Effects of Jackson Lake Dam and Tributaries on the Snake River in ...

<strong>Effects</strong> <strong>of</strong> <strong>Jackson</strong> <strong>Lake</strong> <strong>Dam</strong> <strong>and</strong>
<strong>Tributaries</strong> on the Snake River in Gr<strong>and</strong>
Teton National Park: a continuing analysis
John (Jack) Schmidt, Nicholas Nelson,
Susannah Erwin
Department <strong>of</strong> Watershed Sciences
Utah State University
(funded by National Park Service <strong>and</strong> U. S.
Geological Survey

•What are the geomorphic impacts <strong>of</strong> JLD on
the Snake River? Have these impacts
changed through time? Do these impacts
change with distance downstream from
<strong>Jackson</strong> <strong>Lake</strong> <strong>Dam</strong> <strong>and</strong> from significant
tributaries?

History <strong>of</strong> <strong>Jackson</strong> <strong>Lake</strong> <strong>Dam</strong>
•1908 – log crib dam completed
•1910 – failure <strong>of</strong> log crib dam
•1916 – permanent concrete dam completed
– raised lake level 39 ft
•1980s – dismantled <strong>and</strong> rebuilt
Construction <strong>of</strong> Palisades <strong>Dam</strong>: 1958

Predicting channel change downstream
from dams
sediment load
water discharge

Snake River below <strong>Jackson</strong> <strong>Lake</strong> <strong>Dam</strong>
Snake River below <strong>Jackson</strong> <strong>Lake</strong> <strong>Dam</strong>
1.2 10 4 2-Mar 3-May 4-Jul 4-Sep 5-Nov
1 10 4
A
1916-1958 - regulated
1916-1958 - estimated unregulated
1959-2005 - regulated
1959-2005 - estimated unregulated
8000
6000
4000
2000
0
Date

Reduction in floods immediately
below dam
<strong>Jackson</strong> <strong>Lake</strong> reservoir
capacity
0.85 x 10 6 af
actual
no dam
1917-1956: actual flood regime is -4% (2-yr) <strong>and</strong> -6% (10-yr) less than without JLD
1957-2002: actual flood regime is -42% (2-yr) <strong>and</strong> -36% (10-yr) less than without JLD

25000
Snake River at Moose
DISCHARGE, IN CUBIC FEET PER SECOND
20000
15000
10000
5000
Snake
River
at
Flagg
Ranch
Buffalo Fork
Pacific Creek
1996
Snake River near Moran
0
1/1/96 3/3/96 5/4/96 7/5/96 9/5/96 11/6/96 1/7/97

Contributions from tributaries
Pacific Creek
64% 13% 23%
Buffalo Fork
width <strong>of</strong> arrow
proportional
to total annual
water flow

Snake River below Buffalo Fork
Discharge, cubic feet per second
1 10 10 44
8000
6000
4000
1916-1958 - estimated unregulated
1959-2005 - estimated unregulated
1916-1958 - regulated
1959-2005 - regulated
1.2 10 44 2-Mar 3-May 4-Jul 4-Sep 5-Nov
2000
00
Date

Snake River below Buffalo Fork
Snake River below Buffalo Fork
2.5 10 4
3 10 4 1920 1940 1960 1980 2000
regulated
estimated unregulated
B
Estimated 2-yr
flood
2 10 4
1.5 10 4
1 10 4
5000
0
2-yr flood: 1916-1958 1959-2005
regulated: 12,002 9753
est. unregulated: 15,340 15,316
Year
Regulated 2-yr flood
1958: completion <strong>of</strong> Palisades <strong>Dam</strong> -22% 1916-1958
-36% 1959-2005

Absolute changes in the sediment supply are unknown.
?
?
Sediment comes
from tributaries
<strong>and</strong> channel banks

Balance <strong>of</strong> Water <strong>and</strong> Sediment
sediment
degrade aggrade
water
Assuming that there have been no long term changes
in sediment supply, there should be accumulation <strong>of</strong>
sediment in channel <strong>and</strong> floodplain.

Anticipated channel changes where sediment surplus
<strong>and</strong> reduced floods
•Cross-section
•Channel narrowing; inset floodplains
How do they vary downstream?
What flows are necessary to inundate the alluvial deposits?
•Planform
–Has the rate <strong>of</strong> avulsion <strong>and</strong> channel change increased or decreased
with time? Where?
Before flow regime change
terrace floodplain Channel at
bank full
terrace
After flow regime change
ab<strong>and</strong>oned
floodplain
Channel at
new bank full
inset
floodplain

Analysis <strong>of</strong>
channel change
using aerial
photographs:
•Aerial photographs
•1945
•1969
•1990/1991
•2002

1945-1969
Many reaches with high channel
activity
Most reaches more deposition
than erosion
2 reaches more narrow; 1 reach
wider
Several reaches more braided
Changes occurred close <strong>and</strong> far
from tribs
Cottonwood Creek
Schwabachers L<strong>and</strong>ing
19
18
Moose
W
17
N
16
Ditch Creek
Downstream <strong>of</strong> JLD
Deadmans Bar
-
1 2
0 0.5 1 2 3 4 5 6 7 8
15
12
14
N
13
3
11
9
4
6
W
Kilometers
10
5
8
Pacific Creek to
Buffalo Fork
7
Pacific Creek
Buffalo Fork
Spread Creek
1 dam release > post-1958 10-yr RI
Blw BF: 1 flood > post-1958 10-yr RI
N
Channel activity
Change in braid
index
Change in width

1969-1990
Many reaches with high activity
Reaches either had more erosion or no net
change
2 reaches wider
1 reach less braided; no significant
changes elsewhere
Changes occurred close <strong>and</strong> far from tribs
Downstream <strong>of</strong> JLD
1 2
12
14
15
3
W
9
11
13
4
6
10
5
8
Pacific Creek to
Buffalo Fork
7
Pacific Creek
Buffalo Fork
Spread Creek
16
Deadmans Bar
Cottonwood Creek
Schwabachers L<strong>and</strong>ing
18
19
Moose
17
W
Ditch Creek
-
0 0.5 1 2 3 4 5 6 7 8
4 dam release > post-1958 10-yr RI
Blw BF: 3 flood > post-1958 10-yr
RI
1974 big trib flood
Kilometers

1990-2002
Many reaches with high activity
More reaches with net loss <strong>of</strong> floodplain
1 reach wider; 1 reach narrower
2 reaches more braided; 1 reach less
braided
Downstream <strong>of</strong> JLD
1 2
12
14
13
3
11
9
4
6
10
5
8
Pacific Creek to
Buffalo Fork
7
Pacific Creek
Buffalo Fork
Spread Creek
Changes occurred close <strong>and</strong> far
from tribs
16
15
Deadmans Bar
Schwabachers L<strong>and</strong>ing
17
W
2 dam release > post-1958
10-yr RI
Cottonwood Creek
-
Blw BF: 3 flood > post-1958
10-yr RI
19
18
N
Ditch Creek
1999 big trib flood
Moose
0 0.5 1 2 3 4 5 6 7 8
Kilometers

Lower Floodplain
Before 1958: 3-5 yr recurrence <strong>of</strong>
inundation
1-3m
0.25-0.75m
Post-1958: 9-17 yr recurrence
Upper Floodplain
Inundated ~1/yr before
1958
Now inundated ~ 1.5 yrs

Where does the sediment come from?
?
Sediment comes
from tributaries
<strong>and</strong> channel
banks
?

Bed mobility
•Bed mobility – determining what moves <strong>and</strong> what flows are
necessary to mobilize the bed
•ability to move gravels increases downstream
•Bed not fully mobile immediately below dam
•River is capable <strong>of</strong> fully mobilizing portions <strong>of</strong> the bed below
tributaries

How do we calculate sediment transport rates?
Qs = aQ b
Pacific Creek Sediment Rating Curve
10000
1. Transport formulae
q
s
( s −1)
gD
Inputs:
- grain size
- channel dimensions
- channel slope
⎛
= 8⎜
⎝
τ
−
τ
c
3
( s −1)
ρgD
( s −1)
ρgD
⎞
⎟
⎠
3/ 2
Transport Rate (tons/d
1000
100
10
2. Direct Measurement
1
100 1000 10000
Discharge (cfs)

2006 Bedload Transport Measurements
Pacific Creek:
- 24 measurements
3500
3000
2500
2000
1500
1000
500
0
April May June July
Buffalo Fork:
- 37 measurements
4000
3500
3000
2500
2000
1500
1000
500
0
April May June July

TR2 – Toutle River Bedload Sampler
• Designed for sampling under conditions <strong>of</strong> high velocity, turbulence <strong>and</strong> high
sediment transport rate
• Improved bedload sampling efficiency relative to Helley-Smith
• Provides most representative bedload samples for sediment 1.0mm to
128.0mm (Ryan, 2003)

Concluding thoughts…
• Hydrology has been modestly affected
• Absence <strong>of</strong> long term progressive channel
change
• Proposals for changes in dam operations to alter
physical habitat must be based on analysis <strong>of</strong>
sediment delivered from tributaries.