Minnesota River Turbidity TMDL and Lake Pepin TMDL Meeting
Southern Research and Outreach Center, Waseca
Thursday, July 20, 2006
Causes
and Consequences
Stream Instability
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Karen Gran
Presenter
name
Stream “Instability”?
Copyright © Steve Baxter 2002
USGS
Stream “Instability”?
Multi-thread, braided stream
High sediment load, High slope
Relatively coarse grain size,
High lateral mobility rates
USGS
Stream “Instability”?
Copyright © Steve Baxter 2002
Add in photos of braided river
Wolman and Leopold, 1957
Streams DO change, but they can
also be out of “equilibrium”.
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Natural processes can
be altered by changing
inputs to the system
When development
encounters an active
stream, sometimes it is
forced to be “stable”
Lane’s Balance for Alluvial Channels
Grain Size Distribution
Sediment Supply
Slope
Water Discharge
From Lane 1955
Ex. Channelization
1) Straighten a
meandering channel
2) Add more runoff
Slope
Qw
Photo courtesy NRCS
- Increase slope
- Increase shear stress (τ=ρgDS)
- Increase bed erosion leading to degradation locally;
extra sediment leads to aggradation downstream
- Increased discharge could lead to channel widening/bank erosion
- Channel may attempt to “undo” the straightening, and create a
meandering form in straightened channel
Channel Evolution Model
Adapted from Simon 1994; USACOE 1990
Independent Variables
imposed on the system
Grain Size Distribution
Sediment Supply
Vegetation
Water Discharge
Dependent Variables
Resulting from the combination of
sediment and water supply, grain size, and vegetation
Width, Depth, Slope, Sinuosity, Planform (braided/meandering)
Surface grain size, organization, bedforms
Lateral migration rates, Vertical changes (aggradation/degradation)
Vegetation
How long will it take for the channel to respond
to imposed changes in water or sediment supply?
From Knighton 1998
Major forcings on the Minnesota River at different spatial and temporal scales.
Holocene
(<10,000 yr)
“Anthropocene”
(<~200 yr)
From Knighton 1998
How do new fluvial landscapes
form and develop?
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Head-cutting;
Knick-point migration
Network Extension
Drainage Integration
From Les Hasbargen
How do fluvial landscapes
form and develop?
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Head-cutting;
Knick-point migration
Network Extension
Drainage Integration
Initially High Sediment Yields
decaying “exponentially”
Pasig-Potrero River
300
250
Sediment Yield (mcm)
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200
150
100
50
0
0
5
10
Years after Eruption
15
Minnesota River
Landscape Evolution
Since valley formation by
Glacial River Warren:
‹ Adjustments to
mainstem slope
‹ Knick-point migration
on tributaries
‹ Drainage integration in
uplands – artificially
enhanced (draining of
wetlands and lakes,
ditching and tiling)
Minnesota River
Since valley formation by
Glacial River Warren:
‹ Adjustments to
mainstem slope
‹ Knick-point migration
on tributaries
‹ Drainage integration in
uplands – artificially
enhanced (draining of
wetlands and lakes,
ditching and tiling)
Sediment Yield
Landscape Evolution
?
Time
Anthropogenic signal swamps
natural background rates
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Post-settlement rates of
sedimentation in Lake
Pepin have increased by
a factor of 10.
Most of the sediment
comes from the
Minnesota River.
Rates have increased
due to land-clearing,
drainage modification,
and development.
From Engstrom and Almendinger, 2000
Major post-settlement changes
‹ Land-clearing,
mid-1800s
‹ Drainage modification (draining
wetlands, ditching, tiling)
‹ How do the two interact?
‹ How do changes in hydrology affect
sediment loading?
How might land-clearing affect the balance?
Aggradation
Driftless Area of Wisconsin
1.5 meters of aggradation
Original floodplain
Copyright © Robert Pavlowsky 2002
New Diggins Branch, Galena River
What happens after aggradation?
Whitewater River, Driftless Area of Minnesota
690’
680’
1992 ground surface
1989 ground surface
1939 ground surface
Pre-agricultural ground surface
670’
Data provided by Natural Resources Conservation Service
Slide courtesy Jason Moeckel, DNR
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Aggradation from land clearing
Channel incision
Formation of new floodplain within elevated
“floodplain”
What happens if you then increase peak flows
through drainage modification?
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Increased peak flows can move more sediment
Higher shear stress on banks and bed can erode
more sediment
Creation of wider floodplain
If this new floodplain is inset into higher elevation
aggraded surface, there will be a net loss of
sediment to the stream (bank erosion will not be
balanced by point bar deposition)
Additional effects from concentrated flow
Gullying
Focused scour
on banks and
bed
Photo courtesy Carrie Jennings
Photo courtesy NRCS
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How much sediment can be attributed specifically to
gullies? To concentrated flow entering rivers?
We will see some of these features on the field trip.
Possible consequences of stream
disequilibrium in Minnesota River?
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Recent hydrologic changes could mobilize “legacy” sediment
from post-glacial times to land-clearing
If channel is incised, floodplain can become a sediment
source (not balanced by deposition)
Ditches and channelized reaches may still be adjusting to
inputs, through degradation and widening, mobilizing more
sediment from banks and bed.
Excess sediment can create habitat degradation, fill in
floodplain wetlands and depressions reducing storage, and
lower overall water quality
Sometimes, unstable banks have an obvious, local cause…
Copyright © Harriet Orr 2002
Sometimes, unstable banks have an obvious, local cause…
Source: NRCS
Other times, locally unstable reaches are caused
by basin-scale changes in hydrology or land use,
and thus require basin-scale remediation.
Reminder: Streams are not inherently “unstable”, but they may be
out of equilibrium with imposed conditions, esp. if change was
rapid or recent. In some cases, the adjustment phase may
mobilize a lot of sediment…
Unanswered questions on
Minnesota River
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What are the primary sources of sediment
entering the mainstem and Lake Pepin?
– Upland sources from overland flow and rilling
– Gullying in areas of concentrated flow
– Stored legacy sediment in floodplains
– Bluff erosion in incised reaches
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What is the dominant driver for excess
erosion? How much can be attributed to
changes in hydrology?
On-going research by
Minnesota Geological Survey,
National Center for Earth-surface Dynamics,
St. Croix Watershed Research Station, et al.
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Developing new techniques for sediment
fingerprinting to separate upland from stream
bank sources (Le Sueur River; Beauford Creek)
Sediment fingerprinting in Seven Mile Creek to
determine contributions from uplands, bluffs, and
stream bank/floodplain sources
Century-scale sedimentation record on Redwood
Creek upstream of dam