 

Cableway

940945950955960

0 50 100 150 200 250 300

Elevation, in meters

Distance, in meters

940945950955960

30 35 40 45

5

0

5

5

6/96/5/5/2

Elevation, in meters

Distanc

e, i

n met

ers

pred

a

m aggr

a

datio

n

Stabl

e during post-dam perio

d
Mapping from Air 

Photos
Erosion of pre-dam terr

1952 to 1984

0 7 35

0 14

0

M

eters

region of 1959-1965

bed degradation

1952 edge of w

ater

Leg

end

Er

o

sio

n

Erosion - t

errac

e

Ero

sion - D

epo

sition

O

verlay M

a

p

S

eptemb

er 1952 to O

ctober 1984

perche

d gr

a

vel

d

e

p

o

sit

945955965975

-300 -250 -200 -150 -100 -50 0

Range 11a, 10.3 km downstream from Glen Canyon Dam

Z-11A-1956Z-11A-1959Z-11A-1965Z-11A-1975

Z-11A-1983Z-11A-1990Z-11A-2000Z-11A-gravel

Elevation, in meters

Distance from baseline, in meters

Left Right
Erosion and “perching” of cha

deposits: 1952 to 198

0 8 40

0 16

0 Met

ers

Ra

n

g

e 7

region of 1956-2000

bed degradation

Le

g

end

Er

o

sio

n

D

epo

sition - flu

ctuating flow

Ero

sion - D

epo

sition

O

verlay M

a

p

S

eptemb

er 1952 to O

ctober 1984

D

epo

sition - p

o

st-dam floo

d

region

s of "appare

nt"

940 s

and bar depositio

n

950960970

-250 -200 -150 -100 -50 0 50 100

Range 7, 15.8 km downstream from Glen Canyon Dam

Z-7-1956Z-7-2000

Elevation, in meters

Distance from baseline, in meters

Right

Left

Left Right

“perched deposits”
Sketch of Pre- and Post-Dam 

Cross-Section

pre-damaverage flood

pre-dam channel

post-dam channel

fluctuatinflow dep

post-damaverage flood 

pre-damterrace

eroded deposits perch

gravel bar
Changes in the area of alluvial 

1952 to 1996

0

20406080

100

1950 1960 1970 1980 1

Da

t

e

Area, in hectarespre-da

m t

e

r

r

a

c

e

hi

g

h

- and l

o

w

-elevation san

d
Downstream distribution of c

alluvial deposits: 1956 to 

05

10152025

0

5 10 15 

Distance dow

nstream from Glen C

a

n

yon D

a

m, i

n 

Area, in hectares

p

e

r

c

h

e

d g

ravel

e

r

osi

o

n (

t

e

r

race)

e

rosion (

s

and)

pe

r

ched s

and

e

r
Conclusions I

•

Sediment evacuation from Glen Canyon ha

since 1975, but at a much reduced rate

–

Has not significantly decreased mean bed elevat

–

Most degradation since 1975 occurred in pools

–

Magnitude of degradation in riffles decreases in 

direction and extends downstream to 2 mi above 

–

Magnitude of degradation in pools does not corr

downstream and extends below Lees Ferry

•

The stage discharge relations have dropped 

the upstream end of the reach and have not downstream end of the reach
Conclusions II

•

Massive terrace erosion at localized sites du

channel cleaning flows, not widespread throand current rates of erosion are very low

•

Channel-side sand deposits and gravel depo

above active channel and stabilized by vege
Prognosis

• Continued evacuation of sediment fro

primarily during floods

• No further degradation of channel co

(riffles), even during floods

• Possible aggradation of riffles over ti

absence of scouring floods

• Subtle changes to channel-side depos

– Sporadic and localized terrace erosion– Small-scale erosion and deposition at sa