U.S. Department of the Interior

U.S. Geological Survey

Analysis of Suspended-Sediment 

Concentrations and Radioisotope Levels in 

the Wild Rice River Basin, Northwestern 

Minnesota, 1973–98

By M.E. Brigham1, C.J. McCullough1, and P. Wilkinson2

Water-Resources Investigations Report 01–4192

Prepared in cooperation with the Legislative Commission on Minnesota Resources

1

 U.S. Geological Survey

2 Freshwater Institute, Department of Fisheries and Oceans, Winnipeg, Manitoba
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Use of trade, product, or firm names in this report is for identification purposes only and does not

constitute endorsement by the U.S. Government.

Mounds View, Minnesota, 2001

For additional information write to:

District Chief

U.S. Geological Survey, WRD

2280 Woodale Drive

Mounds View MN 55112

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver CO 80225

For more information on the USGS in Minnesota, you may connect to the Minnesota District home page at:

http://mn.water.usgs.gov

For more information n all USGS reports and products (including maps mages, and computerized data), call 1-888-ASK-USGS

Water-Resources Investigations Report 01–4192
 iii

CONTENTS

Abstract.................................................................................................................................................................................... 1

Introduction.............................................................................................................................................................................. 1

Environmental setting ....................................................................................................................................................... 2

Hydrologic conditions during 1998 sampling................................................................................................................... 2

Acknowledgements ........................................................................................................................................................... 2

Analysis of historical suspended-sediment data ...................................................................................................................... 2

Suspended-sediment data .................................................................................................................................................. 2

Relation to streamflow, season, and time.......................................................................................................................... 2

Background................................................................................................................................................................. 2

Regression model and results of sediment analysis.................................................................................................... 3

Sediment loads .................................................................................................................................................................. 5

Radioisotope levels of sediments............................................................................................................................................. 7

Background on use of radioisotope methods .................................................................................................................... 7

Sources of isotopes ..................................................................................................................................................... 7

Behavior of 210Pb, 137Cs, and 7Be in soils................................................................................................................. 8

Radioisotope study approach...................................................................................................................................... 8

Sampling methods............................................................................................................................................................. 9

Laboratory analysis of radioisotopes................................................................................................................................. 9

Radioisotope levels of sediments ...................................................................................................................................... 9

Implications for sediment sources................................................................................................................................... 12

Implications and limitations of study..................................................................................................................................... 17

Summary................................................................................................................................................................................ 20

References.............................................................................................................................................................................. 20

ILLUSTRATIONS

Figure 1. Map showing Wild Rice River Basin, major tributaries, ecoregions, and location map; detailed hydrography 

including ditches and minor tributaries.......................................................................................................................... 3

Figure 2. Suspended-sediment relations, Wild Rice River at Twin Valley, Minnesota, water years 1973–98, 

suspended-sediment concentration versus streamflow, flow-adjusted suspended-sediment concentration 

versus julian date, and suspended-sediment load versus streamflow ............................................................................ 5

Figures 3–8. Graphs showing:

3. Estimated annual suspended-sediment loads, Wild Rice River at Twin Valley, Minnesota, and at Hendrum, 

Minnesota ................................................................................................................................................................ 6

4. Excess lead-210 activity, cesium-137 activity, and beryllium-7 activities in suspended-sediment samples 

from the Wild Rice River at Twin Valley, Minnesota and at Hendrum, Minnesota; in surficial soils from

 cultivated fields; and estimated activities contributed from erosion of streambank material .............................. 13

5. Suspended-sediment, excess lead-210, cesium-137, and beryllium-7 versus time, overlain on hydrograph for 

Wild Rice River at Twin Valley, Minnesota, spring-summer 1998...................................................................... 14

6. Suspended-sediment, excess lead-210, cesium-137, and beryllium-7 versus time, overlain on hydrograph 

for Wild Rice River at Hendrum, Minnesota, spring-summer 1998 ..................................................................... 15

7. Model estimates of the fraction of suspended-sediment in the Wild Rice River at Twin Valley, Minnesota, 

that originates from erosion of surficial cultivated soils and by vertical erosion of streambank material 

determined from excess lead-210, cesium-137, and beryllium-7, and stream hydrograph.................................. 18

8. Model estimates of the fraction of suspended-sediment in the Wild Rice River at Hendrum, Minnesota, 

that originates from erosion of surficial cultivated soils and by vertical erosion of streambank material 

determined from excess lead-210, cesium-137, and beryllium-7, and stream hydrograph................................... 19
 iv

TABLES

Table 1. Radioisotope activities and inventories from reference cores in the Wild Rice River Basin.................................. 10

2. Radioisotope activities in suspended sediments of the Wild Rice River................................................................ 11

3. Radioisotope activities in the upper 1 cm of cultivated soils of the Wild Rice River Basin, sampled after autumn 

harvest and plowing, September-November 1998 ................................................................................................. 12

4. Fraction of suspended sediment originating from soil erosion, as determined from excess lead-210, cesium-137, 

and beryllium-7 activities in sediments and source areas ...................................................................................... 17

CONVERSION FACTORS

Multiply By To obtain

Millimeters (mm 0.0393 inch

Centimeter (cm) 0.393 inch

Square centimeters (cm2) 0.154 square inches 

Meter (m) 3.281 foot (ft)

Gram (g) 0.35 ounce (avoirdupois) 

Gram per cubic centimeter (g/cm3) 1 metric ton per cubic meter (t/m3)

Liter (L) 0.264 gallon

Square kilometer (km2) 0.3861 square mile (mi2)

Cubic feet per second (ft3/s) 0.0283 cubic meters per second (m3/s)

Millibecquerels (mBq) 16.67 disintegrations per minute (dpm)

Millibecquerels (mBq) 37 picocuries (pCi) 

Meters per kilometer (m/km) 10 percent slope

Centimeters per year (cm/yr) 100 centimeters per century

Metric ton per day (t/d) 1,000 kilograms

Radioactivity is reported in millibecquerels (mBq). A becquerel is one radioactive disintegration (decay) per second (dps). Activities 

are expressed on a dry-mass basis—millibecquerels per gram of dry sediment or soil (mBq/g). Inventories of radioisotopes are 

expressed as the amount of isotope contained in a vertical soil profile under a 1 cm2 area, in millibecquerels per square centimeter 

(mBq/cm2). 
1

Analysis of Suspended-Sediment 

Concentrations and Radioisotope Levels 

in the Wild Rice River Basin, Northwestern 

Minnesota, 1973–98

By M.E. Brigham, C.J. McCullough, and P. Wilkinson

ABSTRACT

We examined historical suspended-sediment data and activities of fallout radioisotopes (lead-210 [210Pb], cesium-137 [137Cs], and 

beryllium-7 [7

Be]) associated with suspended sediments and source-area sediments (cultivated soils, bank material, and reference soils) 

in the Wild Rice River Basin, a tributary to the Red River of the North, to better understand sources of suspended sediment to streams 

in the region. Multiple linear regression analysis of suspended-sediment concentrations from the Wild Rice River at Twin Valley, Minnesota indicated significant relations between suspended-sediment concentrations and streamflow. Flow-adjusted sediment concentrations tended to be slightly higher in spring than summer-autumn. No temporal trends in concentration were observed during 1973–98. 

The fallout radioisotopes were nearly always detectable in suspended sediments during spring-summer 1998. Mean 210Pb and 7

Be activities in suspended sediment and surficial, cultivated soils were similar, perhaps indicating little dilution of suspended sediment from lowisotopic-activity bank sediments. In contrast, mean 137Cs activities in suspended sediment indicated a mixture of sediment originating 

from eroded soils and from eroded bank material, with bank material being a somewhat more important source upstream of Twin Valley, 

Minnesota; and approximately equal fractions of bank material and surficial soils contributing to the suspended load downstream at Hendrum, Minnesota. This study indicates that, to be effective, efforts to reduce sediment loading to the Wild Rice River should include 

measures to control soil erosion from cultivated fields. 

INTRODUCTION

Soil erosion and sediment loading to streams are concerns in 

the Red River of the North Basin (RRB) in Minnesota, North 

Dakota, South Dakota, and Manitoba, Canada. Soil erosion may 

reduce cropland fertility. Agricultural drainage ditches fill with 

eroded sediment over time, and require costly ditch maintenance. 

High suspended-sediment concentrations can also adversely 

affect aquatic ecosystems (Waters, 1995). Water utilities that use 

water from the Red River of the North (Red River) as a source of 

drinking water must spend more to treat water that has high sediment concentrations. Lake Winnipeg, in Manitoba, receives most 

of its tributary sediment loading from the Red River. Eutrophication in southern Lake Winnipeg due to sediment and nutrients 

(some portions of which are originally sediment-bound) is a concern. To better understand the dominant sources of sediment—

channel processes or upland soil erosion—to streams in the RRB, 

the U.S. Geological Survey (USGS) in cooperation with the Legislative Commission on Minnesota Resources studied soils and 

suspended sediments of the tributary Wild Rice River Basin of 

northwestern Minnesota. 

The RRB is set in glacial-lake-bed, glaciofluvial, and 

morainal topography. Nearly all of the streams flow through glacial deposits or glacial lake-bed sedimentary deposits, and exhibit 

channel meanders, cut banks, and point bars, and often fairly turbid waters. Much of the RRB—particularly in the Red River Valley—is cultivated cropland, and soil erosion from cropland also 

contributes to the sediment load in streams. It is widely accepted 

that sediment sources in streams in such settings are comprised of 

sediment that originates both from eroded soil and from erosion of 

stream-bank sediments (Colby, 1963). The relative amounts from 

these two sources in a given stream is seldom known. It is important for natural resource managers to gain a better understanding 

of sediment sources, so that management efforts can be targeted 

accordingly.

The study objectives were to: (1) analyze suspended-sediment concentrations at stream sites within the Wild Rice River 

Basin for possible relations to streamflow, seasonality, and longterm trends, and calculate sediment loads; (2) determine the relative importance of soil erosion versus streambank erosion as 

potential sources of sediment to the Wild Rice River, based on 

activities of the fallout radioisotopes lead-210 (210Pb), cesium137 (137Cs), and, secondarily, beryllium-7 (7

Be) in suspended 

sediments in transport and in potential sources; and (3) determine 

gross erosion rates from upland source areas using radioisotope 

analysis of soil cores from cultivated and undisturbed settings. 

This report describes results related to objectives (1) and (2). 

Objective (3), an analysis of soil-core isotopic inventories measured to determine historical upland erosion rates, is being 

addressed separately (C.J. McCullough,U.S. Geological Survey, 

written commun., 2001). 

This report summarizes suspended-sediment concentration 

data collected during 1973–98. Radioisotope samples were collected during 1998. Sampling for isotopic activities was targeted 

toward runoff events, with minimal sampling during base flow.
2

Environmental setting

The Wild Rice River Basin (4,170 

km2) drains parts of three ecoregions 

(Omernik and Gallant, 1988; Stoner and 

others, 1993) (fig. 1). The headwaters are 

in the Northern Lakes and Forests ecoregion, which is predominantly forested 

land with lakes and minimal agriculture 

(mostly grazing). Stream slopes in this 

ecoregion average 0.95 m/km (Stoner and 

others, 1993). Farther west, the middle 

reaches span the North Central Hardwood 

Forest ecoregion, where the land cover is 

mixed forest, grazing, cropland, with 

some lakes. Stream slopes in this ecoregion are 0.5–1.7 m/km. Lower reaches of 

the basin are in the Red River Valley 

ecoregion, where about 80 percent of the 

land is cultivated. This ecoregion is the 

lake bed of Glacial Lake Agassiz, and has 

very low-gradient land surface with predominantly silt and clay soils. Numerous 

beach ridges from glacial Lake Agassiz, 

and a highly branched tributary system 

(fig. 1b) punctuate the transitional zone 

between North Central Hardwood Forest 

and Red River Valley ecoregions. In the 

Red River Valley ecoregion, an extensive 

network of drainage ditches (fig. 1b) augments natural drainage to more quickly 

remove excess water from cultivated 

fields. 

Soils the Wild Rice River Basin are 

frozen in winter months, and probably 

have limited erodability during springsnowmelt runoff, when the soils begin to 

thaw. Also, in early spring, soils have 

probably settled from fall tillage. Soils are 

potentially most erodible during late 

spring, after thawing, but before significant crop cover exists. Soils are less erodible during summer due to crop cover and 

during autumn because less precipitation 

typically falls than during spring.

Erodible cut banks are prevalent in 

the basin. The following are estimates of 

bank heights at cut bank areas along the 

Wild Rice River: along lower reaches of 

the river (from about 13 km [8 miles] east 

of Ada, Minn. to the confluence with Red 

River), bank heights are about 0.9–1.2 m 

(3–4 ft); farther upstream (about 9.6 km [8 

miles] east of Ada to east of Twin Valley, 

Minn.), bank heights are about 7.5–12 m 

(25–40 ft); farther upstream (western 

Mahnomen County), bank heights are 

3.3–4.5 m (10–15 ft); and in the headwaters (eastern Mahnomen County), bank 

heights are about 0.9–1.2 m (3–4 ft) (estimates from Curtis Borchert, Norman 

County Soil and Water Conservation District, oral commun., 2001). Separate estimates of typical bank heights are: 1.8–3.6 

m (6–12 ft) near Ada, with some 3.6–4.4 

m banks (12–14 ft); and maximum bank 

heights of 12 m (40 ft) near Twin Valley 

(Jerry Bents, Houston Engineering, Fargo, 

N. Dak., oral commun., 2001). Lastly, 

typical bank heights were estimated at 

1.8–3.6 m (6–12 ft) downstream of Twin 

Valley; about 2.4 m (8 ft) upstream of 

Twin Valley; and 1.2–1.8 m (4–6 ft) farther upstream, in Mahnomen County 

(Henry Van Offelen, Minnesota Department of Natural Resources, oral commun., 

2001). From these estimates, a typical 

height of eroded cut banks along the Wild 

Rice River is about 4 m, possibly as low as 

2–3 m according to some estimates. 

Hydrologic conditions during 

1998 sampling

A very early snowmelt occurred in 

1998. Large spring storms and runoff 

were followed by dry conditions from mid 

to late summer. May 1998 was unusually 

rainy, resulting in considerable runoff. 

May rainfall occurred prior to significant 

crop cover—any leaf growth that had 

started was insufficient to shelter soils 

from raindrop impact—thus, soils were 

highly susceptible to erosion. Heavy rains 

in early July fell on more settled soils and 

established crops. 

Acknowledgments

Glenn Skuta and Nolan Baratono 

(Minnesota Pollution Control Agency) 

helped initiate this study. The following 

individuals offered helpful advice during 

the planning of the study: Desmond Walling (University of Exeter, United Kingdom); Jerry Ritchie (U.S. Department of 

Agriculture); Edward Nater and Jay Bell 

(University of Minnesota); Daniel Engstrom and Shawn Schottler (St. Croix 

Watershed Experiment Station, Science 

Museum of Minnesota); and Waite 

Osterkamp and Greg Payne (U.S. Geological Survey). Fred Wrona, Carol Casey, 

and Bruce Holiday (National Hydrology 

Research Institute, Saskatoon, Sask., Canada; and Environment Canada, Regina, 

Sask., Canada) loaned equipment that was 

critical to the study. We thank James Fallon (U.S. Geological Survey), and Sharon 

Fitzgerald (U.S. Geological Survey) for 

helpful reviews of this report. 

ANALYSIS OF HISTORICAL 

SUSPENDED-SEDIMENT 

DATA

Suspended-sediment data

Considerable data exist for the Wild 

Rice River at Twin Valley (site 05062500, 

fig 1a). During some years in the 1970’s, 

approximately daily sediment samples 

were collected during open water season; 

less frequent samples were collected in the 

mid-1990’s. Few historical sediment data 

exist for the Wild Rice River at Hendrum 

(site 05064000, fig. 1a); therefore, we collected additional samples at this site. Sediment samples, both historical and recent, 

were collected and analyzed by traditional 

USGS methods (Guy, 1969; Guy and Norman, 1970; Tornes and others, 1997). 

Sediment-concentration data were 

retrieved from the USGS water-quality 

data base (QWDATA) and from paper 

files. Many historical data exist in the 

USGS daily-values data base (ADAPS); 

however, daily data are typically 

smoothed to produce daily-average 

records, and rounded, removing some laboratory precision. Thus, raw data 

extracted from paper files were preferred 

for statistical analyses performed herein. 

The large historical data set for Twin Valley includes samples collected by several 

slightly different methods (single-vertical, 

multiple-vertical, equal-width-increment), 

and different sampling equipment. Data 

were analyzed without regard to specific 

sampling methods or equipment.

Relation to streamflow, 

season, and time

Background

Suspended-sediment concentrations 

are often related to streamflow. Higher 

stream velocities, which correspond to 

higher streamflows, are potentially more 

erosive and can carry greater sediment 

loads than slower-moving water. Also, 

soil erosion contributes sediment to overland runoff, and higher streamflows result 

from overland runoff compared to base
Sand

Hill

River

Marsh

River

Wild

Rice

River

South

Branch

Buffalo

River

Red

River

of

the

North

Twin

Lakes

White

Earth

Lake

Upper

Rice

Lower Lake

Rice

Lake

Bagley

Mahnomen Ada

Hendrum

Twin

Valley

Moorhead

POLK COUNTY

NORMAN

COUNTY

MAHNOMEN

COUNTY

CLEARWATER

COUNTY

CLAY COUNTY

BECKER

COUNTY

TRAILL CO. CASS CO. NORTH DAKOTA

USGS streamflow

gage 05064000

USGS streamflow

gage 05062500

96°30' 96° 95°30'

47°30'

47°

Sand

Hill

River

Marsh

River

Wild

Rice

River

South

Branch

Buffalo

River

Red

River

of

the

North

Twin

Lakes

White

Earth

Lake

Upper

Rice

Lower Lake

Rice

Lake

96°30' 96° 95°30'

47°30'

47°

MINNESOTA

Wild Rice

River

Basin

0 10 20 30 KILOMETERS

0 10 20 30 MILES

(a) Wild Rice River Basin, major tributaries, ecoregions, and location map. (b) detailed hydrography including

ditches and minor tributaries. (Ecoregion boundaries from Omernik, 1987 and Stoner and others, 1993.)

EXPLANATION

Ecoregion boundary

Wild Rice River Basin boundary

(a)

(b)

Red River Valley

Ecoregion

North Central Hardwood

Forest Ecoregion Northern Lakes

and Forests Ecoregion

Cultivated soil sampling site

Reference soil sampling site

Bank material sampling site

EXPLANATION

Base from U.S. Geological Survey,

Digital data, 1:100,000, 1993,

U.S. Albers projection.

Base from U.S. Geological Survey,

Digital data, 1:100,000, 1993,

U.S. Albers projection.

Figure 1.

3
4

 flow. Often, both streamflow and suspended-sediment data for a site are 

approximately log-normally distributed. 

Thus, log-transformed concentration and 

streamflow data are typically used in data 

analysis. 

Often, the sediment concentrations 

exhibit hysteresis with respect to streamflow—concentrations are higher during 

periods of rising stage, and lower during 

periods of falling stage during a single 

runoff event. Colby (1963, p. A23) notes 

that: “Peak concentration of fine material 

early in the runoff is consistent with the 

idea that loose soil particles at the beginning of a storm will be eroded by the first 

direct runoff of appreciable amount.” 

Typically in the Wild Rice River, multiple 

samples were not collected over single 

runoff events, so there are insufficient data 

to determine hysteresis effects in the Wild 

Rice River.

Seasonality in sediment data could be 

related to several physical factors: cropping practices, wind erosion, water erosion, and frozen soils during winter. In the 

RRB, typical cropland practice is: plant in 

the spring (winter wheat is planted in the 

fall), and harvest and till soil in the fall. 

Bare soil, susceptible to both wind and 

water erosion, covers large areas from fall 

until the next season’s crops grow. Fall 

months tend to be drier than growing season months, with few heavy rainstorms. 

Freezing and snow cover minimize, but do 

not eliminate, erosion during the winter. 

High winds erode snow and topsoil, which 

tend to be deposited in ditches and vegetated areas. A portion of winter-eroded 

soil, particularly that deposited in ditches, 

may be readily transported to streams. 

Wind erosion of soils may be more acute 

during spring months, particularly dry 

springs, than in winter because frozen 

soils are less erodible and runoff is uncommon during most winters in the RRB. 

These considerations combine to result in 

the greatest soil-erosion rates (from water 

erosion) expected during runoff events in 

the spring (before plant growth stabilizes 

the soils and crop canopy protects soils 

from the effects of precipitation) and 

autumn (after harvesting and tillage, when 

soils are most disturbed if rainfall is 

greater than normal. Conversely, the lowest soil erosion rates are expected during 

base flows of winter months, and possibly 

during mid-summer, when crop vegetation 

minimizes the erosive effects of direct 

impact of raindrops. Hence, seasonality is 

considered in data analysis.

Long-term trends in sediment concentrations could be caused by large-scale 

changes in tillage practices; changes in 

rainfall-runoff relations due to changes in 

land use; and construction of dams that 

would tend to trap sediments. Therefore, 

time is included in data analysis to assess 

long-term (multi-year or decadal) trends. 

Regression model and results of 

sediment analysis

Data from 913 suspended-sediment 

samples from the Wild Rice River at Twin 

Valley were analyzed in detail (maximum 

of one sample per day). Only 35 values 

exist for the Wild Rice River at Hendrum; 

therefore detailed exploratory data analysis was not performed for data from that 

station. Instead, it was assumed that the 

form of the regression model developed 

for the Twin Valley station (discussed 

below) was suitable for the Hendrum station. Because of the limited data for the 

Hendrum station, model validity cannot 

adequately be assessed, and load estimates 

from that site are concomitantly less certain. 

Trends in suspended-sediment concentration with respect to streamflow, season, and time were analyzed with multiple 

linear regression analysis, using the REG 

procedure of SAS software (version 8, 

SAS Institute, Cary, N.C.). The regression 

model is expressed by the following equation:

log10S = a + blog10Q + c(log10Q)2 + 

dQ0.5 + esine(2πt) + fcosine(2πt) + gt + ε

 (1)

where log10 is the base-10 logarithm 

function; 

S is suspended-sediment concentration, in milligrams per liter; 

Q is streamflow, in cubic meters per 

second;

 t is time, in years; 

ε is the error, or residual, which is the 

difference between the predicted value 

and an individual observation; and 

a-g are regression coefficients. 

The trigonometric functions (sine and 

cosine) of time account for the seasonal 

component of variability in sediment concentrations. Significant temporal trends 

are indicated if the time term (t) is significant (that is, if the coefficient g differs significantly (αcri t = 0.05) from zero). 

Residuals from a simpler model, excluding the (log10Q)2

 and Q0.5 terms, exhibited heteroscedasticity (changing variance 

over the range in log10Q values); these 

terms were therefore included. Exclusion 

of the Q0.5 term resulted in overprediction 

of loads at the highest load values, thus 

overestimating annual loads for two highflow years for which nearly daily sediment 

samples were collected (1978–79). 

An attempt to account for hysteresis, 

by comparing suspended-sediment concentrations to the change in gage height 

over a four-hour period prior to sampling, 

produced no significant relation. Thus, no 

term to account for hysteresis was 

included in the regression equation. 

The streamflow component of equation 1 (plotted as the smooth line in fig. 2a) 

fits log-transformed sediment concentrations reasonably well, although there is 

considerable variability in the sediment 

data. There is a significant, but small seasonal influence on suspended-sediment 

concentrations: flow-adjusted values 

[residuals from regression of log10S versus log10Q, (log10Q)2

, and Q0.5] are greatest in the spring and early summer, and are 

least during late-summer through autumn 

(fig. 2b). 

To analyze for temporal trends, data 

were trimmed to a maximum of one sample per week (by selecting the first sample 

in a one-week period), resulting in 220 

samples used in the analysis. This was 

done for two primary reasons: (1) the 

sampling frequency was much greater 

during the 1970’s than 1990’s, and spurious trends can result from unbalanced data 

sets, and (2) spurious “significant” trends 

can also result from large numbers of samples. No significant temporal trends were 

observed; therefore, time (t) was removed 

from the regression analysis for load computations. The other regression coefficients (a-f in equation 1) were nearly 

unchanged regardless of which data set 

(all data or trimmed data set) was used.
5

 Sediment loads

A regression equation similar to equation 1 was used to calculate suspendedsediment loads:

log10L = a + blog10Q + c(log10Q)2 + 

dQ0.5 + esine(2πt) + fcosine(2πt) + ε (2)

where L = suspended-sediment load 

(the product of measured concentration 

and daily mean streamflow, converted to 

metric tons per day), and the time term has 

been dropped. This approach produces 

regression estimates of log-transformed 

loads (fig. 2c). Loads were calculated 

using the program Estimator (version 

96.04 and Estimator2000; written by Timothy Cohn, U.S. Geological Survey), 

which uses the minimum variance unbiased estimator (MVUE) to correct for 

back-transformation bias from log space to 

linear space (Cohn and others, 1992; Cohn 

and others, 1989; Helsel and Hirsch, 

1992). 

Regression-predicted loads were calculated using daily mean flow for each 

gaging station. Predicted daily loads were 

then summed over the entire year to produce estimates of annual sediment loads. 

Annual loads of suspended sediment (fig. 

3) vary greatly from year to year, as 

expected from the highly variable flow 

conditions. 

The mean annual suspended-sediment 

load for water years 1973–98, excluding 

years for which no flow data exist 

(denoted by “nd” in fig. 3), is 31,500 metric tons per year at Twin Valley, and 

60,000 metric tons per year at Hendrum. 

Sheet erosion of soil from agricultural

SUSPENDED-SEDIMENT CONCENTRATION, AS BASE-10

LOGARITHM OF MILLIGRAMS PER LITER

RESIDUAL, AS BASE-10 LOGARITHM UNITS

3.5

3.0

2.5

2.0

1.5

1.0

0.5 -1.5

-1.0

-0.5

0

0.5

1.0

1.5

JAN.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

Suspended sediment relations, Wild Rice River at

Twin Valley, Minnesota, water years 1973-98, (a) suspendedsediment concentrations versus streamflow, (curved line represents predicted values, from regression of log (sediment concentrations), versus log (flow), log (flow) squared, and square

root of flow), (b) flow-adjusted suspended sediment concentrations (residual from regression described in [a]) versus julian

date, (curved line is trigonometric function from regression of

log-transformed suspended sediment concentrations versus

flow and seasonal terms, as described in text), and (c) suspended-sediment load versus streamflow. Curved line represents predicted values from the regression of log (sediment

load) versus log flow), log (flow) squared, and square root

of flow. [Concentrations are in milligrams per liter; loads are

in metric tons per day; and streamflow is in cubic meters per

second.]

10

10 10

10

10 10 (

(a) (b)

SUSPENDED-SEDIMENT LOAD, AS BASE-10

LOGARITHM OF METRIC TONS PER DAY

LOG STREAMFLOW, IN CUBIC METERS PER SECOND 10

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

4

3

2

1

0

-1

(c)

Figure 2.
SUSPENDED-SEDIMENT LOAD, IN METRIC TONS PER YEAR

WATER YEAR

250,000

200,000

150,000

100,000

50,000

0

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Estimated annual suspended-sediment loads, Wild Rice River at Twin Valley, Minnesota, and at Hendrum, Minnesota. NDNDNDNDNDNDNDND

64

0

29

0

120

0

3

0

232

0

208

0

193

4

1

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

18

0

9

1

9

0

6

0

3

1

18

23

5

Twin Valley

Hendrum

Load

ND Not determined for water year

95 Percent confidence interval for sediment load

4 Number of samples

Figure 3.

6
7

 fields would have to erode, on average, 

the upper 0.022 mm/yr of soil (2.2 mm per 

century) to supply the mean sediment load 

at Twin Valley, and 0.025 mm/yr (2.5 mm 

per century) to supply the mean sediment 

load at Hendrum. This estimate results 

from dividing the sediment load (t/d) by 

average bulk density (1.2 g/cm3

, or 1.2 

metric tons per m3

); then dividing by the 

area of agricultural land (drainage area at 

Twin Valley = 2,405 km2

, 50 percent of 

which is agricultural, including cropland 

and pasture (Lorenz and Stoner, 1996); 

drainage area at Hendrum = 4,038 km2

, 

and assuming the same percentage of agricultural land as the basin upstream of Twin 

Valley, although the western portion of 

basin is more highly cultivated). 

A parallel estimate can be developed 

if one assumes all suspended sediment in 

the river came from streambank erosion. 

Assuming a mean bank height of 4 m, 

channel length of 341 km (the sum of all 

channel lengths in the Wild Rice River 

Basin fig. 1a), and cut banks occurring 

only on one side of the river, then net bank 

erosion of 3.7 cm/yr (3.7 m per century) 

would be required to supply the sediment 

load at Hendrum. That is, entire vertical 

sections of erodible cut bank would 

recede, or erode, by an average of 3.7 

cm/yr, with no redeposition, to supply the 

sediment load. If bank erosion occurring 

along 25 percent of the river banks, then 

approximately 15 cm/yr (or 15 m per century) (3.7 ÷ 0.25) of net bank erosion 

would be required to supply the sediment 

load. 

Delivery (or yield) of sediment from 

large watersheds is known to be a small 

fraction of total erosion (Walling, 1983), 

although that fraction varies widely 

among watersheds. Much of the eroded 

sediment is redeposited on the land surface (soils) and in the channel environment, including flood plain, point bars, 

and stream-bottom sediments (soils and 

bank materials). Assuming a delivery 

ratio of 0.1 (10 percent of eroded material 

is delivered from the basin as suspended 

sediment, and 90 percent is redeposited), 

which is reasonable for watersheds larger 

than 1,000 km2

 (Walling, 1983), then total 

erosion of soils of about 25 mm (2.5 cm, or 

about 1 inch) per century would be 

required, whereas total horizontal bank 

cutting of 37–150 m per century would be 

required. Thus, net rates of erosion (total 

erosion minus redeposition) needed to 

supply the annual sediment loads in the 

Wild Rice River are quite small if soil is 

the dominant source. Total rates of bank 

erosion required to supply net delivery of 

suspended sediments are very high if one 

assumes that about 90 percent of eroded 

sediment is redeposited in the channel 

environment. Redeposition of sediments 

from bank erosion would be minimal for 

fine (<63 μm) sediments, but could be 

substantial for coarser sediments 

(>63 μm)

RADIOISOTOPE LEVELS OF SEDIMENTS

Background on use of 

radioisotope methods

Radioisotope methods offer promise 

in soil erosion and sediment-source studies, and researchers continue to refine 

appropriate models. Since early studies 

by Ritchie and others (Ritchie and 

McHenry, 1975; Ritchie and others, 

1974), fallout radioisotopes have been 

used increasingly in soil erosion and sedimentation studies. Early studies relied 

exclusively on 137Cs, whereas more 

recently, various radioisotopes have been 

used, sometimes in conjunction with other 

geochemical measurements. 

The basis for the use of fallout radioisotopes in sediment studies can be summarized as follows: 210Pb, 137Cs, and 7Be 

are deposited on the earth’s surface as 

atmospheric fallout at rates that are 

thought to be spatially uniform, within a 

limited geographic range (similar latitude 

and annual precipitation). These isotopes 

sorb to soil particles and decay at rates 

described in terms of half-life (t1/2), which 

is the time that it takes for one-half of an 

isotope to decay producing a daughter 

product. Because decay of these fallout 

isotopes is rapid, with respect to geologic 

time scales, deeper soils are essentially 

devoid of fallout isotope activities. The 

areal input of these isotopes can be measured in soil profiles taken from areas 

known to be non-erosional and non-depositional (other than from the atmosphere). 

Comparing areal isotopic inputs to measured isotopic inventories in study soil 

profiles and understanding how these 

inventories change in soil profiles can be 

used to assess redistribution of surficial 

soils, and to assess sources of suspended 

sediment, assuming appropriate end members can be identified. 

Sources of isotopes

210Pb (t1/2=22.3 yr) is a naturally 

occurring isotope, formed in the uranium238 decay series (Turekian and others, 

1977). Radium-226 (226Ra), also a member of this series, decays to form radon222 (222Rn, t1/2=3.82 d), a gaseous species. A portion of 222Rn escapes from the 

earth’s crust and is distributed in the atmosphere, where it quickly decays through a 

series of short-lived isotopes to form 

210Pb. 210Pb is effectively removed from 

the atmosphere by wet and dry deposition 

(Baskaran and others, 1993). Because not 

all the 222Rn escapes the earth’s crust 

before it decays, a portion of the 210Pb 

present in the earth’s crust (for example, 

soil and sediment) is formed in situ as a 

product of this decay series. This portion, 

termed supported 210Pb, has an activity 

equal to that of 226Ra, owing to a secular 

equilibrium between 226Ra and its daughter products. Excess (unsupported) 210Pb 

is that portion of 210Pb that is derived from 

atmospheric fallout (from decay of atmospheric 222Rn gas and subsequent fallout 

of the daughter isotope). Subtracting measured 226Ra activity from measured (total) 

210Pb activity yields excess 210Pb activity. 

137Cs (t1/2=30.2 yr) is formed as a byproduct of thermonuclear reactions, and 

was introduced to the atmosphere primarily from above-ground nuclear device 

testing. Significant inputs of 137Cs into 

the atmosphere began in 1954, peaked in 

1963, and afterwards decreased sharply 

(Delaune and others, 1978; He and others, 

1996) due to the atmospheric test ban 

treaty. Since 1968, atmospheric 137Cs 

deposition has been negligible. Fallout of 

137Cs resulting from the Chernobyl explosion of 1986 was mainly limited to 

Europe (Antonopoulos-Domis and others, 

1995; Owens and others, 1996; Ritchie 

and McHenry, 1990; Rosén and others, 

1998). 

7

Be is a short-lived (t1/2=53.3 d), naturally occurring isotope produced by cosmic ray bombardment of oxygen and 

nitrogen in the atmosphere (Lal and others, 1958; Rangarajan and Gopalakrish-
8

nan, 1970). Like 210Pb and 137Cs, 7Be 

strongly sorbs to particles in the atmosphere and rapidly falls out to the earth’s 

surface (Todd and others, 1989; and references therein).

The behavior of 210Pb, 137Cs, and 7Be 

is well documented (reviewed by Ritchie 

and McHenry, 1990; Todd and others, 

1996; Wallbrink and Murray, 1993; Walling and others, 1999). Wet deposition is 

the predominant fallout mechanism, 

although some dry deposition of aerosolbound radioisotopes occurs. Rates of fallout, therefore, are highly dependant on 

precipitation patterns. Deposition rates, 

and hence areal burdens in soils (inventories), of the longer-lived isotopes, 210Pb 

and 137Cs, are considered to be regionally 

uniform because short-term variability in 

precipitation (deposition) is smoothed 

over time by many precipitation events. 

Conversely, 7Be may exhibit considerable 

spatial variability within a region because 

individual precipitation events are not 

evenly distributed, and the isotope decays 

rapidly. 

Although small amounts of these fallout isotopes may be taken up by plants and 

removed with harvested crops, most of the 

deposited isotope remains in the particle 

phase. The primary mechanisms by which 

isotopic activities are diminished are 

assumed to be radioactive decay, dilution 

by limited penetration of surficial activities into the soil column, and erosion of the 

soil from a site. 

The source functions for the three isotopes, with respect to their decay rates, are 

quite different. 210Pb deposition is 

thought to be relatively constant over time 

(it is naturally replenished in the atmosphere via degassing of 222Rn). The 210Pb 

inventory is large and the decay rate is 

slow, relative to seasonal variability in 

210Pb deposition. 137Cs can be viewed as 

a pulse input in the early 1960’s. Its inventory is decreasing over time, with minute 

new inputs. The short half life of 7Be, 

combined with seasonally varying fallout 

rates, results in strong seasonal dependence of soil-bound 7

Be activities. 

Behavior of 210Pb, 137Cs, and 

7Be in soils

All three isotopes strongly sorb to soil 

and suspended-sediment particles (Hawley and others, 1986; Jenne and Wahlberg, 

1968; Ritchie and McHenry, 1990; Wallbrink and Murray, 1993; Walling and others, 1999), and can act as tracers of eroded 

surficial soils. In undisturbed soils, excess 

210Pb and 137Cs activities extend to soil 

depths of about 5–40 cm, depending on 

soil conditions, with peak activities typically in the upper few centimeters (Olsen 

and others, 1985; Wallbrink and Murray, 

1993, 1996a; Walling and Quine, 1994; 

Walling and Woodward, 1992). In contrast, 7Be tends to be detected only in the 

upper 1–2 cm of undisturbed soils (Wallbrink and Murray, 1993, 1996b); it’s rapid 

decay rate precludes substantial downward migration. 

In cultivated settings, plowing evenly 

distributes 210Pb and 137Cs throughout the 

plowed layer, typically the upper 20 cm; 

activities rapidly diminish at greater 

depths (Walling and Quine, 1994; Walling 

and Woodward, 1992). 7

Be is not longlived enough to become homogenized in 

the plow layer from annual plowing. Even 

in cultivated settings, 7Be is typically 

detected only at the surface, with highest 

activities in the upper 1–2 mm.

Radioisotopes have been used extensively to assess upland soil erosion from 

agricultural settings (reviewed in Ritchie 

and McHenry, 1975; Ritchie and 

McHenry, 1990; Walling and Quine, 1992, 

1994). Long-term fallout of radioisotopes 

should be spatially uniform within a 

region—that is, where latitude and annual 

precipitation do not vary significantly. 

Hence, the inventory of excess 210Pb and 

137Cs in undisturbed soils should be 

regionally uniform. Herein, inventory is 

defined as activity per unit area of a soil 

column (in mBq/cm2

), and is determined 

as the product of activity (mBq/g) and the 

dry mass per unit area (g/cm2), summed 

over the depth of detectable isotope activities. 

Inventories are useful for comparing 

isotopic data for reference (undisturbed) 

sites within a region.Whereas activitydepth relations may vary somewhat 

among sites, depending on soil characteristics, reference inventories should be relatively uniform within a region. 

Reference inventories are typically determined by analyzing radioisotope profiles 

in several soil cores from undisturbed settings. In disturbed settings (cultivated 

fields) isotopic inventories vary considerably depending on whether a specific core 

is from an erosional or a depositional setting. Erosional settings generally contain 

isotopic inventories less than the reference 

inventory, because some soil (and soilbound radioisotope) has been eroded from 

the site. Depositional settings generally 

contain isotopic inventories greater than 

the reference inventory. Radioisotope 

inventories that are nearly equal to the reference inventory indicate little net movement of soil—either total movement is 

minimal, or erosion is balanced by deposition of soils that were eroded upslope from 

the core site. 

Radioisotope study approach

Most radioisotope studies of soil erosion and fluvial sediment sources have 

been on small watersheds—many at the 

scale of a single farm field—allowing 

detailed, spatially focused measurements 

of radioisotope inventories to assess net 

erosion rates of fields. This study, in contrast, attempts to describe sediment 

sources on the scale of a 4,038 km2

 basin. 

Thus our approach was to compare radioisotope activities in suspended sediments 

with those of potential source areas (soil 

erosion from cultivated land and bank erosion), by comparing radioisotope inventories from reference cores to soil cores 

collected along hill slope transects from 

selected field sites across the basin (C.J. 

McCullough, U.S. Geological Survey, 

written commun., 2001). 

Radioisotope activities of suspended 

sediments reflect the weighted average of 

the radioisotope levels of sediments from 

each type of source area. Two potential 

sources of sediment would be expected to 

predominate in the Wild Rice River Basin: 

eroded soil from cultivated farm land and 

eroded streambank sediments. Both 

sources are known to occur, but the relative magnitude of soil erosion versus bank 

erosion in the Wild Rice River Basin is 

unknown. Resuspension of stream-channel deposits may occur, but the source of 

channel deposits is presumed to be predominantly eroded sediment from soil or 

bank material. Soil erosion in grassland, 

forest, pasture, and wetlands is negligible. 

Within the basin, there is extensive cultivation, particularly in the central and 

western portions. Cut banks—as high as 
9

12 m in some reaches—evidencing bank 

erosion are prevalent on the tributaries and 

main stem of the Wild Rice River. 

Surficial cultivated soils would be 

expected to contain measurable quantities 

of excess 210Pb and 137Cs unless the 

inventory at a site has been eroded away 

completely. Bank material is assumed to 

contain excess 210Pb and 137Cs only in the 

upper 5–40 cm. With that assumption, and 

assuming erosion of a 4-m high cut bank—

an estimated average for the river—then, 

the isotope activities from bank erosion 

would be diluted by a factor of 10–80 for a 

vertically eroded bank. 7

Be activities, 

contained in the upper 1–2 cm, would be 

diluted by a factor of 200–400 for a 4-m 

cut bank. Suspended sediments in transport should have isotopic activities similar 

to surficial soils if soils are the dominant 

sediment source; should be highly diluted 

(likely undetectable) if bank erosion is the 

dominant sediment source; or reflect modest dilution of soil-bound isotopic activities if the two sources are approximately 

equal in importance. 

Sampling methods

Bulk quantities (tens of grams) of suspended sediment were separated from 

stream water via continuous-flow centrifugation. Water was pumped from near the 

center of the channel from the downstream 

side of a bridge, from about 0–0.5 m below 

stream surface, with a submersible centrifugal pump. Water was forced, through 

Tygon tubing, into an Alfa-Laval MAB103B centrifuge at a rate of 4 L/min. Nearsurface pump deployment likely excluded 

a portion of large-diameter particles 

(heavier sands), which preferentially 

travel near the streambed-water interface. 

Detailed descriptions of a comparable 

Alfa-Laval centrifuge have been given by 

Ongley and Thomas (1989) and Horowitz 

and others (1989). Horowitz and others 

(1989) determined that the Alfa-Laval 

centrifuge recovered 94–99 percent of 

inflowing suspended sediment. Following 

sampling, bulk sediment was transferred 

from the centrifuge bowl using a stainless 

steel scraper, into polypropylene jars (Nalgene). Internal surfaces (bowl and discs) 

were thoroughly cleaned of sediment 

between uses, using tap water.

Surficial soil samples were collected 

by scraping the upper 1 cm of soil into 

polyethylene bags. The sample locations 

were adjacent to cores collected for radioisotope inventories and soil morphology 

(Carolyn McCollough, U.S. Geological 

Survey, written commun., 2001), which 

were located along a hill slope sequence at 

the summit, shoulder, backslope, footslope, and toeslope when these positions 

were determinable. Sampling points were 

determined by observing changes in slope 

and surface soil color. All the samples, 

except the reference sites, were taken after 

harvest and fall tillage on fields used for 

wheat, soybeans, or sugar beets. Reference cores were collected for two purposes: (1) to compare with isotopic 

inventories determined on soil cores from 

cultivated settings (Carolyn McCullough, 

U.S. Geological Survey, written commun., 

2001), and (2) to be representative of 

potentially erodible bank material. Reference cores were sampled with a truckmounted, hydraulic powered Giddings 

probe (model GSRP-S-M), equipped with 

a 4.44-cm (1.75-inch) diameter, 122-cm 

long coring tube and a 5.1-cm quick relief 

bit. Core samples were collected and 

stored in Zero-Contamination tubes (Giddings Machine Co., Fort Collins, Colo.) 

with plastic liners. Reference sites were 

unused and supposedly undisturbed areas 

of cemeteries in the basin. Reference cores 

were assumed to represent bank-sediment 

isotopic levels. In addition, two cores were 

collected from streambanks using a stainless steel knife to transfer sediment to 

polypropylene jars. Bank-material cores 

were only analyzed (for two depth increments from each core, to confirm that surficial and deeper isotopic levels were not 

anomalous; table 1). Soil coring was conducted during the fall, after crops were 

harvested. During the fall, soils are typically drier (and thus accessible by truck) 

than during the spring, and this time is 

least disruptive to landowners.

In cultivated settings, the longer-lived 

fallout isotopes (210Pb and 137Cs) were 

assumed to be uniformly distributed in the 

upper 20 cm, due to mixing from annual 

plowing. Thus, surface activities are presumed to not vary greatly before and after 

a given year’s tillage. In contrast, the 

short-lived 7

Be isotope is concentrated 

within the upper 1 cm (Wallbrink and Murray, 1996b), and decays too rapidly to be 

measurably distributed in the plow layer. 

Because soil samples were collected after 

fall tillage, surface 7

Be activities may have 

been diluted by as much as 20-fold 

(assuming uniform mixing in the plow 

layer).

Laboratory analysis of 

radioisotopes

Bulk sediment and soil samples were 

analyzed for radioisotopes at the Freshwater Institute, Department of Fisheries and 

Oceans, in Winnipeg, Manitoba. Six to 

eighteen grams of dried sediment were 

sealed in 60 x 15 mm plastic petri dishes, 

aged for 30 days, and counted for 48 hours 

each on a gamma spectrometer equipped 

with Ge (Li) [germanium(lithium)] or 

HPGe (hyperpure germanium) coaxial 

detectors, for the determination of 137Cs 

and 226Ra (Joshi, 1987) and 7Be and potassium-40 (40K). Sample activities were 

corrected for counting efficiencies, which 

were calculated using National Institute of 

Standards and Technology (NIST) standard reference material (SRM) spiked 

clay, soil and sediments. Counting efficiencies were also cross-checked through 

participation in the Environmental Measurements Laboratory Quality Assurance 

Program. 

One- to three-gram samples of dried 

sediment were leached with 6-normal 

hydrochloric acid in the presence of an 

added polonium-209 tracer. Polonium was 

autoplated onto a silver disc (Flynn, 1968), 

which was then counted for 48 hours on an 

alpha spectrometer to determine 210Pb 

from its polonium-210 daughter. 226Ra 

was determined by the radon de-emanation technique (Mathieu, 1977; Wilkinson, 

1985). Excess 210Pb was determined in 

each sample by subtracting the 226Ra 

activity from the total 210Pb activity. All 

activities are corrected to the sampling 

date.

Radioisotope levels of 

sediments

All suspended-sediment samples had 

excess 210Pb, and nearly always had measurable activities of 137Cs (table 2). Mean 

activities of each isotope were slightly 

higher at the downstream site (Hendrum), 

and could reflect greater inputs of finegrained sediments from the Red River Valley ecoregion. Activities of excess 210Pb
10

 Table 1. Radioisotope activities and inventories from reference cores in the Wild Rice River Basin. 

 [Raw data, with counting errors, are reported by Carolyn McCullough, U.S. Geological Survey, written commun., 2001.; n.d., not detected; --, not calculated or 

measured; (*), sample exceeded holding time for 7

Be; cm, centimeter; g, gram; mBq, millibecquerel; cm2, square centimeter]

Sample

identification

Depth

increment

(cm)

Dry mass/unit 

area (g/cm2

) 

(increment)

Dry mass/unit 

area (g/cm2

) 

(cumulative)

Excess 210Pb 137Cs 7Be

activity

(mBq/g)

inventory

(mBq/cm2

)

activity

(mBq/g)

inventory

(mBq/cm2)

activity

(mBq/g)

inventory

(mBq/cm2)

Core Identification: Reference Core 5-1, Landstad Church, 47°11'51" North 96°43'57" West

R5-1-2 0-2 1.32 1.32 86.1 113.2 29.5 38.7 31.9 42.0

R5-1-4 2-4 1.80 3.12 45.9 82.7 27.4 49.3 n.d. --

R5-1-6 4-6 1.71 4.83 23.4 40.0 19.5 33.3 n.d. --

R5-1-8 6-8 1.77 6.60 14.1 25.1 16.8 29.8 n.d. --

R5-1-10 8-10 2.41 9.01 10.5 25.3 11.6 27.8 n.d. --

R5-1-12 10-12 1.89 10.90 7.9 14.9 7.7 14.5 n.d. --

R5-1-14 12-14 2.17 13.07 n.d. -- 4.7 10.3 n.d. --

R5-1-16 14-16 2.06 15.13 n.d. -- 3.4 7.1 n.d. --

R5-1-18 16-18 2.17 17.30 n.d. -- 2.0 4.2 n.d. --

R5-1-20 18-20 2.64 19.94 n.d. -- 1.5 4.0 n.d. --

R5-1-24 20-24 3.76 23.70 n.d. -- n.d. -- n.d. --

R5-1-28 24-28 5.06 28.76 n.d. -- n.d. -- n.d. --

R5-1-32 28-32 4.00 32.76 n.d. -- n.d. -- n.d. --

R5-1-36 32-36 5.48 38.24 n.d. -- n.d. -- n.d. --

R5-1-40 36-40 5.30 43.54 n.d. -- n.d. -- n.d. --

Cumulative (total) inventory 301.1 219.1 42.0

Core Identification: Reference Core 3-3, Skandinavia Church, 47°15'08" North 96°26'02" West

R3-3-2 0-2 2.29 2.29 32.7 74.8 9.6 21.9 n.d. --

R3-3-4 2-4 1.47 3.76 17.4 25.6 6.2 9.1 n.d. --

R3-3-6 4-6 2.08 5.84 14.1 29.3 10.4 21.6 n.d. --

R3-3-10 6-10 4.82 10.66 13.2 63.7 8.8 42.5 n.d. --

R3-3-15 10-15 6.60 17.26 11.9 78.7 7.4 48.6 n.d. --

R3-3-20 15-20 6.41 23.67 12.0 77.1 10.3 65.8 n.d. --

R3-3-25 20-25 6.49 30.16 5.8 37.7 7.6 49.6 n.d. --

R3-3-30 25-30 5.79 35.95 0.6 3.5 6.0 34.9 n.d. --

R3-3-35 30-35 5.46 41.41 n.d. -- n.d. -- n.d. --

Cumulative (total) inventory: 390.4 294.0

Core Identification: Reference Core 6-2, Bejou, 47°27'06" North 95°59'00" West

R6-2-2 0-2 1.99 1.99 40.1 79.9 5.7 11.3 n.d. --

R6-2-4 2-4 2.25 4.24 19.8 44.5 6.1 13.7 n.d. --

R6-2-6 4-6 2.29 6.53 10.2 23.4 3.7 8.4 n.d. --

R6-2-10 6-10 4.66 11.19 5.4 25.3 3.4 15.8 n.d. --

R6-2-14 10-14 4.97 16.16 n.d. -- n.d. -- n.d. --

R6-2-18 14-18 5.18 21.34 0.4 2.1 n.d. -- n.d. --

R6-2-22 18-22 5.28 26.62 0.4 2.3 n.d. -- n.d. --

R6-2-26 22-26 4.99 31.61 2.0 10.2 n.d. -- n.d. --

R6-2-30 26-30 5.54 37.15 1.6 8.6 n.d. -- n.d. --

R6-2-34 30-34 5.35 42.50 n.d. -- n.d. -- n.d. --

R6-2-38 34-38 5.26 47.76 n.d. -- n.d. -- n.d. --

Cumulative (total) inventory: 196.4 49.3

Core identification: Bank 1, 47°16'54" North 96°06'05"West

Bank 1-10 0-10 -- -- 77.8 -- 34.4 -- (*) --

Bank 1-40 35-40 -- -- n.d. -- n.d. -- (*) --

Core identification: Bank 2, 47°17'19" North 96°22'17" West

Bank 2-10 0-10 -- -- 16.3 -- 8.1 -- (*) --

Bank 2-140 130-140 -- -- 0.17 -- n.d. -- (*) --
11

 in stream sediments closely matched 

those of cultivated, surficial soils in the 

study area (table 3, fig. 4a). There was 

considerable overlap in values of suspended versus soil-bound 137Cs activities, 

although mean suspended 137Cs activities 

were about one-half the mean activity in 

surficial soils from cultivated fields (fig. 

4b). One sample at each stream site had 

no detectable 137Cs activitiy. Correlations 

between suspended excess 210Pb or 137Cs 

and streamflow (figs. 5b, 5c, 6b, and 6c) or 

suspended-sediment concentration (figs. 

5a and 6a) were not significant (p>0.05). 

7Be was detected in most suspendedsediment samples, which is a strong indication of inputs of recently eroded surficial soils (table 1, figs 4c, 5d, and 6d). 

 Table 2. Radioisotope activities in suspended sediments of the Wild Rice River. 

 [A, activities in millibecquerels per gram dry sediment; σ, counting error of +/- one standard deviation; -, falling stage; ~, stable stage; +, rising stage; n.d., not detected; 

*, exceeded holding time for 7 Be; mean values are based on detected values (means with zero substituted for the undetected values are given in parentheses)] 

Date Stage

Total 210Pb 226Ra excess 210Pb 137Cs 7Be 40K

A σ A σ A A σ A σ A σ

Wild Rice River at Twin Valley, Minnesota

3-Mar-98 - 28.5 1.5 19.9 1.9 8.60 2.87 0.78 n.d. 428 13

26-Mar-98 ~ 50.0 2.2 34.9 4.6 15.1 4.96 1.8 n.d. 578 17

11-May-98 + 66.7 2.6 26.9 2.7 39.8 5.68 2.2 571 68 976 20

13-May-98 + 29.5 1.6 24.4 1.9 5.02 1.93 1.3 122 23 501 15

14-May-98 + 30.7 1.5 23.5 1.9 7.25 6.90 2.3 145 33 501 20

18-May-98 - 32.0 1.6 23.6 2.6 8.43 3.58 1.4 87 29 518 16

19-May-98 - 30.0 1.1 21.6 2.0 8.42 n.d. n.d. 667 13

27-May-98 - 36.6 1.2 28.2 2.1 8.43 4.31 0.6 42 12 416 12

22-Jun-98 - 29.9 1.1 22.9 3.0 7.07 1.47 0.82 53 11 507 10

25-Jun-98 - 25.5 0.93 19.8 3.2 5.77 2.35 0.85 88 12 408 12

29-Jun-98 - 28.1 1.0 21.2 1.6 6.94 4.48 0.81 92 10 662 13

2-Jul-98 - 31.9 1.1 26.6 2.0 5.34 2.37 1.3 77 19 500 10

9-Jul-98 + 41.6 1.3 30.5 3.8 11.1 6.93 1.2 92 8.2 445 13

14-Jul-98 + 80.1 3.2 16.3 1.5 63.8 11.49 1.3 (*) 285 9

5-Aug-98 - 55.2 3.0 22.2 1.6 33.0 n.d. (*) 353 7

Mean 15.6 4.56 (3.95) 137 (105) 516

Standard deviation 16.8 2.75 155 164

Mean for falling or stable stream stage 10.7 2.90 (2.60) 73.2 (48.8) 504

Mean for rising stage 25.4 6.59 233 541

Wild Rice River at Hendrum, Minnesota

25-Mar-98 ~ 55.8 1.7 34.9 5.0 20.9 5.51 0.88 n.d. 456 14

12-May-98 + 47.7 1.6 28.6 3.6 19.2 2.08 0.56 119 23 837 17

13-May-98 + 46.4 1.6 29.8 2.5 16.5 9.57 1.9 265 40 565 17

14-May-98 + 45.6 1.6 28.7 3.0 16.8 5.38 0.97 105 19 493 15

18-May-98 + 54.8 1.7 36.6 3.4 18.2 8.82 1.2 194 27 489 15

19-May-98 + 51.6 1.3 28.2 4.8 23.4 9.33 1.2 174 38 606 18

27-May-98 - 44.9 1.2 33.4 2.9 11.6 4.35 1.4 43 18 475 14

23-Jun-98 - 38.0 1.2 30.1 3.7 7.9 n.d. 56 20 383 7.7

25-Jun-98 - 50.0 1.3 40.4 4.1 9.6 7.99 0.16 182 18 767 23

30-Jun-98 + 33.2 1.0 25.8 3.7 7.4 3.76 1.5 167 25 637 19

2-Jul-98 - 40.8 1.4 30.5 2.7 10.3 5.02 0.9 111 10 460 14

7-Jul-98 - 46.7 2.3 24.4 2.0 22.3 3.53 1.3 (*) 428 13

9-Jul-98 + 52.2 2.0 35.0 4.2 17.2 6.80 0.88 240 14 482 14

6-Aug-98 - 43.3 2.2 19.5 1.2 23.8 n.d. (*) 474 9

Mean 16.1 6.01 (5.15) 151(138) 539

Standard deviation 5.7 2.47 71 131

Mean for falling or stable stream stage 15.2 4.40 (3.80) 98.0 (78.4) 492

Mean for rising stage 17.0 6.53 181 587
12

7Be was not detected in suspended-sediment samples collected in March. These 

samples were analyzed in late July-early 

August, about 2.25–2.65 half-lives after 

collection, and it is possible that relatively 

low 7Be activities would have decayed to 

less-than-detectable activities during that 

time. Accordingly, 7Be data for March are 

not used for analysis of source sediments. 

The lack of 7Be detections (except 1 sample) in surficial soils collected in the fall 

can be explained by the post-tillage collection of soil samples, which likely diluted 

surface-accumulated 7

Be to less-than 

detectable levels in all but one sample. 

Radioisotope activities and inventories in 

reference soil cores are given in table 3. 

Implications for sediment 

sources

To assess the relative contributions of 

surficial soils versus streambank sediments as potential sources of suspended 

sediment to the Wild Rice River, certain 

assumptions were made. First, radioisotope activities in the upper 1 cm of surficial, cultivated soils are representative of 

erodible soils. Second, activities of 

excess 210Pb and 137Cs are approximately 

uniform in the plow layer (upper 20 cm), 

implying that the measured activities may 

be useful to assess sheet, rill, and minor 

gully erosion. 7

Be, in contrast, is assumed 

to be concentrated in the upper few mm, 

and would be greatly diluted by minor 

gully erosion (gullies greater than a few 

cm depth). Third, bank erosion is 

assumed to occur along a vertical plane. 

Fallout radioisotopes in bank material are 

detected only in the upper portion (table 

1), and thus would be diluted by isotopefree sediment as a vertical section of a cut 

bank sloughs into the river. The degree to 

which radioisotope inventories would be 

diluted is directly related to the height of 

the bank. Fourth, suspended isotopic 

activities are assumed to be a mixture of 

activities found in these potential sources. 

This is a simplification because radioisotope activities tend to be higher in finegrained sediments (fines) than in coarser 

sediments, whereas coarser sediments 

would be redeposited more quickly than 

fines (and hence not be collected as suspended load). The samples are thus some-

 Table 3. Radioisotope activities in the upper 1 cm of cultivated soils of the Wild Rice River Basin, sampled after autumn harvest and 

plowing, September-November, 1998. 

[A, Activities in millibecquerels per gram dry sediment; σ, counting error of +/- one standard deviation; n.d., not detected; mean values are based on detected values 

(means with zero substituted for the undetected values are given in parentheses).] 

Sample 

identification

Latitude

(North)

Longitude

(West)

Total

210Pb

226Ra

Excess 

210Pb

137Cs 7Be 40K

activity σ activity σ activity activity σ activity σ activity σ

HO-1-S 47°12'14" 96°06'36" 24.0 0.9 20.6 2.1 3.4 6.9 1.4 11.3 10.3 558 11.2

HO-3-S 47°12'14" 96°06'36" 30.3 1.0 22.0 2.0 8.3 11.0 1.2 n.d. n.d. 489 9.8

HO-5-S 47°12'14" 96°06'36" 47.4 1.4 24.8 2.2 22.5 14.1 2.1 n.d. n.d. 746 14.9

KR-1-S 47°05'38" 96°19'27" 22.5 0.9 20.7 2.7 1.7 9.2 1.4 n.d. n.d. 439 13.2

KR-2-S 47°05'38" 96°19'27" 18.6 0.8 14.9 1.3 3.7 11.0 1.5 n.d. n.d. 532 16.0

KR-3-S 47°05'38" 96°19'27" 28.0 1.0 12.9 1.2 15.1 5.0 1.0 n.d. n.d. 382 7.6

KL-1-S 47°04'54" 96°07'45" 25.4 0.9 28.1 2.8 -2.6 6.1 1.6 n.d. n.d. 707 14.1

KL-3-S 47°04'54" 96°07'45" 34.0 1.1 22.2 2.7 11.8 16.3 2.1 n.d. n.d. 674 20.2

KL-5-S 47°04'54" 96°07'45" 40.7 1.4 24.3 1.7 16.4 9.5 1.2 n.d. n.d. 464 9.3

SK-1-S 47°20'27" 96°08'06" 35.6 1.2 18.4 1.1 17.2 10.6 0.8 n.d. n.d. 423 8.5

SK-2-S 47°20'27" 96°08'06" 31.6 1.1 20.7 2.1 10.9 7.5 1.4 n.d. n.d. 665 20.0

SK-6-S 47°20'27" 96°08'06" 38.8 1.2 24.1 1.7 14.7 10.5 1.6 n.d. n.d. 618 12.4

BO-1-S 47°10'45" 96°47'57" 43.8 1.2 30.4 1.5 13.5 13.1 1.0 n.d. n.d. 749 15.0

BO-3-S 47°10'45" 96°47'57" 44.8 1.4 30.1 1.2 14.7 11.9 0.8 n.d. n.d. 747 14.9

BU-1-S 47°20'18" 95°59'25" 34.6 1.0 22.9 2.1 11.6 9.3 1.7 n.d. n.d. 682 20.5

BU-2-S 47°20'18" 95°59'25" 34.4 1.1 21.5 1.5 12.9 10.5 1.3 n.d. n.d. 629 12.6

BU-3-S 47°20'18" 95°59'25" 42.5 1.5 27.1 1.4 15.4 10.6 1.1 n.d. n.d. 753 15.1

GE-1-S 47°10'45" 96°33'43" 39.5 1.6 25.7 1.5 13.8 10.8 1.2 n.d. n.d. 705 14.1

GE-2-S 47°10'45" 96°33'43" 36.7 1.1 24.9 1.5 11.8 12.1 1.3 n.d. n.d. 544 10.9

GE-3-S 47°10'45" 96°33'43" 44.7 1.2 23.5 1.2 21.2 13.1 1.0 n.d. n.d. 529 10.6

Mean 11.9 10.4 n.d. n.d. 602

Standard deviation 6.3 2.7 n.d. n.d. 121
EXCESS LEAD-210 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

BERYLLIUM-7 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

CESIUM-137 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

TWIN VALLEY

TWIN VALLEY

HENDRUM TWIN VALLEY

HENDRUM BANK

MATERIAL

BANK

MATERIAL

BANK

MATERIAL

SURFICIAL SOILS HENDRUM

(cultivated fields)

SURFICIAL SOILS

(cultivated fields)

SURFICIAL SOILS

(cultivated fields)

70

300

571

400

60

40

200

50

250

30

150

10

50

20

100

0

-10

0

0

5

10

15

20

(a) Excess lead-210, (b) cesium-137, and (c) beryllium-7 activities in suspended-sediment samples from the Wild Rice River

at Twin Valley, Minnesota and at Hendrum, Minnesota; in surficial soils from cultivated fields; and estimated activities contributed

from erosion of streambank material (calculated by diluting reference isotopic inventories with 2 m, 4 m, and 5 m [meters] of bankmaterial sediments). (For beryllium-7, note the following: surface-soil samples were collected after fall plowing, thereby diluting

most values to undetectable levels (note 1); one low level activity was detected (note 2); one value from a reference core (upper 2

cm) is shown (note 3). Estimated ranges of beryllium-7 activities are based on literature deposition rates (see text). For surface soils,

steady-state inventories were assumed to be distributed in the upper 1.0 cm (note 4) or upper 0.2 cm (note 5). Dilution of steady-state

inventories by bank-material sediments resulted in estimated beryllium-7 activities of less than 0.34 millibecquerels per gram for all

bank heights of 2 m or greater (note 6).)

(a)

(c)

(b)

1

2

3

4

4

4

5

5

5

6

2m

2m

4m

4m

5m

5m

4

Activity

EXPLANATION

Mean

Estimated activity

contributed from

stream-bank

sediments, assuming

mean bank heights

of 2, 4, and 5 meters

Activity estimated from

literature (see text)

2m 4m

5m

Figure 4.

13
(a)Suspended-sediment; (b) excess lead-210; (c) cesium-137; and (d) beryllium-7 verses time, overlain on hydrograph,

for Wild Rice River at Twin Valley, Minnesota, spring-summer 1998.

SUSPENDED-SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

EXCESS LEAD-210 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

CESIUM-137 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

BERYLLIUM-7 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

STREAMFLOW,

IN CUBIC METERS PER SECOND

1,000

750

500

250

0

70

60

50

40

30

20

10

0

-10

20

15

10

5

0

600

500

400

300

200

100

0

100

100

100

100

75

75

75

75

50

50

50

50

25

25

25

25

0

0

0

0

MARCH APRIL MAY JUNE JULY AUGUST

1998

(a)

(b)

(c)

(d)

Figure 5.

STREAMFLOW,

IN CUBIC METERS PER SECOND

STREAMFLOW,

IN CUBIC METERS PER SECOND

STREAMFLOW,

IN CUBIC METERS PER SECOND

14
(a) Suspended-sediment; (b) excess lead-210; (c) cesium-137; and (d) beryllium-7 verses time, overlain on hydrograph,

for Wild Rice River at Hendrum, Minnesota, spring-summer 1998.

SUSPENDED-SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

EXCESS LEAD-210 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

CESIUM-137 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

BERYLLIUM-7 ACTIVITY,

IN MILLIBECQUERELS PER GRAM

1,000

750

500

250

0

70

60

50

40

30

20

10

0

-10

20

15

10

5

0

600

500

400

300

200

100

0

125

125

125

125

100

100

100

100

150

150

150

150

175

175

175

175

200

200

200

200

75

75

75

75

50

50

50

50

25

25

25

25

0

0

0

0

MARCH APRIL MAY JUNE JULY AUGUST

1998

(a)

(b)

(c)

(d)

Figure 6.

STREAMFLOW,

IN CUBIC METERS PER SECOND

STREAMFLOW,

IN CUBIC METERS PER SECOND

STREAMFLOW,

IN CUBIC METERS PER SECOND

STREAMFLOW,

IN CUBIC METERS PER SECOND

15
16

what biased toward fines, relative to sediment sources. 

To estimate bank contributions of isotopic activities, reference inventories are 

adjusted for dilution by an “average” 

height of erodible cut banks. Typical 

heights of erodible cut banks range from 

about 1–12 m, with 4 m being a mid-range 

estimate for the basin. Dividing reference 

isotopic inventories by bank height of 4 m 

(400 cm), and dividing the result by a bulk 

density of 1.2 g/cm3, yields estimates of 

the mean activity that bank-eroded sediment would contribute to the suspended 

load. Excess 210Pb inventories from the 

three reference cores yield a mean activity 

of 0.62 mBq/g for a 4-m bank (fig. 4a). 

For 137Cs, only cores 5–1 and 3–3 were 

used (the activity measured for core 6–2 

was an outlier), yielding a mean activity of 

0.53 mBq/g for a 4-m bank (fig. 4b). 

These values do not change markedly if 

other reasonable estimates of mean cutbank heights are used (for example 2 m 

and 5 m; see fig. 4). 

Although only one 7Be inventory was 

determined (42 mBq/cm2

, core 5–1, table 

1), it agrees with reported depositional 

rates and measured activities in surficial 

soils. Todd and others (1989) reported 

7

Be depositional rates of 12.0–12.9 

dpm/cm2

/yr in southeastern Virginia, 

which convert to steady-state inventories1 

(assuming constant inputs, and accounting 

for decay) of 42.1–45.3 mBq/cm2

, or a 

mean of about 44 mBq/cm2

.

Similarly, Baskaran (1995) reported 

annual rates of 12.00–23.62 dpm/cm2

/yr 

in Galveston, Texas, which convert to 

steady-state inventories of 42.1 (drier 

year) to 82.8 (wetter year) mBq/cm2. 

Steady-state 7Be inventories should 

typically be lower in northwestern Minnesota than in Virginia or the Gulf Coast of 

Texas, due to lower annual precipitation in 

Minnesota. Instantaneous inventories, 

however, could be relatively high after 

precipitation events, and decay to lower 

values during dry periods. Given these 

caveats, and comparison to literature data, 

our measured 7

Be inventory could underestimate or overestimate typical instantaneous inventories significantly, perhaps 

by a factor of about 2. 7

Be inventories in 

the Wild Rice River Basin are, therefore, 

expected to be in the range of about 20–80 

mBq/cm2

, with 40 mBq/cm2 as a midrange estimate.

Such inventories would contribute 

7

Be activities of about 40 mBq/g (range of 

20–80 mBq/g), if averaged over the surficial 1 cm of soil, assuming a dry-mass 

density of about 1–1.2 g/cm3

 for surficial 

soils. For comparison, Walling and Woodward (1992, p. 157) detected 7

Be in the 

upper 1 cm of soil cores from pasture and 

cultivated sites in Devon, England, with 

activities ranging from about 6–52 mBq/g 

(mean for upper 1 cm ranged from about 

22–33 mBq/g). Walling and others (1999, 

p. 3868) reported activities of about 21–80 

mBq/g for surficial increments (less than 1 

cm depth) of soil cores in Devon, England. 

Wallbrink and Murray (1996b) reported 

near-surface (0–4 mm) 7

Be activities of 

about 23–75 mBq/g for soils in Tasmania, 

and about 30–60 mBq/g in bare surficial 

soils in Black Mountain, Australia. 

Because much of the fallout 7

Be is contained in the upper few mm of soil, activities in the surface 2 mm (the layer most 

readily eroded) are potentially higher than 

activities averaged over the upper 1 cm. 

Also, inventories and surficial activities 

are likely much higher directly following 

precipitation events. 

Averaging 7Be inventories over a theoretical 4-m cut bank yields an estimated 

mean 7Be activity contributed from bank 

erosion of about 0.08 mBq/g (range of 

0.04–0.16 mBq/g), assuming a dry mass 

density of 1.2 g/cm3 for the soil column. 

A mean bank height of 2 m yields an estimated 7Be activity from bank material 

equal to only 0.085–0.34 mBq/g, suggesting that suspended 7Be activities greater 

than 1 mBq/g are strongly indicative of 

upland soil inputs. 

The estimates of mean activities of 

excess 210Pb, 137Cs, and 7

Be contributed 

from bank material (when reasonable estimates of mean heights of cut banks are 

used) are less than levels that can be reliably detected. This indicates that if bankerosion is the sole, or dominant source of 

suspended sediments to the river, then suspended isotopic activities would be undetectable.

A simple mixing model can be 

applied to measured suspended isotopic 

activities, assuming two sources of suspended sediments: upland soil and streambank. This model is represented 

mathematically as:

Ased,i = fsoil Asoil,i + fbank Abank,i 

(3)

where Ased,i is the activity of isotope i

in suspended-sediment samples; 

Asoil,i is the mean activity of isotope i

in surficial soils from cultivated fields 

(excess 210Pb and 137Cs are means from 

data in table 2; 7

Be is assumed to be 40 

mBq/g, as previously described); 

Abank,i is the estimated mean activity 

of isotope i contributed from bank erosion 

(assuming dilution of isotopic inventories 

with bank material, as described above); 

fsoil is the fraction of suspended sediment that originates from soil erosion; and 

fbank is the fraction of suspended sediment that originates from bank erosion.

In equation 3, all quantities are measured or estimated except fsoil and fbank. 

The fractions of sediment originating from 

different sources must sum to 1. Therefore, substituting the quantity (1- fsoil) for 

fbank the equation can be solved for fsoil:

fsoil = (Ased,i - Abank,i) / (Asoil,i - Abank,i

 (4)

The quantity fbank can be solved similarly or, as noted above, fbank = 1 - fsoil. It 

follows that an fsoil value of 1 indicates 

that soil erosion contributes all suspended 

sediment to the stream, whereas an fbank 

value of 1 indicates that bank erosion contributes all suspended sediment to the 

stream. Values of fsoil greater than 1 may 

result (and, hence, fbank values are forced 

to negative values) when stream sediment 

samples have higher isotopic activities 

than the mean values measured (or estimated, for 7

Be) in surficial soils, which 

indicates it is possible that a third source 

of suspended sediment (with high isotopic 

activities) exists. Because this is consid1Steady-state inventory (I) is the total amount of radioisotope under a specific area. I = F/k, where F = fallout rate (in mBq/cm2/yr), and k = radioactive-decay rate constant (in yr-1). For 7

Be, k = 4.75 yr-1 (k = [ln(2)/t1/2], where ln is the natural logarithm function and t1/2 is the half life of 7Be in 

years (53.3 d, or 0.1459 yr). The steady-state assumption is an oversimplification, because inputs of 7

Be vary seasonally, and the half life is much less 

than 1 yr.
17

ered unlikely, for summary purposes 

(table 4), all f-values that exceed 1 were 

set equal to one; accordingly all f-values 

that were less than zero were set equal to 

zero. This adjustment keeps f-values 

within their physically meaningful 

domains (0–1), and precludes one or a few 

extreme values from biasing the mean. 

Model-estimated values of fsoil and 

fbank vary considerably among sediment 

samples collected during spring-summer 

1998 (fig. 7 and 8). Consistent with the 

patterns in activities, the downstream site 

(Hendrum) tended to yield higher values 

of fsoil (table 4), indicating that a greater 

fraction of suspended sediment in the 

downstream portion of the river originates 

from soil erosion, compared to upstream 

of Twin Valley, where bank erosion 

appears to be of greater relative importance. 

Values of fsoil determined from excess 

210Pb and 7Be tend to indicate predominantly erosion of surficial soils, whereas 

137Cs indicated more of a mixed source, 

with bank material being of somewhat 

greater importance than soil erosion. The 

reason for this disagreement is unknown. 

A possible explanation is that there are 

upland soil areas that contribute sediment 

to the stream that have been partially 

depleted of 137Cs, through erosion, that 

are underrepresented in our sampling. 

More actively eroding areas could be disproportionately depleted of 137Cs, compared to 210Pb and 7Be, because 137Cs was 

mainly a pulse input in the 1960’s, 

whereas 210Pb and 7Be inputs are relatively constant over time. This discrepancy highlights the importance of 

obtaining multiple indicators of sediment 

sources, rather than relying on a single 

indicator. 

Activities of fallout isotopes tend to 

be greater when stream stages are rising, 

compared to falling or stable stages (table 

2). Accordingly, fsoil values (particularly 

for excess 210Pb and 137Cs) tend to be 

greatest during rising stages, whereas 

f bank values tend to be greatest during 

falling stages (figs. 7 and 8). This pattern, 

although somewhat variable and based on 

limited coverage of a range of flow conditions, is consistent with the concept that 

the greatest soil-erosion rates should occur 

with large (runoff-producing) rain events, 

whereas the greatest bank-erosion rates 

should occur when streambanks may be 

more saturated and stage is decreasing.

CONCLUSIONS AND LIMITATIONS OF STUDY

This study provides evidence that 

upland soil erosion from cultivated fields 

contributes the majority of the suspendedsediment load in the Wild Rice River. 

Therefore, efforts to control or minimize 

upland soil erosion will likely reduce instream suspended-sediment concentrations. Bank erosion also contributes to the 

suspended-sediment load, although 210Pb 

and 7Be data indicate that bank erosion is 

not the dominant source of suspended sediment. Efforts to minimize bank erosion 

may help reduce suspended-sediment concentrations, but might not be as effective 

as controlling upland soil erosion.

This study applied fallout radioisotope techniques, which have traditionally 

been applied in very small areas such as 

single farm fields, to a larger basin (4,038 

km2). Accordingly, sampling was distributed over a large area, and source-soils 

were not characterized in great spatial 

detail. Suspended sediments sampled for 

radioisotope analysis were collected over 

a limited time frame and were targeted 

toward runoff events, when suspendedsediment concentrations tend to be greatest. Isotopic activities of suspended sediments during base flow, when suspendedsediment concentrations tend to be low, 

are represented by few samples. Also, 

data collection for this study was focused 

on determining sources of suspended sediments; the data are not useful for inferring 

the quantity or predominant source of bedload sediments. Bedload, typically composed of coarser sediments than 

suspended sediments, is thought to be 

important in streams draining glaciofluvial sediments. It is possible that bank 

erosion, although not the dominant source 

of suspended sediments, is an important 

contributor to bedload and to channelforming processes (point bar migration, 

and so forth.). 

Fallout radioisotopes were useful 

indicators of sediment sources in the Wild 

Rice River Basin, and may be useful in 

other relatively large basins. Additional 

studies may provide a more detailed characterization of source areas, and suspended isotopic signatures over a greater 

range of basin types and hydrologic conditions. It may be useful, for example, to 

assess suspended radioisotope levels in 

contrasting basins—a basin where severe 

bank erosion is thought to predominate; a 

basin where both soil and bank erosion 

may be prevalent; and an agricultural 

basin where bank erosion is minimal. Suspended radioisotope levels would be 

expected to increase along such a gradient; 

such data may confirm the utility of the 

method for determining sediment sources 

in streams where the dominant source is in 

doubt.

 Table 4. Fraction of suspended sediment originating from soil erosion (fsoil), as determined from excess lead-210, cesium-137, and 

beryllium-7 activities in sediments and source areas (surficial cultivated soils and bank material). 

[Values of fbank are equal to 1-fsoil.; f-values of zero result when the isotope was not detected (n.d.) in a sample; f-values greater than 1 (or less than zero) were set to 1 

(or zero); the quantity fsoil,mean is calculated from mean fsoil values for each isotope.]

Twin Valley Hendrum

Minimum Mean Maximum Minimum Mean Maximum

fsoil,Pb-210 0.39 0.71 1.0 0.60 0.92 1.0

fsoil,Cs-137 0 (n.d.) 0.34 1.0 0 (n.d.) 0.47 0.91

fsoil,Be-7 0 (n.d.) 0.91 1.0 1.0 1.0 1.0

fsoil,mean(all isotopes) 0.65 0.80
Model estimates of the fraction of suspended-sediment in the Wild Rice River at Twin Valley, Minnesota, that originates from

erosion of surficial cultivated soils and by vertical erosion of streambank material, determined from (a) excess lead-210; (b) cesium-137;

and (c) beryllium-7, (For bank sources, the model assumes dilution of reference activities by 4 meters of bank-material sediments), and

(d) stream hydrograph. FRACTIONS OF SUSPENDED-SEDIMENT IN STREAM ORIGINATING FROM SOIL EROSION AND STREAMBANK EROSION CESIUM-137 EXCESS LEAD-210 BERYLLIUM-7

1.0

1.0

1.0

0.5

0.5

0.5

0

0

0

MARCH APRIL MAY JUNE JULY AUGUST

1998

(a)

(b)

(c)

(d)

S

B B B B B

S = 1.3 S = 3.5

B = 1.05 S = 1.1 B = 1.05

S = 5.6 S = 2.9

S

S

S S S

S

S

S

S

S

B

B

B B B

B

B

B

B

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

B

B

S

B

S

S

B

B

S

S

B

B

S

B

S

S BS

B

S

B

S

B

S

B

S

S

B

S = 14.3

S = 3.1

S = 3.6

S = 2.1

S = 1.05

B = 1.0

S = 2.3

S = 1.9

S = 2.3

S = 2.2

S = 1.3

B

B

B

B S B B B

B

B

B

100

75

50

25

0

Figure 7. STREAMFLOW, IN CUBIC METERS PER SECOND

18
Model estimates of the fraction of suspended-sediment in the Wild Rice River at Hendrum, Minnesota, that originates from

erosion of surficial cultivated soils and by vertical erosion of streambank material, determined from (a) excess lead-210; (b) cesium-137;

and (c) beryllium-7. (For bank sources, the model assumes dilution of reference activities by 4 meters of bank-material sediments.), and

(d) stream hydrograph. FRACTIONS OF SUSPENDED-SEDIMENT IN STREAM ORIGINATING FROM SOIL EROSION AND STREAMBANK EROSION CESIUM-137 EXCESS LEAD-210 BERYLLIUM-7

1.0

1.0

1.0

0.5

0.5

0.5

0

0

0

125

150

175

200

100

75

50

25

0

MARCH APRIL MAY JUNE JULY AUGUST

1998

(a)

(b)

(c)

(d)

B B B B

B

B

B

B

B

B = 1.05 B = 1.05

S = 1.8 S = 1.5 S = 1.6 S = 1.9 S = 2.0

S = 1.4

S = 1.4

S = 1.5

S = 2.0 S

S S

S

S

S

S

B

B

B

B

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

S - Fraction of sediment

from surficial soil

erosion

B - Fraction of sediment

from streambank

erosion

B

S

B

B

B S

S

S

S

B

B

S

B

S

B

S

B

S

S

S

S

B

B

B

B

S = 3.0

S = 6.6

S = 2.6

S = 4.9

S = 4.4

S = 1.1 S = 1.4

S = 4.6

S = 4.2

S = 2.8

S = 6.0

B

B

B

B

B B

B

B

B

B B

Figure 8. STREAMFLOW, IN CUBIC METERS PER SECOND

19
20

SUMMARY

Sedimentation is a significant resource concern in the Red 

River of the North Basin (RRB). The Wild Rice River, a tributary 

to the Red River of the North, and its drainage basin were studied 

to assess sediment concentrations, loads, and potential sources of 

suspended sediment. We examined historical suspended-sediment 

data and activities of fallout radioisotopes (lead-210 [210Pb], 

cesium-137 [137Cs], and beryllium-7 [7

Be]) associated with suspended sediments and source-area sediments (cultivated soils, 

bank material, and reference soils) in the Wild Rice River Basin to 

better understand sources of suspended sediment to streams in the 

region.

Multiple linear regression analysis of suspended sediment 

concentrations measured from 1973–98 (Wild Rice River at Twin 

Valley, Minn.) exhibit the following: strong dependence on 

streamflow; limited seasonal dependence, with flow-adjusted 

concentrations slightly higher in the spring than summer-autumn; 

and no significant temporal trend. Suspended-sediment load estimates were developed for the Wild Rice River at Twin Valley, 

Minn. and at Hendrum, Minn., based on 913 and 35 samples, 

respectively. The mean loads for 20 years of data (1973–84; 

1989–98) were 31,500 and 60,000 metric tons per year, respectively, for the two sites. Relatively low net rates of upland soil 

erosion (about 2.2–2.5 millimeters [mm] of soil per century) could 

support these suspended loads; similarly, moderately high rates of 

net stream-bank erosion (about 3.7–15 meters per century of horizontal erosion of vertical cut bank) could account for the entire 

suspended load. In both cases, net erosion is thought to be a small 

percentage of total erosion, due to redeposition in the upland (soil 

only) and riparian (soil and bank sediments) environments. 

Activities of the fallout radioisotopes 210Pb, 137Cs, and 7Be 

were measured in samples of suspended and source-area (soil, reference, and bank material) sediments during spring-summer 1998. 

The fallout radioisotopes were nearly always detectable in suspended sediments during spring-summer 1998. Mean 210Pb and 

7Be activities in suspended sediment and surficial, cultivated soils 

were similar, perhaps indicating little dilution of suspended sediment from low-isotopic-activity bank sediments. In contrast, 

mean 137Cs activities in suspended sediment indicated a mixture 

of sediment originating from eroded soils and from eroded bank 

material, with bank material being a somewhat more important 

source upstream of Twin Valley, Minnesota; and approximately 

equal fractions of bank material and surficial soils contributing to 

the suspended load downstream at Hendrum, Minnesota. In general, 210Pb and 7

Be data indicated that suspended sediments originate primarily from upland soil erosion, whereas 137Cs data 

indicated a mixed source, with streambank sediments slightly 

more important. Application of fallout radioisotopes appears to 

be a useful tool to resolve ambiguity in cases where the dominant 

source of sediments—upland soil erosion or stream-bank erosion—is unknown, and both sources are thought to contribute. 

This study indicates that, to be effective, efforts to reduce sediment loading to the Wild Rice River should include measures to 

control soil erosion from cultivated fields.

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