Causes and Consequences of Stream Instability

Home , Degradation (geology), Point bar

Minnesota River Turbidity TMDL and Lake Pepin TMDL Meeting Southern Research and Outreach Center, Waseca Thursday, July 20, 2006

TitleTitleCauses (View(View and ConsequencesMenu/ViewMenu/View ofMasterMaster Stream Instabilitytoto edit)edit) PresenterKaren Granname StreamStream “Instability”?“Instability”?

USGS StreamStream “Instability”?“Instability”?

Multi-thread, braided stream

High sediment load, High slope Relatively coarse grain size, High lateral mobility rates

USGS StreamStream “Instability”?“Instability”?

Add in photos of braided river

Wolman and Leopold, 1957 StreamsStreams DODO change,change, butbut theythey cancan alsoalso bebe outout ofof “equilibrium”.“equilibrium”.

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 Slope

Sediment Supply Water Discharge

From Lane 1955 Ex.Ex. ChannelizationChannelization 1)1) StraightenStraighten aa meanderingmeandering channelchannel 2)2) AddAdd moremore runoffrunoff Slope

Photo courtesy NRCS - Increase slope - Increase shear stress (ττ=ρρgDS) - 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 Water Discharge

Vegetation 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 HowHow dodo newnew fluvialfluvial landscapeslandscapes formform andand develop?develop?

Head-cutting; Knick-point migration Network Extension Drainage Integration

From Les Hasbargen HowHow dodo fluvialfluvial landscapeslandscapes formform andand develop?develop?

Head-cutting; Initially High Sediment Yields Knick-point migration 300decaying “exponentially” Network Extension Pasig-Potrero River Network Extension 250 Drainage Integration 200

150

100

Sediment Yield (mcm) 50

0 0 5 10 15 Years after Eruption MinnesotaMinnesota RiverRiver 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) MinnesotaMinnesota RiverRiver Landscape Evolution

Since valley formation by Glacial River Warren: Adjustments to mainstem slope Knick-point migration on tributaries Drainage integration in uplands – artificially Sediment Yield ? enhanced (draining of wetlands and lakes, Time ditching and tiling) AnthropogenicAnthropogenic signalsignal swampsswamps naturalnatural backgroundbackground ratesrates

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 MajorMajor postpost--settlementsettlement changeschanges

LandLand--clearing,clearing, midmid--1800s1800s DrainageDrainage modificationmodification (draining(draining wetlands,wetlands, ditching,ditching, tiling)tiling) HowHow dodo thethe twotwo interact?interact? HowHow dodo changeschanges inin hydrologyhydrology affectaffect sedimentsediment loading?loading? How might land-clearing affect the balance? Aggradation

Driftless Area of Wisconsin

1.5 meters of aggradation

Original floodplain

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 Aggradation from land clearing Channel incision Formation of new floodplain within elevated “floodplain”

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

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) AdditionalAdditional effectseffects fromfrom concentratedconcentrated flowflow

Gullying

Focused scour on banks and bed Photo courtesy Carrie Jennings Photo courtesy NRCS

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. PossiblePossible consequencesconsequences ofof streamstream disequilibriumdisequilibrium inin MinnesotaMinnesota River?River?

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…

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… UnansweredUnanswered questionsquestions onon MinnesotaMinnesota RiverRiver

WhatWhat areare thethe primaryprimary sourcessources ofof sedimentsediment enteringentering thethe mainstemmainstem andand LakeLake Pepin?Pepin? – Upland sources from overland flow and rilling – Gullying in areas of concentrated flow – Stored legacy sediment in floodplains – Bluff erosion in incised reaches WhatWhat isis thethe dominantdominant driverdriver forfor excessexcess erosion?erosion? HowHow muchmuch cancan bebe attributedattributed toto changeschanges inin hydrology?hydrology? OnOn--goinggoing researchresearch byby MinnesotaMinnesota GeologicalGeological Survey,Survey, NationalNational CenterCenter forfor EarthEarth--surfacesurface Dynamics,Dynamics, St.St. CroixCroix WatershedWatershed ResearchResearch Station,Station, etet al.al.

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