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Causes and Consequences of Stream InstabilityKaren Gran

Minnesota River Turbidity TMDL and Lake Pepin TMDL MeetingSouthern Research and Outreach Center, Waseca

Thursday, July 20, 2006

Stream “Instability”?Stream “Instability”?

Copyright © Steve Baxter 2002

USGS

Stream “Instability”?Stream “Instability”?

USGS

Multi-thread, braided stream

High sediment load, High slopeRelatively coarse grain size,High lateral mobility rates

Stream “Instability”?Stream “Instability”?

Add in photos of braided river

Copyright © Steve Baxter 2002

Wolman and Leopold, 1957

Streams DO change, but they can Streams DO change, but they can also be out of “equilibrium”.also be out of “equilibrium”.

Natural processes can Natural processes can be altered by changing be altered by changing inputs to the systeminputs to the system

When development When development encounters an active encounters an active stream, sometimes it is stream, sometimes it is forced to be “stable” forced to be “stable”

Lane’s Balance for Alluvial Channels

From Lane 1955

Sediment Supply Water Discharge

Grain Size Distribution Slope

1) Straighten a 1) Straighten a meandering channelmeandering channel

2) Add more runoff2) Add more runoff

Ex. Ex. ChannelizationChannelization

-- Increase slopeIncrease slope-- Increase shear stress (Increase shear stress (ττ==ρρgDSgDS))-- Increase bed erosion leading to degradation locally; Increase bed erosion leading to degradation locally;

extra sediment leads to extra sediment leads to aggradationaggradation downstream downstream -- Increased discharge could lead to channel widening/bank erosionIncreased discharge could lead to channel widening/bank erosion-- Channel may attempt to “undo” the straightening, and create a Channel may attempt to “undo” the straightening, and create a

meandering form in straightened channelmeandering form in straightened channel

Slope

Qw

Photo courtesy NRCS

Adapted from Simon 1994; USACOE 1990

Channel Evolution Model

Independent Variables imposed on the system

Sediment Supply Water Discharge

Grain Size Distribution

Vegetation

Dependent VariablesResulting 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.

From Knighton 1998

Holocene(<10,000 yr)

“Anthropocene”(<~200 yr)

How do new fluvial landscapes How do new fluvial landscapes form and develop?form and develop?

HeadHead--cutting; cutting; KnickKnick--point migrationpoint migrationNetwork ExtensionNetwork ExtensionDrainage IntegrationDrainage Integration

From Les Hasbargen

How do fluvial landscapes How do fluvial landscapes form and develop?form and develop?

0

50

100

150

200

250

300

0 5 10 15

Years after Eruption

Sed

imen

t Yie

ld (m

cm)

Pasig-Potrero River

Initially High Sediment Yieldsdecaying “exponentially”

HeadHead--cutting; cutting; KnickKnick--point migrationpoint migrationNetwork ExtensionNetwork ExtensionDrainage IntegrationDrainage Integration

Minnesota RiverMinnesota RiverLandscape EvolutionLandscape Evolution

Since valley formation by Since valley formation by Glacial River Warren:Glacial River Warren:Adjustments to Adjustments to mainstemmainstem slopeslopeKnickKnick--point migration point migration on tributarieson tributariesDrainage integration in Drainage integration in uplands uplands –– artificially artificially enhanced (draining of enhanced (draining of wetlands and lakes, wetlands and lakes, ditching and tiling)ditching and tiling)

Minnesota RiverMinnesota RiverLandscape EvolutionLandscape Evolution

Since valley formation by Since valley formation by Glacial River Warren:Glacial River Warren:Adjustments to Adjustments to mainstemmainstem slopeslopeKnickKnick--point migration point migration on tributarieson tributariesDrainage integration in Drainage integration in uplands uplands –– artificially artificially enhanced (draining of enhanced (draining of wetlands and lakes, wetlands and lakes, ditching and tiling)ditching and tiling)

?Sedi

men

t Yie

ld

Time

Anthropogenic signal swamps Anthropogenic signal swamps natural background ratesnatural background rates

PostPost--settlement rates of settlement rates of sedimentation in Lake sedimentation in Lake Pepin have increased by Pepin have increased by a factor of 10.a factor of 10.Most of the sediment Most of the sediment comes from the comes from the Minnesota River.Minnesota River.Rates have increased Rates have increased due to landdue to land--clearing, clearing, drainage modification, drainage modification, and development.and development.

From Engstrom and Almendinger, 2000

Major postMajor post--settlement changessettlement changes

LandLand--clearing, midclearing, mid--1800s1800sDrainage modification (draining Drainage modification (draining wetlands, ditching, wetlands, ditching, tiling)tiling)How do the two interact?How do the two interact?How do changes in hydrology affect How do changes in hydrology affect sediment loading?sediment loading?

Copyright © Robert Pavlowsky 2002

1.5 meters of aggradation

Original floodplain

New Diggins Branch, Galena River

How might land-clearing affect the balance? Aggradation

Driftless Area of Wisconsin

Whitewater River, Driftless Area of Minnesota

670’

680’

690’

1992 ground surface

1989 ground surface

1939 ground surface

Pre-agricultural ground surface

What happens after aggradation?

Data provided by Natural Resources Conservation ServiceSlide courtesy Jason Moeckel, DNR

AggradationAggradation from land clearingfrom land clearingChannel incisionChannel incisionFormation of new floodplain within elevated Formation of new floodplain within elevated “floodplain”“floodplain”

What happens if you then increase peak flows What happens if you then increase peak flows through drainage modification?through drainage modification?

Increased peak flows can move more sedimentIncreased peak flows can move more sedimentHigher shear stress on banks and bed can erode Higher shear stress on banks and bed can erode more sedimentmore sedimentCreation of wider floodplainCreation of wider floodplainIf this new floodplain is inset into higher elevation If this new floodplain is inset into higher elevation aggraded surface, there will be a net loss of aggraded surface, there will be a net loss of sediment to the stream (bank erosion will not be sediment to the stream (bank erosion will not be balanced by point bar deposition)balanced by point bar deposition)

Additional effects from concentrated flowAdditional effects from concentrated flow

GullyingGullying

Focused scour Focused scour on banks and on banks and

bedbed

How much sediment can be attributed specifically to How much sediment can be attributed specifically to gullies? To concentrated flow entering rivers? gullies? To concentrated flow entering rivers? We will see some of these features on the field trip.We will see some of these features on the field trip.

Photo courtesy Carrie JenningsPhoto courtesy NRCS

Possible consequences of stream Possible consequences of stream disequilibrium in Minnesota River?disequilibrium in Minnesota River?

Recent hydrologic changes could mobilize “legacy” sediment Recent hydrologic changes could mobilize “legacy” sediment from postfrom post--glacial times to landglacial times to land--clearing clearing

If channel is incised, floodplain can become a sediment If channel is incised, floodplain can become a sediment source (not balanced by deposition)source (not balanced by deposition)

Ditches and Ditches and channelizedchannelized reaches may still be adjusting to reaches may still be adjusting to inputs, through degradation and widening, mobilizing more inputs, through degradation and widening, mobilizing more sediment from banks and bed.sediment from banks and bed.

Excess sediment can create habitat degradation, fill in Excess sediment can create habitat degradation, fill in floodplain wetlands and depressions reducing storage, and floodplain wetlands and depressions reducing storage, and lower overall water qualitylower 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 Other times, locally unstable reaches are caused by basinby basin--scale changes in hydrology or land use, scale changes in hydrology or land use, and thus require basinand thus require basin--scale remediation. scale remediation.

Reminder: Streams are not inherently “unstable”, but they may bReminder: Streams are not inherently “unstable”, but they may be e out of equilibrium with imposed conditions, esp. if change was out of equilibrium with imposed conditions, esp. if change was rapid or recent. In some cases, the adjustment phase may rapid or recent. In some cases, the adjustment phase may mobilize a lot of sediment…mobilize a lot of sediment…

Unanswered questions on Unanswered questions on Minnesota RiverMinnesota River

What are the primary sources of sediment What are the primary sources of sediment entering the entering the mainstemmainstem and Lake Pepin?and Lake Pepin?–– Upland sources from overland flow and Upland sources from overland flow and rillingrilling–– GullyingGullying in areas of concentrated flowin areas of concentrated flow–– Stored legacy sediment in floodplainsStored legacy sediment in floodplains–– Bluff erosion in incised reachesBluff erosion in incised reaches

What is the dominant driver for excess What is the dominant driver for excess erosion? How much can be attributed to erosion? How much can be attributed to changes in hydrology?changes in hydrology?

OnOn--going research by going research by Minnesota Geological Survey, Minnesota Geological Survey,

National Center for EarthNational Center for Earth--surface Dynamics, surface Dynamics, St. Croix Watershed Research Station, et al.St. Croix Watershed Research Station, et al.

Developing new techniques for sediment Developing new techniques for sediment fingerprinting to separate upland from stream fingerprinting to separate upland from stream bank sources (Le Sueur River; bank sources (Le Sueur River; BeaufordBeauford Creek) Creek)

Sediment fingerprinting in Seven Mile Creek to Sediment fingerprinting in Seven Mile Creek to determine contributions from uplands, bluffs, and determine contributions from uplands, bluffs, and stream bank/floodplain sourcesstream bank/floodplain sources

CenturyCentury--scale sedimentation record on Redwood scale sedimentation record on Redwood Creek upstream of damCreek upstream of dam