Sources of Suspeded Sediment in the Le Sueur River

St. Croix Watershed Research Station / SMMDept. of Soil, Water & Climate / UMN

National Ceter for Earth Surface Dynamics

What’s the problem?1. Turbidity impairment of water quality2. Accelerated infilling of Lake Pepin

What do we what to know?1. What is the source of the sediment?2. How has that changed over time?

Lake Pepin Sedimentation

1500

1600

17001800

1820

1840

1860

1880

1900

1920

1940

1960

1980

2000

0 200 400 600 800 1000

0 1 2 3 4 5 6 7 8 9

103 t yr-1

kg m-2yr-1

1,500,000 m3 per year

Natural (background)

Why the Le Sueur?• Between 80-90% of the TSS entering Lake Pepin

arises in the Minnesota River Basin

• On average 65% of the TSS leaving the Minnesota River Basin arises from the Le Sueur and Blue Earth River watersheds

TSS Loads (tons/yr)

0200000400000600000

2000

2001

2002

2003

2004

2005

Avera

ge

Blue Earth

Le Sueur

MN River Average 809,000 t/yr

Geologic ContextSource of Relief in Lower Le Sueur River• The draining of Glacial Lake Agassiz 11,500

C14 yr BP carved the Minnesota River valley• Incision of the MN River spawned

knickpoints, ~60 m high at the mouth of the Blue Earth/Le Sueur Rivers.

• Knickpoints are still migrating upstream.• The migration of knickpoints upstream

leads to high relief seen in lower Le Sueur.

Glacial Lake

Agassiz

Source of Fine-grained sediment• Le Sueur River drains the area

covered by Glacial Lake Minnesota• Fine-grained lacustrine silts and clays

cap ~3/4 of the watershed. • These sediments are easily erodible.

Tan color:Glacial lake sediment

Three Cooperative Studies(w/different approaches)

Sediment Budget & Routing ModelNational Center for Earth Surface Dynamics / Univ. of MN (and

others)

Ravines and Bluffs InventorySoil, Water & Climate and Biosystems Engineering / Univ. of MN

Sediment FingerprintingSt. Croix Watershed Research Station / Science Museum of MN

Sediment budget and routing model, Le Sueur River Watershed

Patrick Belmont, Stephanie Day, Karen Gran, Carrie Jennings, Andrea Johnson, J. Wesley Lauer, Gary Parker, Lesley Perg, Peter Wilcock

Goals and Approaches for studyPrimary Goal:

Develop a sediment budget and routing model for the Le Sueur River watershed that incorporates-- Contributions from bluffs, ravines, uplands, and stream banks-- Sediment exchange in and out of storage in floodplains

Approach:– Geomorphological assessment, surficial mapping, granulometry– Measurement of contemporary sediment fluxes at gaging stations – Annual surveys of bluffs– Meander migration and bluff retreat from air photos (65 year record)– Sediment provenance study with cosmogenic radionuclides– Sediment routing model

Slope-Area Analysis shows presence of two major knickpoints

Maple R.

Cobb R.

Le Sueur R.

Changes in slope mark the locations of major knickpoints.

The biggest knickpoint is located 35-40 km upstream of the mouth on all three major branches of the Le Sueur River.

This knickpoint has migrated an average of 3 m/yr upstream for the past 11,500 yr.

The major knickpoint at 35-40 river km on all 3 branches essentially separates the rivers into two sections. Major bluffs and ravines are primarily confined to the river below the knickpoint.

A smaller knickpoint is located far upstream on all three branches. This knick moved an avg. of 10 m/yr upstream, and may have been eroding glacial lake sediments and loose tills only. This knick triggered smaller ravines and bluffs.

Ele

vatio

n (m

)

Distance upstream (km)

Blue Earth County LiDAR was used to measure the total volume eroded from major ravines and river valleys.

Ravines

ValleysMinimum volume eroded: 1.9 x 109 MgIf divide by 11,500 years: 1.6 x 105 Mg/yr

This is similar to current rates of TSS.

However, rates were likely NOT the same for the past 11,500 yrs. We need to understand changes in sediment flux over this time to determine what pre-settlement sediment flux rates were.

Much of this history is recorded in river terraces in the lower valleys.

Le Sueur River

River Elevation (m)250 260 270 280 290 300 310 320 330

Trea

d El

evat

ion

(m)

260

270

280

290

300

310

320

330

Cobb River

River Elevation (m)260 270 280 290 300 310 320 330

Trea

d E

leva

tion

(m)

260

270

280

290

300

310

320

330

Maple River

River Elevation (m)260 270 280 290 300 310 320 330

Trea

d E

leva

tion

(m)

260

270

280

290

300

310

320

330

Terraces havebeen mapped

on all 3 majorchannels. They record incisionhistory.

3 sets of paired gages record changes in sediment flux as the river passes through the lower knick zone.

The paired gaging stations show large increases in sediment load with minor increases in discharge.

These paired gages will be used to help constrain inputs to the routing model.

Data from MPCA

40510522788TSS yield (lbs/acre)

33,3913,96622,3267,875TSS(Mg)

182,00083,000217,000197,000Drainage Area (acres)

LowerUpperLowerUpperCobbMaple2006

Measuring sediment flux through the knick zones with paired gages

200725 scans12 sites

Other constraints on sediment inputs from different sources

Ground-based side-scanning LiDAR:Used to directly measure annual bluff retreat rates.

2003 1938

Air photo analyses from 1938 to 2003 to measure bluff retreat and channel migration rates over 65 years.

0 0.5 1 Kilometers

1938

2003

Mouth of Le Sueur R.Average channel

migration rate on mainstem LS River: 20 cm/yr

Grain size analyses of floodplain deposits along the 3 major channels.

Other constraints on sediment inputs from different sources

Cosmogenic nuclides to get the bluff contributions as a relativeproportion of total sediment budget.

Al26 and Be10 are produced near the surface at a ratio of 6.1.

If they are buried, these nuclides decay, and the ratio declines.

Work by Balco et al. (2005) indicates buried tills in the Minnesota River Valley have ratios of 3.5.

We can use this as a tracer of bluff contributions.

Slide from Balco et al.

Ravines and River Bluffs in the Le Sueur Watershed

David Mulla, Joel Nelson, John Nieber, Bruce Wilson, Brad Hansen

University of Minnesota

Background• A few studies have been conducted to estimate

TSS loads in the Blue Earth River watershed arising from stream bluffs, none have been conducted in the Le Sueur River watershed

• No studies have been conducted to quantify the TSS loads arising from ravines in the Minnesota River Basin

• HSPF modeling for the Minnesota River Basin sediment TMDL requires estimates of sediment contributions from ravines, bluffs and streambanks

Ravine Identification -Methodology

• ESRI ArcGIS 9.2 Software Suite• USGS 30 m digital elevation model (DEM)• Terrain attributes

– Specific catchment area– Slope– Stream power index– Profile curvature

Blue Earth County Ravines

Le SueurRiver Watershed

Blue EarthRiver Watershed

WatonwanRiver Watershed

Preliminary Ravine Results –Le Sueur River Watershed

• Area– Watershed area 288,072 ha– Preliminary ravine area 2,083 – 2,734 ha (0.7-

0.9% of watershed)• Sediment Loss

– Watershed TSS load 283,000 t/yr– Ravine TSS load TBD

Le Sueur and Blue Earth River Bluff Locations

Blue Earth Bluffs216 sites

139 sites198 sites

19 sites

258 sites

Le Sueur Bluffs614 sites

Maple River

Conclusions• Ravines can be accurately located using GIS

and terrain analysis with 30 m DEMs

• Ravines have an impact on TSS loads in the Le Sueur River watershed that is out of proportion with their areal coverage (0.7-0.9%)

• The number of stream bluffs ranges from 216 in the Blue Earth River watershed to 614 in the Le Sueur River watershed

• Stream bluff slumping contributes large quantities of TSS to the Blue Earth and Le Sueur River watersheds

A new method for Fingerprinting Riverine Suspended Sediments

Shawn Schottler, Dylan Blumentritt, Daniel EngstromSt. Croix Watershed Research Station / Science Museum of Minnesota

0

0.5

1

1.5

2

2.5

0 0.05 0.1 0.15 0.2 0.25

Reference Lakes

Dia

mon

d

Geo

rge

Hen

ders

onLo

ng

Hoo

k

Sta

hl

Dun

ns Ric

hard

son

Kre

ighl

eS

agat

agan

Duc

k

Geo

rge

Bas

s

Fish

Bea

ver

y = 0.65 + 4.8x R 2= 0.58

Mea

n 21

0 Pb F

lux

(pC

i/cm

2 /yr)

BeFePbCsHg

BeFePbCsHg

BeFePbCsHg

BeFePbCsHg

BeFePbCsHg

Minimum Tillage

Tile DrainageConventionalTillage

Streambank

Pasture

The Question:How much from upland field erosion?

vs.How much from non-field sources?

The Challenge:How to quantify…

..and believe the answer?

Bank (etc.) Sediment

Cultivated Field

Suspended Sediment 0

0.05

0.1

0.15

0.2

0.25

Pb-210

Act

ivity

(bq/

g)

Constant Exposure to Atmosphere and Rain

Minimal Exposure to Atmosp. and Rain

…Fingerprinting Fundamentals

• No incoming rivers

• Time integrated

• Upland source integrated…… sheet + rill + gully

Seepage lakes as integrated reference systems

George Lake, Blue Earth Co.

Archive

of Field Eros

ion Fing

erprint

Mankato

St. Peter

Kasota Pond0 0.5 1 1.5 2 2.5 3 3.5 4

0

50

100

150

200

250

300

Pb-210 Cs-137

Kasota Pond Conc. (pCi/g)

Dep

th (c

m)

Riverine Depositional (Integrator) Sites… other integrator sites

Sites like Kasota Pond:

• collect and store suspended sediment

• integrate upstream erosion processes

Excess 210Pb = 1.4 pCi/g

Relative Field Contribution to Riverine SedimentAverage of Cs and Pb methods

22%

S. Fork Crow =

31%

32%

20% 42%

18%11%

27%

11% TSS-Snowmelt

36%22%Lake Pepin =

Source of Sediment to Pepin---Why change over time???

Field Non-field

1996 25% 75%

1964 33% 67%

1940 83% 17%

Pre~1860 0% 100%

• Field constant (or decreasing)

• Non-field constant until c. 1940

• Non-field accelerating since 1940

… if you express as loading,some sources not really changing

0

2

4

6

8

10

1840 1940 1964 1996

Lake Pepin- Sediment Loading

FieldNon-Field

Load

(g/c

m2 -y

r)Prairie Ag.

Project Timelines

Sediment Budget & Routing Model

Ravines and Bluffs Inventory

Sediment Fingerprinting