U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5109

Prepared in cooperation with the Chester County Water Resources Authority 

and the Chester County Health Department

Estimated Suspended-Sediment Loads and Yields 

in the French and Brandywine Creek Basins, Chester County, 

Pennsylvania, Water Years 2008–09
Cover. West Branch Brandywine Creek near Honey Brook, Pennsylvania, overflowing its banks on April 15, 2007. The streamgage and 

water-quality monitoring station is to the right of the photograph.
Estimated Suspended-Sediment Loads and 

Yields in the French and Brandywine Creek 

Basins, Chester County, Pennsylvania, 

Water Years 2008–09

By Ronald A. Sloto and Leif E. Olson

Prepared in cooperation with the Chester County Water Resources Authority 

and the Chester County Health Department

Scientific Investigations Report 2011–5109

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Sloto, R.A., and Olson, L.E., Estimated suspended-sediment loads and yields in the French and Brandywine Creek 

Basins, Chester County, Pennsylvania, water years 2008–09: U.S. Geological Survey Scientific Investigations 

Report 2011–5109, 31 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Methods of Data Collection .........................................................................................................................5

Station Instrumentation .......................................................................................................................5

Turbidity Monitoring .............................................................................................................................5

Suspended-Sediment Sampling.........................................................................................................5

Quality Assurance.................................................................................................................................6

Regression Models........................................................................................................................................6

Estimation of Suspended-Sediment Loads and Yields ..........................................................................11

Limitations of the Investigation .................................................................................................................29

Summary and Conclusions.........................................................................................................................29

References Cited..........................................................................................................................................30

Figures

1. Map showing water-quality-monitoring stations on French and Brandywine Creeks, 

Chester County, Pennsylvania....................................................................................................3

2. Photograph showing turbidity sensor deployed at East Branch Brandywine Creek 

below Downingtown, Pennsylvania ..........................................................................................5

3. Graph showing regression relations of turbidity and suspended-sediment 

concentration for French Creek near Phoenixville, Pennsylvania.......................................8

4. Graph showing regression relations of turbidity to suspended-sediment 

concentration for West Branch Brandywine Creek near Honey Brook, 

Pennsylvania .................................................................................................................................8

5. Graph showing regression relations of turbidity to suspended-sediment 

concentration for West Branch Brandywine Creek at Modena, Pennsylvania.................9

6. Graph showing regression relations of turbidity to suspended-sediment 

concentration for East Branch Brandywine Creek below Downingtown, 

Pennsylvania .................................................................................................................................9

7. Graph showing discharge and turbidity measured at West Branch Brandywine 

Creek near Honey Brook, Pennsylvania, December 16–26, 2008 .......................................10

8. Graph showing discharge and turbidity measured at French Creek 

near Phoenixville, Pennsylvania, August 2–16, 2009 ............................................................10

9. Graphs showing suspended-sediment loads for storms at French Creek 

near Phoenixville, Pennsylvania, 2008 and 2009 water years .............................................13

10. Graphs showing relation of storm suspended-sediment load to peak discharge at 

French Creek near Phoenixville, Pennsylvania, 2008 and 2009 water years ....................14

11. Graphs showing suspended-sediment loads for storms at West Branch Brandywine 

Creek near Honey Brook, Pennsylvania, 2008 and 2009 water years ................................17
iv

12. Graph showing relation of storm suspended-sediment load to peak discharge 

at West Branch Brandywine Creek near Honey Brook, Pennsylvania, 

2008 and 2009 water years ........................................................................................................18

13. Graphs showing suspended-sediment loads for storms at West Branch Brandywine 

Creek at Modena, Pennsylvania, 2008 and 2009 water years .............................................21

14. Graph showing relation of storm suspended-sediment load to peak discharge 

at West Branch Brandywine Creek at Modena, Pennsylvania, 2008 and 2009 

water years..................................................................................................................................22

15. Graphs showing suspended-sediment loads for storms at East Branch Brandywine 

Creek below Downingtown, Pennsylvania, 2008 and 2009 water years............................25

16. Graph showing relation of storm suspended-sediment load to peak discharge 

at East Branch Brandywine Creek below Downingtown, Pennsylvania, 

2008 and 2009 water years ........................................................................................................26

Tables

1. Water-quality monitoring stations on French and Brandywine Creeks, 

Chester County, Pennsylvania....................................................................................................4

2. Basin characteristics, French and Brandywine Creeks, Chester County, 

Pennsylvania .................................................................................................................................4

3. Regression models developed for French and Brandywine Creeks, Chester County, 

Pennsylvania, with suspended sediment concentration as the response variable..........7

4. Estimated annual suspended-sediment loads and yields for French and Brandywine 

Creeks, Chester County, Pennsylvania, 2008 and 2009 water years ..................................12

5. Daily sediment load for French Creek near Phoenixville, Pennsylvania, 

2008 water year...........................................................................................................................15

6. Daily sediment load for French Creek near Phoenixville, Pennsylvania, 

2009 water year...........................................................................................................................16

7. Daily sediment load for West Branch Brandywine Creek near Honey Brook, 

Pennsylvania, 2008 water year.................................................................................................19

8. Daily sediment load for West Branch Brandywine Creek near Honey Brook, 

Pennsylvania, 2009 water year.................................................................................................20

9. Daily sediment load for West Branch Brandywine Creek at Modena, 

Pennsylvania, 2008 water year.................................................................................................23

10. Daily sediment load for West Branch Brandywine Creek at Modena, 

Pennsylvania, 2009 water year.................................................................................................24

11. Daily sediment load for East Branch Brandywine Creek below Downingtown, 

Pennsylvania, 2008 water year.................................................................................................27

12. Daily sediment load for East Branch Brandywine Creek below Downingtown, 

Pennsylvania, 2009 water year.................................................................................................28
v

Conversion Factors and Abbreviations

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile per square mile (mi/mi2) 4.167 kilometer per square kilometer 

(km/km2)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 megagram (Mg) 

ton per stream mile (ton/mi) 1.46 megagram per stream kilometer 

(Mg/km)

ton per square mile (ton/mi2) 2.35 megagram per square kilometer 

(Mg/km2)

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

FNU Formazine nephelometric unit

Q Discharge

SSC Suspended-sediment concentration

SSL Suspended-sediment load

SSY Suspended-sediment yield

T Turbidity
vi

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Estimated Suspended-Sediment Loads and Yields in the 

French and Brandywine Creek Basins, Chester County, 

Pennsylvania, Water Years 2008–09

By Ronald A. Sloto and Leif E. Olson 

Abstract

Turbidity and suspended-sediment concentration data 

were collected by the U.S. Geological Survey (USGS) at 

four stream stations—French Creek near Phoenixville, West 

Branch Brandywine Creek near Honey Brook, West Branch 

Brandywine Creek at Modena, and East Branch Brandywine 

Creek below Downingtown—in Chester County, Pa. Sedimentation and siltation is the leading cause of stream impairment 

in Chester County, and these data are critical for quantifying 

sediment transport. This study was conducted by the USGS in 

cooperation with the Chester County Water Resources Authority and the Chester County Health Department.

Data from optical turbidity sensors deployed at the four 

stations were recorded at 15- or 30-minute intervals by a data 

logger and uploaded every 1 to 4 hours to the USGS database. 

Most of the suspended-sediment samples were collected using 

automated samplers. The use of optical sensors to continuously monitor turbidity provided an accurate estimate of 

sediment fluctuations without the collection and analysis costs 

associated with intensive sampling during storms. Turbidity 

was used as a surrogate for suspended-sediment concentration (SSC), which is a measure of sedimentation and siltation. 

Regression models were developed between SSC and turbidity 

for each of the monitoring stations using SSC data collected 

from the automated samplers and turbidity data collected at 

each station. Instantaneous suspended-sediment loads (SSL) 

were computed from time-series turbidity and discharge data 

for the 2008 and 2009 water years1 using the regression equations. The instantaneous computations of SSL were summed to 

provide daily, storm, and water year annual loads. The annual 

SSL contributed from each basin was divided by the upstream 

drainage area to estimate the annual sediment yield. 

For all four basins, storms provided more than 96 percent of the annual SSL. In each basin, four storms generally 

provided over half the annual SSL each water year. Stormflows with the highest peak discharges generally carried the 

highest SSLs. For all stations, the greatest SSLs occurred 

during the late winter in February and March during the 2008 

water year. During the 2009 water year, the greatest SSLs 

occurred during December and August.

For French Creek near Phoenixville, the estimated annual 

SSL was 3,500 tons, and the estimated yield was 59.1 tons 

per square mile (ton/mi2) for the 2008 water year. For the 

2009 water year, the annual SSL was 4,390 tons, and the 

yield was 74.3 ton/mi2. For West Branch Brandywine Creek 

near Honey Brook, the estimated annual SSL was 4,580 tons, 

and the estimated yield was 245 ton/mi2 for the 2008 water 

year. For the 2009 water year, the annual SSL was 2,300 tons, 

and the yield was 123 ton/mi2. For West Branch Brandywine 

Creek at Modena, the estimated annual SSL was 7,480 tons, 

and the estimated yield was 136 ton/mi2 for the 2008 water 

year. For the 2009 water year, the annual SSL was 4,930 tons, 

and the yield was 90 ton/mi2. For East Branch Brandywine 

Creek below Downingtown, the estimated annual SSL was 

8,900 tons, and the estimated yield was 100 ton/mi2 for the 

2008 water year. For the 2009 water year, the annual SSL was 

7,590 tons, and the yield was 84 ton/mi2.

Introduction

Sedimentation and siltation is the leading cause of stream 

impairment in Pennsylvania. The Pennsylvania Department 

of Environmental Protection state-wide water-quality assessment summary lists siltation as the major cause of impairment 

for 8,271 miles of streams in Pennsylvania (Pennsylvania 

Department of Environmental Protection, 2010, p. 40). In 

Chester County, 16.5 stream miles (33.8 percent) upstream 

from West Branch Brandywine Creek near Honey Brook are 

impaired by siltation (D.E. Crocker, Chester County Water 

Resources Authority, written comm., 2011). Upstream from 

West Branch Brandywine Creek at Modena, 20.3 stream miles 1

A water year is the 12-month period from October 1 to September 30. It is 

designated by the year in which it ends.
2 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

(13.9 percent) are impaired by siltation. Upstream from East 

Branch Brandywine Creek below Downingtown, 22 stream 

miles (8.4 percent) are impaired by siltation. The drainage area 

upstream from French Creek near Phoenixville is not impaired 

by siltation.

Estimation of sediment loading in a stream typically 

requires the use of automated event samplers to collect a 

limited number of samples for laboratory analysis. The discrete samples generally are used in combination with streamdischarge data to estimate sediment concentrations and predict 

loads. Because of budget limitations and the labor intensive 

nature of sediment sampling, sediment loads are often based 

on a small number of samples. The use of optical sensors to 

continuously monitor turbidity provides an accurate estimate 

of sediment fluctuations without the collection and analysis 

costs associated with intensive sampling during storms. 

Turbidity is a principal physical characteristic of water 

and is an expression of the optical property that causes light 

to be scattered and absorbed by particles and molecules rather 

than transmitted in straight lines through a water sample. It 

is caused by suspended matter or impurities that interfere 

with the clarity of the water. These impurities may include 

clay, silt, finely divided inorganic and organic matter, soluble 

colored organic compounds, and microscopic organisms (U.S. 

Environmental Protection Agency, 1999, p. 7–1). Suspended 

solids affect the scattering of light, and turbidity increases as 

the scattering of light increases.

Turbidity or turbidity and discharge are often better indicators of suspended-sediment concentration (SSC) than the 

traditional sediment-transport curve method for some streams. 

Lee and others (2008) compared annual suspended-sediment 

loads (SSL) computed using traditional sediment-transport 

curve methods and a turbidity-SSC model at stations near 

John Redmond Reservoir in Kansas. They found that the SSL 

calculated from the turbidity-SSC model had an error of 1.1 to 

3.2 percent, whereas the SSL calculated from sediment-transport curves had an error of 16 to 20 percent.

Turbidity can serve as a surrogate for SSC. In-stream 

continuous turbidity and discharge data, calibrated with measured SSC data, can be used to compute a time-series of SSC 

and SSL at a stream station. Turbidity data were collected by 

the U.S. Geological Survey (USGS) at four stream stations in 

Chester County, Pa., in cooperation with the Chester County 

Water Resources Authority and the Chester County Health 

Department, with the intent to develop time-series SSC and 

SSL estimates. Data collection began in April 2007 and is 

continuing (2011). 

Purpose and Scope

This report presents the relations between turbidity and 

SSC at four stream stations in Chester County, Pa.—French 

Creek near Phoenixville, West Branch Brandywine Creek near 

Honey Brook, West Branch Brandywine Creek at Modena, 

and East Branch Brandywine Creek below Downingtown. 

Regression models were developed to use turbidity and 

discharge data to compute time-series SSC at the four stations 

for the 2008 and 2009 water years. Estimated daily, storm, 

and annual loads and estimated annual yields are presented for 

each station.

Description of Study Area

The study area encompasses four subbasins in Chester 

County, Pa. (fig. 1, table 1). Chester County is a 760-mi2 area 

in southeastern Pennsylvania. The county lies in the Piedmont 

Physiographic Province. This area is characterized by rolling hills underlain by sedimentary rock and deeply weathered 

crystalline rock. The largest stream in Chester County is 

Brandywine Creek, which drains 290 mi2 (38 percent) of the 

county. Three of the study subbasins are in the Brandywine 

Creek Basin. Brandywine Creek is a tributary to the Christina 

River, which drains to the Delaware River. The fourth station is in the 70.7-mi2

 French Creek Basin. French Creek is a 

tributary to the Schuylkill River, which is a major tributary to 

the Delaware River.

Chester County is undergoing rapid urbanization. 

Suburban development is concentrated in the eastern and 

central parts of the county. The northern and western parts 

of the county are rural with more farm land and pasture than 

the eastern and central parts. Major crops are hay and corn. 

Dairy and equestrian farms are also common. The East Branch 

Brandywine Creek near Honey Brook subbasin has the most 

agricultural and pasture land (59.7 percent), whereas the 

West Branch Brandywine Creek subbasin between the Honey 

Brook and Modena water-quality monitoring stations has the 

least (26.4 percent) (table 2). The French Creek subbasin has 

the most forest land (44.6 percent), whereas the East Branch 

Brandywine Creek near Honey Brook subbasin has the least 

(17.6 percent). The West Branch Brandywine Creek subbasin between the Honey Brook and Modena stations and 

East Branch Brandywine Creek below Downingtown subbasins are the most urbanized (31.5 and 31.9 percent urban 

land, respectively).
Introduction 3

Delaware

River

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

EXPLANATION

Base from U.S. Geological Survey digital data,1972,1:2,000,000 Albers Equal-Area Conic

Projection.Standard parallels 29°30'N, central meridian 75°00'W. 

01472157

Extent of town or city

Basin boundary

Subbasin boundary

Water-quality monitoring 

station and U.S. Geological 

Survey identification number

Location of

study area

76° 75°50' 75°40' 75°30'

39°

50'

40°

40°

10'

01472157

01480300

01480617

01480870

CHESTER COUNTY

BERKS COUNTY

MARYLAND

DELAWARE

PENNSYLVANIA

Modena

Phoenixville

Honey 

Brook

Downingtown

LANCASTER COUNTY

DELAWARE COUNTY

MONTGOMERY COUNTY

Christina River

Schuylkill River

French Cre e k

Brandywine

Creek

East Branch Brandywine Creek

West Bran

ch Brandywine Creek

Figure 1. Water-quality-monitoring stations on French and Brandywine Creeks, Chester County, 

Pennsylvania.
4 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

Table 1. Water-quality monitoring stations on French and Brandywine Creeks, Chester County, Pennsylvania.

[mi2, square miles]

U.S. Geological Survey 

station-identification 

number 

Water-quality monitoring station Latitude Longitude

Drainage 

area 

(mi2)

Measured water-quality 

characteristics

01472157 French Creek near Phoenixville, Pa. 40°09 ́05 ̋ 75°36 ́06 ̋ 59.1 Turbidity

01480300 West Branch Brandywine Creek 

near Honey Brook, Pa.

40°04 ́22 ̋ 75°51 ́40 ̋ 18.7 Turbidity

01480617 West Branch Brandywine Creek at 

Modena, Pa.

39°57 ́42 ̋ 75°48 ́06 ̋ 55 Turbidity, pH, dissolved oxygen, 

conductivity, temperature

01480870 East Branch Brandywine Creek 

below Downingtown, Pa.

39°58 ́07 ̋ 75°40 ́25 ̋ 89.9 Turbidity, pH, dissolved oxygen, 

conductivity, temperature

Table 2. Basin characteristics, French and Brandywine Creeks, Chester County, Pennsylvania. 

[Data provided by D.E. Crocker (Chester County Water Resources Authority, written comm., 2011) from 2005 land-use data.] 

French Creek near 

Phoenixville

West Branch 

Brandywine Creek 

near Honey Brook

West Branch 

Brandywine Creek 

at Modena

Area between West 

Branch Brandywine 

Creek near Honey 

Brook and Modena

East Branch 

Brandywine Creek 

below Downingtown

Drainage area (square 

miles)

59.1 18.7 55 36.3 89.9

Percent of area covered by 

forest

44.6 17.6 27.0 31.7 27.7

Percent of area covered by 

agriculture and pasture

33.0 59.7 37.4 26.4 29.8

Percent of area covered by 

urbanization

16.1 13.8 25.6 31.5 31.9

Percent of area covered by 

lakes, ponds, reservoirs, 

and wetlands

3.6 3.9 3.0 2.6 4.5

Total stream length (miles) 174.6 49 146.2 97.2 261

Stream density (stream 

miles per square mile)

2.95 2.62 2.66 2.68 2.90
Methods of Data Collection 5

Methods of Data Collection

Four water-quality-monitoring stations equipped with turbidity sensors were in operation in the French and Brandywine 

Creek Basins during water years 2008 and 2009 (table 1). 

Discharge data were collected at all stations. Published data 

for these stations are available at http://pa.water.usgs.gov/. 

Turbidity data were collected in accordance with the 

maintenance and calibration protocols described by Wagner 

and others (2006). Most turbidity data were collected using 

Yellow Spring Instrument (YSI) Optical Monitoring System 

(OMS) 600 series and YSI 6920V2 sondes. Analite NEP395G 

turbidity sensors were installed at the West Branch Brandywine Creek at Modena and East Branch Brandywine Creek 

below Downingtown stations prior to the study; however, 

they were replaced with YSI sensors. All turbidity sensors 

had wipers to reduce biofouling and clean the probe prior to 

all measurements. 

The Analite sensor has a range of 0 to 1,000 Formazine 

Nephelometric Units (FNU) with an accuracy of ±0.2 FNU 

at 1,000 FNU. A YSI 6136 turbidity sensor was used in both 

types of YSI sondes. The YSI sensor has a range of 0 to 

1,000 FNU with an accuracy of ±2 percent of measurement 

or 0.3 FNU, whichever is greater. Both brands of turbidity sensors use near-infrared wavelengths with a single 

90-degree detector. 

Station Instrumentation

YSI 6136 turbidity sensors were installed on November 

7, 2006, at French Creek near Phoenixville and West Branch 

Brandywine Creek near Honey Brook. An Analite NEP395 

turbidity sensor was installed on September 22, 2005, at West 

Branch Brandywine Creek at Modena. It was replaced with a 

YSI 6136 turbidity sensor on February 28, 2007. An Analite 

NEP395 turbidity sensor was installed on September 26, 2005, 

at East Branch Brandywine Creek below Downingtown. It was 

replaced with a YSI 6136 turbidity sensor on August 7, 2008. 

Turbidity sensors were installed inside 4-in. diameter polyvinyl chloride housings (fig. 2) drilled with circulation holes. 

Turbidity Monitoring

The Analite sensors recorded turbidity data in whole 

numbers, and the YSI sensors recorded data to 0.1 FNU. All 

turbidity measurements were recorded at 15- or 30-minute 

intervals by a data logger and uploaded every 1 to 4 hours 

to the USGS database. Data from the in-situ sensors were 

compared with annual cross-sectional data using the technique 

described in Wagner and others (2006) to verify that the in-situ 

sensors were installed in representative locations in the stream. 

No bias was noted for any of the stations.

In order to calculate SSC from turbidity, a complete 

record of turbidity and discharge is needed. Missing data 

occurred because of spikes, servicing of the sensor, or fouling. 

Figure 2. Turbidity sensor deployed at East Branch Brandywine 

Creek below Downingtown, Pennsylvania. The sensor is installed 

in a perforated 4-inch-diameter polyvinyl chloride housing for 

protection.

If field meter values were available, they were used to replace 

missing data. When the period of time between missing values 

was small, interpolation between measured data points was 

used to estimate missing values. Where there were large gaps 

in the record, data from nearby turbidity sensors and discharge 

and rainfall data were used to estimate missing values. Missing turbidity values were estimated for the purpose of estimating SSC but were not published or stored in the database.

Suspended-Sediment Sampling

Most of the suspended-sediment samples were collected 

using automated samplers. Several grab samples were collected at stations prior to the installation of an automated 

sampler at that station, and attempts were made to collect 

equal width integrated (EWI) samples as described by Wilde 

and others (1999) at all stations concurrently with automated 

samples to determine mixing. Eighteen EWI samples were 

collected—1 at French Creek near Phoenixville, 8 at West 

Branch Brandywine Creek near Honey Brook, 5 at West 

Branch Brandywine Creek at Modena, and 4 at East Branch 

Brandywine Creek below Downingtown.

Automated samplers were installed on July 23, 2007, at 

French Creek near Phoenixville; on April 11, 2007, at West 

Branch Brandywine Creek near Honey Brook; and on November 15, 2007, at East Branch Brandywine Creek below Downingtown and West Branch Brandywine Creek at Modena. 

Some of the automated samplers had programmable thresholds 

for turbidity and discharge to control sample collection. The 

thresholds were constantly adjusted to better target conditions 

under which the samples were collected. At other stations, 

older automated samplers had to be turned on before each 

storm event; they collected one sample per hour for 24 hours. 
6 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

As more funding became available, the older automated 

samplers were replaced with programmable samplers at all stations, except the West Branch Brandywine Creek near Honey 

Brook station. 

Of the 24 samples collected for a storm, 1 to 6 samples 

were selected for laboratory analysis. These selections were 

made to minimize potential bias from serial correlation and 

reduce the effect of any one event on the overall relation 

between turbidity and SSC. Serial correlation (also called 

autocorrelation) occurs when data are collected close enough 

in time that the regression assumption of data independence 

is violated. Serial correlation caused by using too many 

samples from a single storm can cause underestimation of the 

regression uncertainty. Samples were analyzed at the USGS 

Kentucky Water Science Center Sediment Laboratory and the 

USGS Pennsylvania Water Science Center.

Quality Assurance

Turbidity was measured across the width of the stream 

during the collection of samples and during annual reviews 

of in-stream continuous water-quality installations. The same 

model YSI sonde and turbidity probe were used to collect the 

cross-section measurements as was deployed at the station. 

The measurements were made directly across the stream from 

the in-stream sensors; however, when flows were too high to 

wade the stream, the cross-sections were measured from the 

nearest bridge. Cross-sectional turbidity measurements were 

compared with measurements from in-stream sensors to assess 

the ability of the in-stream sensors to collect data representative of the width of the stream. Because consistent bias was 

not observed in the relation at any monitoring location, values 

from continuous-water-quality monitors are representative 

of stream-water quality across the width of the stream cross 

section. Turbidity records generally were rated good (error of 

5–10 percent), occasionally fair (10–15 percent), and rarely 

poor (greater than 15 percent) on the basis of guidelines developed by Wagner and others (2006). 

Regression Models

Regression models were developed between SSC and turbidity as a means of estimating time-series SSC using turbidity data. Available data from the 2007, 2008, and 2009 water 

years were used. The estimated SSC was used to compute 

daily, storm, and annual SSLs at each station for water years 

2008 and 2009. 

Regression models were developed for the relations of 

SSC to turbidity (T) and discharge (Q) for each station using 

SSC data collected by the automated samplers and turbidity 

and discharge data collected at each station. The methodology was described by Rasmussen and others (2009), Helsel 

and Hirsch (2002), and Bragg and others (2007). Each station 

was evaluated independently, and regression equations were 

developed for each station using model building approaches 

explained by Helsel and Hirsch (2002).

Six regression models were initially developed for each 

station: (1) SSC in relation to T, (2) SSC to Q, (3) SSC to T

and Q, (4) log10SSC to log10T, (5) log10SSC to log10Q, and (6) 

log10SSC to log10T and log10Q. Statistics, including coefficient 

of determination (R2), adjusted R2, standard error, prediction 

error sum of squares (PRESS), Mallow’s Cp, variance inflation factor, probability plot correlation coefficient, and model 

standard percentage error, were used to evaluate the models 

(table 3). The statistics are appropriate only for comparing 

models with the same response variable units. The coefficient 

of determination is the fraction of the variance explained by 

the regression model. The adjusted coefficient of determination is adjusted for the number of degrees of freedom to allow 

comparison of regression models with differing numbers of 

explanatory variables. The standard error is an estimate of 

the variation from the average. The prediction error sum of 

squares estimates error by using n – 1 observations in the 

regression model to estimate the value left out; a lower PRESS 

indicates less model error. Mallow’s Cp is used to assess the 

fit of a regression model by minimizing bias and standard 

error; the best model is the one with the lowest Cp. The variance inflation factor estimates how much the variance of an 

estimated regression coefficient in a multiple linear regression 

model is increased because of collinearity. The probability 

plot correlation coefficient is a test for normal distribution, 

which will have a correlation coefficient close to 1.0. As data 

depart from normality, the coefficient drops below 1.0. The 

model standard percentage error is the root mean squared 

error expressed as a percentage. It is a measure of the variance between observed values and values computed by the 

regression model. Uncertainty of regression estimates was 

determined using 90-percent prediction intervals (Helsel and 

Hirsch, 1992). 

For French Creek near Phoenixville, a linear regression 

model using turbidity as the explanatory variable provided the 

best results (table 3). The regression model selected for French 

Creek was 

SSC = 1.689T – 3.954 . (1)

The relation between SSC and T is shown in figure 3. Turbidity explained 96 percent of the variability in SSC values.

For West Branch Brandywine Creek near Honey Brook, 

the magnitudes of the correlation coefficients among SSC, T, 

and discharge were lower than for the other stations (table 3). 

A multiple linear regression using T and discharge as the 

explanatory variables (adjusted R2 = 0.75) and a simple linear 

regression using T as the explanatory variable (adjusted 

R2 = 0.74) gave approximately the same results. Because of 

the similarity of the coefficients of determination, the simpler 

model was chosen. The regression model selected for the 

Honey Brook station was 

SSC = 1.8729T – 11.5034 . (2)
Regression Models 7

Table 3. Regression models developed for French and Brandywine Creeks, Chester County, Pennsylvania, with suspended sediment 

concentration as the response variable.

[R2, coefficient of determination; PPCC, probability plot correlation coefficient; PRESS, prediction error sum of squares; MSPE, model standard percentage 

error; VIF, variance inflation factor; -- statistic not applicable; n, number of observations; T, turbidity; Q, discharge]

Explanatory 

variable R 2 Adjusted R 2 Standard error

PPCC PRESS Mallow’s 

Cp MSPE VIF

French Creek near Phoenixville (n=106)

T 0.957 0.956 26.0 0.995 77,858 4.64 14.7 --

Q 0.285 0.278 99.6 0.953 1,076,112 1,340 57.0 --

T, Q 0.949 0.948 26.8 0.985 79,437 3 -- 1.34

log10T 0.928 0.928 0.10 0.961 1.03 1.32 22.4 --

log10Q 0.448 0.443 0.27 0.997 7.71 693 65.6 --

log10T and log10Q 0.929 0.927 0.10 0.960 1.04 3 -- 1.88

West Branch Brandywine Creek near Honey Brook (n=74)

T 0.744 0.741 70.6 0.996 371,425 3.93 39.0 --

Q 0.087 0.074 134 0.971 1,391,323 191 73.5 --

T, Q 0.757 0.750 69.6 0.996 372,461 3 -- 1.25

log10T 0.710 0.705 0.21 0.986 3.21 1.00 49.8 --

log10Q 0.398 0.390 0.30 0.977 6.73 74.94 74.6 --

log10T and log10Q 0.710 0.701 0.21 0.986 3.31 3 -- 2.27

West Branch Brandywine Creek at Modena (n=79)

T 0.936 0.935 29.9 0.985 82,901 1.13 18.20 --

Q 0.183 0.172 107 0.849 918,636 893 65.0 --

T, Q 0.936 0.934 30.2 0.988 87,914 3 -- 1.26

log10T 0.503 0.497 0.22 0.992 3.97 466 53.3 --

log10Q 0.928 0.927 0.08 0.991 0.58 3.13 19.5 --

log10T and log10Q 0.930 0.928 0.08 0.991 0.58 3 -- 1.98

East Branch Brandywine Creek below Downingtown (n=55)

T 0.938 0.937 45.7 0.989 132,881 2.42 18.7 --

Q 0.029 0.011 184 0.887 1,911,963 639 75.58 --

T, Q 0.927 0.924 51.1 0.969 169,811 3 -- 1.01

log10T 0.944 0.943 0.09 0.992 0.49 8.98 21.44 --

log10Q 0.369 0.357 0.31 0.993 1.30 5.52 624 --

log10T and log10Q 0.951 0.950 0.09 0.989 0.44 3 -- 1.44
8 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

The relation between SSC and T is shown in figure 4. Turbidity explained 74 percent of the variability in SSC values.

For West Branch Brandywine Creek at Modena, a multiple linear regression using T and discharge as the explanatory variables (adjusted R2

 = 0.93 for untransformed data and 

adjusted R2 = 0.94 for logarithmic base-10 transformation 

data) and a simple linear regression using T as the explanatory 

variable (adjusted R2 = 0.94) gave approximately the same 

results (table 3). Because of the similarity of the coefficients of 

determination, the simpler model was chosen. The regression 

model selected for the Modena station was 

SSC = 1.9879T – 9.6239 . (3)

The relation between SSC and T is shown in figure 5. Turbidity explained 94 percent of the variability in SSC values.

For East Branch Brandywine Creek below Downingtown, a multiple linear regression using T and discharge as the 

explanatory variables (adjusted R2 = 0.95 for logarithmic base10 transformation data) and a simple linear regression using T

as the explanatory variable (adjusted R2 = 0.94 for logarithmic 

base-10 transformation data) gave approximately the same 

results (table 3). Because of the similarity of the coefficients of 

determination, the simpler model was chosen. The regression 

relation model selected for the Downingtown station was 

log(SSC) = 1.0115 log(T) + 0.2596 . (4)

SSC = 1.689T – 3.954

R 2

 = 0.96

Regression fit

90-percent prediction error

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350 400

Turbidity (T), in formazin nephelometric units

Suspended-sediment concentration (SSC), in milligrams per liter

Figure 3. Regression relations of turbidity 

and suspended-sediment concentration 

for French Creek near Phoenixville, 

Pennsylvania. 

Regression fit

90-percent prediction error

SSC = 1.8729T – 11.5034

R 2

 = 0.74

Turbidity (T), in formazin nephelometric units

0

0 50 100 150 200 250 300 350

Suspended-sediment concentration (SSC), in milligrams per liter

100

200

300

400

500

600

700

Figure 4. Regression relations of turbidity 

to suspended-sediment concentration for 

West Branch Brandywine Creek near Honey 

Brook, Pennsylvania.
Regression Models 9

The relation between SSC and T is shown in figure 6. Turbidity explained 94 percent of the variability in SSC values. 

A Duan’s bias correction factor (Duan, 1983) of 1.022 was 

applied to correct for negative bias during retransformation of 

the response variable.

Discharge correlated poorly with SSC. The adjusted R2

values were 0.28, 0.07, 0.17, and 0.01 for the Phoenixville, 

Honey Brook, Modena, and Downingtown stations, respectively (table 3), which indicates that discharge is a poor 

surrogate for SSC. For example, consider three storms measured at West Branch Brandywine Creek near Honey Brook 

during December 16–26, 2009 (fig. 7). For the storm beginning December 19, the use of discharge as a surrogate for SSC

would provide a good approximation of the SSL. However, for 

the storms beginning December 16 and 24, the use of discharge as a surrogate for SSC would substantially overestimate 

the SSL. For French Creek near Phoenixville, discharge is a 

poor surrogate for SSC (fig. 8).

Regression fit

90-percent prediction error

SSC = 1.9879T – 9.6239

R 2

 = 0.94

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250 300 350 400

Turbidity (T), in formazin nephelometric units

Suspended-sediment concentration (SSC), in milligrams per liter

Figure 5. Regression relations of turbidity 

to suspended-sediment concentration for 

West Branch Brandywine Creek at Modena, 

Pennsylvania.

1

10

100

1,000

10,000

1 10 100 1,000

Turbidity (T), in formazin nephelometric units

Suspended-sediment concentration (SSC), in milligrams per liter

Regression fit

90-percent prediction error

LogSSC = 1.0115 logT + 0.2596

R 2

 = 0.94

Figure 6. Regression relations of turbidity 

to suspended-sediment concentration 

for East Branch Brandywine Creek below 

Downingtown, Pennsylvania.
10 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Discharge, in cubic feet per second

10

100

1,000

12/16

Turbidity, in formazin nephelometric units

12/27

0

20

40

60

80

100

120

140

160

180

Water year 2009

12/17 12/18 12/19 12/20 12/21 12/22 12/23 12/24 12/25 12/26

Discharge

Turbidity

Figure 7. Discharge and 

turbidity measured at West 

Branch Brandywine Creek near 

Honey Brook, Pennsylvania, 

December 16–26, 2008.

Discharge, in cubic feet per second

10

100

1,000

10,000

Turbidity, in formazin nephelometric units

0

50

100

150

200

250

300

350

400

Water year 2009

Discharge

Turbidity

8/2 8/3 8/4 8/5 8/6 8/7 8/8 8/9 8/10 8/11 8/12 8/13 8/14 8/15 8/16 8/17

Figure 8. Discharge and 

turbidity measured at French 

Creek near Phoenixville, 

Pennsylvania, August 2–16, 

2009.
Estimation of Suspended-Sediment Loads and Yields 11

Estimation of Suspended-Sediment 

Loads and Yields

The term “load” represents the mass (commonly 

expressed in tons or pounds) of a constituent transported past 

a sampling station during a specified period of time. Loads can 

be computed for various time increments, such as instantaneous, daily, monthly, seasonal, annual, or for storms. Instantaneous loads represent the mass transported at the specific 

sampling time, whereas daily, monthly, seasonal, annual, and 

storm loads represent the cumulative mass transported over a 

prolonged period. 

Annual loads generally are more informative than instantaneous loads measured at the time of sampling because they 

represent the cumulative transport of sediment during a year 

and, thereby, incorporate the potentially large range of daily 

variations. Annual loads also are useful for evaluating differences in sediment transport among stations to identify source 

areas contributing substantial inputs. Differences in annual 

loads transported past sampling stations can result from differences in annual flow volumes, physical basin characteristics, 

current and historical land-use activities, and localized conditions that affect sediment supply or susceptibility to erosion.

Instantaneous suspended-sediment loads (SSL) were 

computed from time-series turbidity and discharge data for the 

2008 and 2009 water years as follows. Regression equations 

(1) through (4) were used to compute instantaneous SSC from 

turbidity data collected every 30 minutes, the common recording time interval for all four stations. SSC values computed 

from regression estimates were multiplied by the corresponding discharge values and a unit conversion to compute instantaneous estimates of SSL in short tons (2,000 lbs). Instantaneous SSL was calculated using the following equation: 

SSLi = SSCi × Qi × C , (5)

where

SSLi is the computed suspended-sediment load, in 

tons per 30-minute interval; 

SSCi is the computed suspended-sediment 

concentration for the ith value, in 

milligrams per liter; 

Qi is the discharge for the ith value, in cubic feet 

per second; and

C is a constant, 5.62 × 10-5, for converting the 

units to tons per 30 minutes.

The instantaneous (30-minute) computations of SSL were 

summed to provide daily, storm, and annual loads. Estimated annual SSL was determined by summing the estimated 

30-minute SSL transported past each of the four sampling 

stations during water years 2008 and 2009 (table 4). The 

annual SSL contributed from each subbasin was divided by the 

upstream drainage area to estimate the annual sediment yield. 

Annual sediment yields were calculated to compare sediment 

loads among basins of different sizes and are not meant to 

imply equal sourcing of sediment throughout the drainage. 

Annual yields per stream mile also were calculated for each 

subbasin. However, the stream density in each basin is similar 

(range of 2.62 to 2.95 mi/mi2

, table 2), and the ratios of annual 

yield per square mile to annual yield per stream mile for all 

four subbasins ranged from 0.34 to 0.38. 

For French Creek near Phoenixville, the estimated 

annual SSL was 3,500 tons, and the estimated yields were 

59.1 ton/mi2

 and 20 tons per stream mile for the 2008 water 

year (table 4). The storm beginning March 19, 2008, provided 

20.4 percent of the annual SSL. The four storms yielding the 

greatest SSL (storms beginning on March 19, February 13, 

September 6, and February 1, 2008, in order of load) provided 

54 percent of the annual SSL. Storms provided 99.5 percent of the annual SSL; 14 of 41 storms that produced more 

than 1 ton of sediment provided 90.2 percent of the annual 

SSL. The highest cumulative SSLs for the 2008 water year 

occurred during the late winter in February and March (fig. 9, 

table 5). For the 2009 water year, the estimated annual SSL

was 4,390 tons, and the estimated yields were 74.3 ton/mi2

and 25.1 tons per stream mile (table 4). The storm beginning 

December 10, 2008, provided 23.2 percent of the annual SSL. 

The four storms yielding the greatest SSL (storms beginning 

on December 10, 2008, and July 27, May 3, and August 2, 

2009, in order of load) provided 56.1 percent of the annual 

SSL. Storms provided 99.6 percent of the annual SSL; 13 of 

43 storms that produced more than 1 ton of sediment provided 

90.7 percent of the annual SSL. The highest cumulative SSLs 

for the 2009 water year occurred during December and the 

summer in July and August (fig. 9, table 6). Storm flows with 

the highest peak discharges generally carried the highest SSLs 

(R2

 = 0.90) (fig. 10).

For West Branch Brandywine Creek near Honey Brook, 

the estimated annual SSL was 4,580 tons, and the estimated 

yields were 245 ton/mi2 and 93.5 tons per stream mile for the 

2008 water year (table 4). The storm beginning March 19, 

2008, provided 42.4 percent of the annual SSL. The four 

storms yielding the greatest SSL (storms beginning on 
12 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Table 4. Estimated annual suspended-sediment loads and yields for French and Brandywine Creeks, Chester County, Pennsylvania, 2008 and 2009 water years. [--, not determined] U.S. Geological Survey station- identification number Water-quality monitoring station Drainage area (square miles) Estimated annual load (tons per year) Estimated annual yield (tons per square mile per year) Estimated annual yield (tons per stream mile per year) Percentage of sediment from stormflow 2008 water year 2009 water year 2008 water year 2009 water year 2008 water year 2009 water year 2008 water year 2009 water year 01472157 French Creek near Phoenixville, Pa. 59.1 3,500 4,390 59.1 74.3 20.0 25.1 99.5 99.6 01480300 West Branch Brandywine Creek near Honey Brook, Pa. 18.7 4,580 2,300 245 123 93.5 46.9 98.5 96.2 01480617 West Branch Brandywine Creek at Modena, Pa. 55.0 7,480 4,930 136 90.0 51.2 33.7 98.7 98.7 Area between West Branch Brandywine Creek near Honey Brook and Modena 36.3 2,900 2,630 79.9 72.5 29.8 27.1 -- -- 01480870 East Branch Brandywine Creek below Downingtown, Pa. 89.9 8,990 7,590 100 84.0 34.5 29.1 96.5 96.4
Estimation of Suspended-Sediment Loads and Yields 13

Suspended-sediment load, in tons

10/1/07 11/1/07 12/1/07 1/1/08 2/1/08 3/1/08 4/1/08 5/1/08 6/1/08 7/1/08 8/1/08 9/1/08 10/1/08

Water year 2008

0

100

200

300

400

500

600

700

800

Water year 2009

Suspended-sediment load, in tons

10/1/08 11/1/08 12/1/08 1/1/09 2/1/09 3/1/09 4/1/09 5/1/09 6/1/09 7/1/09 8/1/09 9/1/09 10/1/09

0

100

200

300

400

500

600

1,000

1,100

Figure 9. Suspended-sediment loads for storms at French Creek near Phoenixville, Pennsylvania, 2008 and 2009 water years. 
14 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

Peak discharge, in cubic feet per second

Suspended-sediment load, in tons

R 2 = 0.90

0

200

400

600

800

1,000

1,200

0 500 1,000 1,500 2,000 2,500

2008 and 2009 water years.

Figure 10. Relation of storm suspended-sediment load to peak discharge at French Creek near Phoenixville, Pennsylvania, 

March 19, February 1, February 13, and July 27, 2008, in 

order of load) provided 61.6 percent of the annual SSL. Storms 

provided 98.5 percent of the annual SSL; 18 of 42 storms that 

produced more than 1 ton of sediment provided 90.3 percent 

of the annual SSL. The highest cumulative SSLs for the 2008 

water year occurred during the late winter in February and 

March (fig. 11, table 7). For the 2009 water year, the estimated 

annual SSL was 2,300 tons, and the estimated yields were 

123 ton/mi2 and 46.9 tons per stream mile (table 4). The storm 

beginning December 10, 2008, provided 17.1 percent of the 

annual SSL. The four storms yielding the greatest SSL (storms 

beginning on December 10, 2008, and August 2, June 13, and 

April 3, 2009, in order of load) provided 43.6 percent of the 

annual SSL. Storms provided 96.2 percent of the annual SSL; 

24 of 44 storms that produced more than 1 ton of sediment 

provided 90.5 percent of the annual SSL. The highest cumulative SSL for the 2009 water year occurred during December 

and August (fig. 11, table 8). Storm flows with the highest 

peak discharges generally carried the highest SSLs (R2

 = 0.75) 

(fig. 12).
Estimation of Suspended-Sediment Loads and Yields 15

Table 5. Daily sediment load for French Creek near Phoenixville, Pennsylvania, 2008 water year.[Loads given in tons; --, not applicable]

Day October November December January February March April May June July August September

1 0.0 0.0 0.0 1.2 172 0.03 0.05 0.03 0.9 0.1 0.4 0.092 0.0 0.0 0.001 0.4 123 0.01 0.05 0.02 0.05 0.06 0.4 0.23 0.0 0.0 1.4 0.2 4.5 0.002 0.009 0.0 0.02 0.02 0.1 0.0034 0.02 0.009 0.8 0.2 0.8 0.02 0.8 0.04 2.5 0.002 0.08 0.05 0.001 0.0 0.2 0.05 0.3 26.8 0.4 0.0 45.0 0.08 0.1 0.06 0.002 0.0 0.0 0.009 0.2 6.3 0.01 0.004 4.1 0.06 0.2 2327 0.003 0.3 0.005 0.006 0.3 3.1 0.0 0.0008 1.2 0.8 0.1 72.08 0.001 0.0 0.07 0.01 0.2 255 0.0 0.04 1.9 0.99 0.1 1.39 0.02 0.0001 0.005 0.003 0.05 33.5 0.0 25.1 1.7 8.7 0.1 2.8

10 0.3 0.0 0.0 0.003 0.002 3.7 0.0 7.7 2.5 3.2 4.5 1.8

11 1.9 0.0 0.006 0.001 0.0008 1.2 0.2 0.6 8.2 0.7 12.2 0.4

12 1.2 0.0 0.06 0.0003 0.0 0.7 119 0.3 1.2 0.7 1.3 0.313 0.2 0.0 2.5 0.0003 482 0.4 2.6 0.6 0.1 1.1 0.5 3.414 0.04 0.0 3.8 0.0 76.7 0.2 0.7 0.09 0.05 18.9 0.4 0.715 0.006 0.6 1.0 0.0 5.3 0.2 0.3 0.03 7.6 2.8 0.4 0.216 0.003 0.9 113 0.0 1.7 0.2 0.08 28.1 1.9 0.1 0.4 0.0817 0.0 0.3 21.5 0.0 0.7 0.2 0.08 11.1 0.6 0.04 0.2 0.0718 0.0 0.02 1.4 0.7 17.3 0.09 0.0 9.7 0.2 0.06 0.2 0.0619 0.001 0.3 0.3 1.3 6.1 13.2 0.1 12.3 0.1 0.04 0.2 0.0420 0.5 1.3 0.2 0.4 0.9 676 0.2 8.3 0.008 0.04 0.2 0.0521 0.05 0.5 0.009 0.1 0.2 20.0 0.4 9.2 0.003 0.1 0.1 0.0422 0.0 0.04 0.0 0.1 0.1 3.5 0.4 1.4 0.0002 0.1 0.09 0.023 0.0 0.0 0.8 0.003 0.04 1.6 0.07 0.6 0.0 27.5 0.09 0.0224 0.0 0.002 25.0 0.02 0.1 0.9 0.0 0.5 0.1 249 0.04 0.0425 0.0 0.03 2.6 0.02 0.1 0.6 0.04 0.2 0.1 3.5 0.1 0.0226 0.1 6.0 0.8 0.02 1.7 0.4 0.06 0.1 0.1 1.0 0.1 0.0427 133 12.0 3.5 0.02 9.1 0.3 0.3 0.1 0.1 28.9 0.04 0.128 9.4 1.0 0.8 0.01 1.3 0.2 70.1 0.1 0.2 6.7 0.04 22.829 0.4 0.1 6.3 0.001 0.2 0.4 45.5 0.03 0.5 2.0 0.03 11.030 0.02 0.001 2.2 0.1 -- 0.05 1.1 0.003 0.4 0.3 0.02 0.731 0.001 -- 4.5 0.1 -- 0.02 -- 2.3 -- 0.5 0.2 --

Monthly total 147 23.4 193 4.98 905 1,050 243 119 81.3 358 22.9 350
16 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Table 6. Daily sediment load for French Creek near Phoenixville, Pennsylvania, 2009 water year. [Loads given in tons; --, not applicable] Day October November December January February March April May June July August September

1 0.3 0.3 3.3 0.0004 0.6 0.002 0.0 6.9 0.04 0.2 14.4 0.092 0.2 0.1 0.7 0.0 1.9 0.0 0.03 3.7 0.05 0.2 393 0.023 0.008 0.09 0.09 0.02 1.5 0.0 79.6 4.1 0.3 0.3 35.6 0.014 0.001 0.07 0.0 0.0 0.3 0.0 23.5 131 7.3 0.3 2.0 0.025 0.002 0.04 0.0 0.0009 0.04 0.0 1.5 71.9 22.7 0.08 0.4 0.036 0.0 0.006 0.0 0.0 0.05 0.0 0.1 8.8 18.0 0.1 0.1 0.07 0.0 0.0002 0.0 28.9 0.3 0.003 0.07 247 1.2 0.2 0.08 0.00078 0.0 0.0008 0.0 10.4 6.6 0.02 0.002 9.4 0.3 0.09 0.004 0.0059 0.0 0.0 0.0 0.9 4.1 0.003 0.07 1.0 159 0.1 40.4 0.03

10 0.001 0.0004 0.006 0.12 0.5 0.0 0.01 0.4 111 0.2 14.4 0.02

11 0.0 0.001 119 0.0 0.5 0.0 1.6 0.09 4.1 0.7 1.0 189

12 0.0 0.002 882 0.001 0.04 0.0 0.8 0.05 2.1 169 5.5 54.313 0.0 0.02 13.9 0.0 0.02 0.0 0.07 0.05 3.1 4.1 413 4.114 0.0 0.07 2.3 0.002 0.0 0.0 0.8 1.4 23.7 0.6 6.1 0.415 0.0 13.8 0.4 0.0 0.0 0.0 3.2 44.1 2.4 0.3 0.7 0.0916 0.003 5.1 0.2 0.0 0.0 0.0 0.9 4.0 0.8 0.2 0.2 0.0617 0.02 0.6 4.6 0.0 0.0 0.6 0.1 6.7 0.5 0.2 0.06 0.0618 0.02 0.05 2.7 0.0 0.0 5.6 0.03 0.4 2.4 0.8 0.02 0.0919 0.05 0.0 81.1 0.0 0.0 0.07 0.03 0.08 2.0 0.5 0.03 0.0220 0.001 0.0 40.9 0.0 0.001 0.02 0.3 0.07 9.7 0.4 0.0 0.0321 0.0 0.0 2.7 0.01 0.001 0.0 2.3 0.04 12.0 0.09 1.6 0.0322 0.0 0.0 0.8 0.0 0.03 0.02 2.4 0.004 1.8 0.08 3.3 0.0323 0.01 0.0 0.3 0.0 0.004 0.0 0.4 0.008 0.6 3.7 0.6 0.124 0.0003 0.0 6.0 0.0 0.001 0.0 0.09 0.1 0.4 6.6 0.03 2.425 0.07 0.001 13.4 0.0 0.001 0.0 0.2 0.05 0.3 0.7 0.03 0.526 1.1 0.0 1.3 0.0 0.002 0.0 0.2 0.4 0.2 1.5 0.004 0.427 0.0 0.0 0.3 0.0 0.002 0.0 0.3 2.1 0.2 1.3 0.02 8.228 25.2 0.0 0.08 0.02 0.006 0.05 0.4 0.04 0.2 534 4.5 1.829 9.9 0.0 0.06 0.004 -- 0.007 0.4 6.0 0.2 17.9 40.3 0.030 1.0 0.09 0.0 0.0 -- 0.02 0.4 1.1 0.2 16.0 13.5 0.031 0.5 -- 0.4 0.0 -- 0.00 -- 0.08 -- 20.8 0.2 --

Monthly total 38.4 20.3 1,180 40.4 16.5 6.42 120 551 387 781 991 262
Estimation of Suspended-Sediment Loads and Yields 17

Suspended-sediment load, in tons

10/1/07 11/1/07 12/1/07 1/1/08 2/1/08 3/1/08 4/1/08 5/1/08 6/1/08 7/1/08 8/1/08 9/1/08 10/1/08

Water year 2008

Water year 2009

Suspended-sediment load, in tons

10/1/08 11/1/08 12/1/08 1/1/09 2/1/09 3/1/09 4/1/09 5/1/09 6/1/09 7/1/09 8/1/09 9/1/09 10/1/09

0

100

200

300

400

0

100

200

300

400

500

2,000

1,900

Figure 11. Suspended-sediment loads for storms at West Branch Brandywine Creek near Honey Brook, Pennsylvania, 2008 and 2009 

water years.
18 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

Peak discharge, in cubic feet per second

Suspended-sediment load, in tons

R 2 = 0.75

0

500

1,000

1,500

2,000

0 500 1,000 1,500 2,000

Figure 12. Relation of storm suspended-sediment load to peak discharge at West Branch Brandywine Creek near Honey 

Brook, Pennsylvania, 2008 and 2009 water years.

mated annual SSL was 7,480 tons, and the estimated yields 

were 136 ton/mi2 and 51.2 tons per stream mile for the 2008 

water year (table 4). The storm beginning March 19, 2008, 

provided 25.5 percent of the annual SSL. The four storms 

yielding the greatest SSL (storms beginning on March 19, 

March 7, February 1, and February 13, 2008, in order of load) 

provided 65.6 percent of the annual SSL. Storms provided 

99.3 percent of the annual SSL; 12 of 45 storms that produced 

more than 1 ton of sediment provided 90.3 percent of the 

annual SSL. The highest cumulative SSLs for the 2008 water 

year occurred during the late winter in February and March 

(fig. 13, table 9). For the 2009 water year, the estimated 

For West Branch Brandywine Creek at Modena, the esti- annual SSL was 4,930 tons, and the estimated yields were 

90 ton/mi2

 and 33.7 tons per stream mile (table 4). The storm 

beginning December 10, 2008, provided 27.5 percent of the 

annual SSL. The four storms yielding the greatest SSL (storms 

beginning on December 10, 2008, and August 2, June 11, and 

August 9, 2009, in order of load) provided 50.8 percent of the 

annual SSL. Storms provided 98.7 percent of the annual SSL; 

17 of 39 storms that produced more than 1 ton of sediment 

provided 90.4 percent of the annual SSL. The highest cumulative SSLs for the 2009 water year occurred during December 

and August (fig. 13, table 10). Storm flows with the highest 

peak discharge generally carried the highest SSL (R2 = 0.67) 

(fig. 14).
Estimation of Suspended-Sediment Loads and Yields 19

Table 7. Daily sediment load for West Branch Brandywine Creek near Honey Brook, Pennsylvania, 2008 water year.[Loads given in tons; --, not applicable]

Day October November December January February March April May June July August September

1 0.0 0.02 0.05 1.0 370 0.3 0.2 0.0 2.0 0.009 0.4 0.22 0.0 0.005 0.3 0.6 79.6 0.6 0.2 0.0 0.3 0.005 0.5 0.23 0.002 0.0006 33.7 0.5 1.7 0.4 0.2 0.01 0.3 0.008 0.5 0.24 0.03 0.0001 1.1 0.3 0.8 0.5 1.3 0.04 47.5 0.4 0.3 0.35 0.04 0.0 0.3 0.4 0.5 77.6 0.2 0.02 26.8 0.9 0.3 0.36 0.04 0.0 0.008 0.3 0.7 2.6 0.04 0.03 0.6 1.3 0.5 47.17 0.04 0.0009 0.0 0.4 0.8 3.8 0.03 0.04 0.6 0.1 0.3 12.08 0.04 0.0009 0.08 0.4 0.4 121 0.06 0.08 0.5 0.08 0.4 0.029 0.2 0.0 0.2 0.2 0.4 18.7 0.2 6.9 0.4 1.7 0.3 14.9

10 2.1 0.01 0.3 0.01 0.4 1.7 0.3 1.1 10.7 2.7 6.4 2.3

11 7.2 0.009 0.1 0.04 0.5 0.8 2.7 0.08 37.9 0.8 2.1 0.8

12 0.3 0.008 0.09 0.04 0.4 0.6 139 0.03 1.4 0.6 0.4 3.913 0.3 0.3 22.2 0.08 249 0.6 0.5 0.02 0.3 0.6 0.3 22.214 0.3 0.9 4.4 0.06 19.4 0.5 0.2 0.02 40.0 6.1 0.3 0.015 0.5 8.2 0.7 0.1 1.8 0.3 0.1 0.03 35.3 0.3 0.4 0.016 0.5 1.5 88.4 0.09 1.1 0.4 0.1 14.1 0.6 0.2 0.3 0.00517 0.3 0.4 5.2 0.1 0.6 0.6 0.2 0.9 0.1 0.2 0.3 0.0418 0.09 0.008 0.4 3.7 69.3 0.3 1.1 6.0 0.1 0.2 0.3 0.0519 0.5 6.8 0.3 1.8 5.8 55.3 0.8 3.0 0.1 0.3 0.3 0.0720 6.4 2.8 0.4 0.4 1.5 1,880 79.9 19.7 0.1 2.0 0.3 0.121 0.3 1.0 0.4 0.3 1.2 4.6 12.2 4.0 0.003 8.5 0.2 0.222 0.07 0.5 0.4 0.1 1.4 0.8 0.6 0.02 0.007 0.6 0.2 0.123 0.05 0.4 56.8 0.1 1.5 0.4 0.02 0.0 0.0 20.6 0.2 0.0524 0.06 0.6 32.9 0.06 1.8 0.1 0.002 0.02 0.003 76.3 0.3 0.0525 0.3 0.5 1.0 0.03 2.6 0.05 0.007 0.1 0.06 1.0 0.3 0.0326 2.6 34.3 1.3 0.03 34.5 0.003 0.02 0.0 0.2 0.7 0.3 0.0527 116 27.0 0.5 0.01 47.0 0.005 0.8 0.7 0.3 123 0.2 0.128 3.3 0.3 0.2 0.01 1.1 0.005 60.9 1.3 0.2 31.3 0.2 0.229 0.2 0.08 11.2 0.03 0.4 0.02 14.4 0.2 0.05 0.6 0.5 0.130 0.05 0.05 1.0 0.2 -- 0.1 0.1 0.08 0.05 0.7 1.0 0.0931 0.03 -- 12.0 0.06 -- 0.05 -- 5.7 -- 0.8 0.3 --

Monthly total 142 85.7 276 11.5 896 2,170 316 64.2 206 283 18.6 106
20 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Table 8. Daily sediment load for West Branch Brandywine Creek near Honey Brook, Pennsylvania, 2009 water year. [Loads given in tons; --, not applicable] Day October November December January February March April May June July August September

1 0.2 0.007 5.9 0.4 0.7 0.04 0.1 1.4 0.1 2.6 1.9 0.012 0.1 0.0 1.7 0.1 38.4 0.2 0.5 2.5 0.1 0.5 254 0.03 0.05 0.002 0.8 0.2 5.0 0.01 130 2.5 4.8 0.3 4.0 0.04 0.03 0.007 0.6 0.1 0.4 0.001 13.3 65.4 57.3 0.2 0.08 0.05 0.04 0.1 0.6 0.1 0.2 0.03 0.9 15.8 22.6 0.3 0.06 0.06 0.07 0.03 0.5 0.4 0.1 0.4 0.2 4.7 4.7 0.4 0.05 0.0017 0.1 0.0 0.5 45.4 24.3 0.5 0.09 93.8 0.07 0.1 0.1 0.08 0.1 0.01 0.6 5.5 12.9 0.4 0.01 5.8 0.01 0.1 0.3 0.0019 0.1 0.1 0.6 0.2 1.5 0.4 0.05 1.0 21.3 0.2 48.7 0.0

10 0.1 0.3 2.6 0.1 0.6 0.3 0.2 0.3 2.7 0.2 8.9 0.03

11 0.03 0.5 87.4 0.1 0.9 0.3 2.1 0.4 0.4 0.2 0.1 29.5

12 0.01 0.3 301 0.1 1.3 0.3 0.8 0.1 0.3 45.0 3.6 3.913 0.08 0.5 2.7 0.1 0.6 0.2 0.3 0.08 74.5 0.8 18.3 0.514 0.04 1.3 0.6 0.2 0.5 0.2 2.2 0.9 131 0.5 0.3 0.215 0.01 33.3 0.2 0.1 0.4 0.2 3.8 26.3 0.6 0.6 0.1 0.316 0.02 18.5 1.0 0.07 0.4 0.2 1.6 3.6 0.1 0.7 0.2 0.417 0.02 1.1 14.2 0.08 0.3 0.3 0.6 1.6 0.03 0.4 0.2 0.318 0.0 0.2 2.7 0.04 0.3 0.3 0.9 0.1 2.3 0.3 0.6 0.419 0.01 0.1 105 0.06 0.4 0.2 1.0 0.2 1.3 0.1 4.7 0.120 0.02 0.2 25.4 0.1 0.2 0.2 0.8 0.2 49.8 0.3 0.3 0.221 0.02 0.2 3.6 0.03 0.1 0.1 30.8 0.3 4.9 0.1 9.1 0.222 0.01 0.2 0.4 0.03 0.08 0.1 2.7 0.3 0.3 0.2 3.8 0.0923 0.02 0.2 0.2 0.02 0.1 0.05 0.5 0.3 0.3 0.3 0.4 0.0224 0.008 0.2 20.6 0.05 0.09 0.03 0.4 1.2 0.1 0.6 0.4 1.025 4.6 1.8 7.1 0.04 0.07 0.006 0.4 1.0 0.2 0.6 0.04 0.00326 2.6 0.7 0.8 0.02 0.1 1.0 0.5 0.3 0.2 1.2 0.02 0.027 0.002 0.3 0.5 0.01 0.09 1.3 0.5 0.2 0.2 2.8 0.08 9.028 9.5 0.3 0.6 0.1 0.2 0.05 0.5 0.2 0.03 124 2.6 1.029 1.3 0.3 0.4 0.2 -- 0.03 0.5 9.1 0.02 14.8 36.3 0.830 0.2 1.6 0.2 0.02 -- 0.05 0.3 1.9 1.7 1.7 4.3 0.731 0.03 -- 0.2 0.0 -- 0.1 -- 0.2 -- 2.7 0.08 --

Monthly total 19.4 62.4 589 54.0 90.2 7.5 197 242 382 203 404 48.7
Estimation of Suspended-Sediment Loads and Yields 21

Suspended-sediment load, in tons

10/1/07 11/1/07 12/1/07 1/1/08 2/1/08 3/1/08 4/1/08 5/1/08 6/1/08 7/1/08 8/1/08 9/1/08 10/1/08

Water year 2008

Water year 2009

Suspended-sediment load, in tons

10/1/08 11/1/08 12/1/08 1/1/09 2/1/09 3/1/09 4/1/09 5/1/09 6/1/09 7/1/09 8/1/09 9/1/09 10/1/09

0

200

400

600

1,300

100

300

500

1,400

0

400

800

1,200

1,600

200

600

1,000

1,400

1,800

2,000

Figure 13. Suspended-sediment loads for storms at West Branch Brandywine Creek at Modena, Pennsylvania, 2008 and 2009 

water years.
22 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

Peak discharge, in cubic feet per second

Suspended-sediment load, in tons

R 2 = 0.67

0

500

1,000

1,500

2,000

0 500 1,000 1,500 2,000

Pennsylvania, 2008 and 2009 water years.

Figure 14. Relation of storm suspended-sediment load to peak discharge at West Branch Brandywine Creek at Modena, 

For the drainage area between stations West Branch 

Brandywine Creek near Honey Brook and West Branch 

Brandywine Creek at Modena (drainage area of 36.6 mi2), the 

estimated annual SSL was 2,900 tons, and the estimated yields 

were 79.9 ton/mi2 and 29.8 tons per stream mile for the 2008 

water year. For the 2009 water year, the estimated annual SSL

was 2,630 tons, and the estimated yields were 72.5 ton/mi2 and 

27.1 tons per stream mile (table 4).

For East Branch Brandywine Creek below Downingtown, 

the estimated annual SSL was 8,990 tons, and the estimated 

yields were 100 ton/mi2

 and 34.5 tons per stream mile for 

the 2008 water year (table 4). The storm beginning February 13, 2008, provided 13.4 percent of the annual SSL. The 

four storms yielding the greatest SSL (storms beginning on 

February 13, February 1, March 19, and July 23, 2008, in 

order of load) provided 48 percent of the annual SSL. Storms 

provided 96.5 percent of the annual SSL; 19 of 51 storms that 

produced more than 1 ton of sediment provided 90.2 percent 

of the annual SSL. The highest cumulative SSLs for the 2008 

water year occurred during late winter in February and March 

(fig. 15, table 11). For the 2009 water year, the estimated 

annual SSL was 7,590 tons, and the estimated yields were 

84 ton/mi2 and 29.1 tons per stream mile (table 4). The storm 

beginning December 11, 2008, provided 14.5 percent of the 

annual SSL. The four storms yielding the greatest SSL (storms 

beginning on December 11, 2008, and August 12, August 2, 

and June 9, 2009, in order of load) provided 50.1 percent of 

the annual SSL. Storms provided 96.4 percent of the annual 

SSL; 19 of 47 storms that produced more than 1 ton of sediment provided 90.5 percent of the annual SSL. The highest 

cumulative SSLs for the 2009 water year occurred during 

December, June, and August (fig. 15, table 12). Storm flows 

with the highest peak discharges generally carried the highest 

SSL (R2

 = 0.75) (fig. 16).
Estimation of Suspended-Sediment Loads and Yields 23

Table 9. Daily sediment load for West Branch Brandywine Creek at Modena, Pennsylvania, 2008 water year.[Loads given in tons; --, not applicable]

Day October November December January February March April May June July August September

1 0.0 0.0 0.0 4.3 504 4.2 0.6 0.04 0.20 0.3 0.1 0.062 0.0 0.0 0.4 1.1 479 2.7 0.0 0 0.0 0.3 0.06 0.053 0.0 0.0 11.5 0.4 12.5 2.1 0.3 0 0.0 0.4 0.05 0.084 0.0 0.0 4.12 0.3 1.9 2.1 1.5 0 1.0 0.02 0.02 0.045 0.1 0.0 0.2 0.2 0.02 297 0.0 0 14.2 0.2 0.004 0.036 0.0 0.0 0.0 0.03 0.0 148 0.0 0 3.2 0.3 0.008 63.97 0.0 0.0 0.01 0.007 0.0 56.5 0.0 0 0.07 0.4 0.006 42.08 0.0 0.0 0.05 0.0 0.0 568 0.0 0.01 0.0 0.6 0.008 1.19 20.3 0.0 0.06 0.1 0.0 718 0.0 12.2 0.0 10.3 0.005 4.6

10 2.1 0.0 0.1 0.0 0.002 69.2 0.0 3.5 5.9 2.4 13.5 2.5

11 16.0 0.0 0.0 0.5 0.04 13.5 1.4 0.2 7.4 0.3 2.1 0.4

12 0.3 0.0 0.0 0.0 0.0 1.3 123 0.02 1.7 0.1 0.1 2.713 0.0 0.1 4.2 0.0 466 0.2 8.1 0 0.4 0.02 0.1 10.714 0.0 0.0 21.3 0.0 115 0.04 0.5 0 24.3 4.1 0.7 1.515 0.0 4.3 1.8 0.0 4.3 0.1 0.2 0 49.2 1.3 0.7 0.00916 0.0 5.4 167 0.0 1.4 0.4 0.5 16.6 4.3 0.4 0.2 0.0617 0.0 1.4 40.2 0.01 0.7 0.3 1.1 5.9 1.2 0.5 0.1 0.118 0.0 0.1 2.4 0.7 21.0 2.3 0.06 5.7 0.4 1.0 0.1 0.0619 5.2 1.0 0.04 0.02 21.4 20.6 0.0 4.1 0.3 1.1 0.2 0.0920 4.5 2.1 0.0 0.001 3.7 1,850 5.4 7.1 0.2 5.4 0.1 0.0521 0.1 0.4 0.0 0.0 3.8 34.7 56.8 8.7 0.07 14.8 0.03 0.0722 0.0 0.6 0.0 0.0 2.8 3.3 4.1 1.2 0.08 2.5 0.06 0.0723 0.0 0.1 7.6 0.0 1.9 0.6 0.005 0 0.01 93.2 0.03 0.0324 0.04 0.1 52.2 0.0 1.8 0.03 0.03 0 0.01 123 0.05 0.0125 0.1 0.3 8.1 0.0 2.0 0.0 0.005 0 0.01 4.2 0.08 0.00726 3.0 18.7 3.5 0.0 4.8 0.05 0.03 0 0.006 0.8 0.03 0.0627 391 25.3 1.9 0.0 23.0 0.1 0.2 3.7 3.2 3.3 0.04 0.128 30.4 1.1 1.2 0.1 6.7 0.2 76.2 1.5 0.51 66.9 0.05 1.929 1.2 0.0 6.3 0.02 4.1 0.6 137 0 0.08 3.0 1.2 0.230 0.0 0.0 4.7 0.5 -- 1.0 2.6 0 0.08 0.5 0.4 0.0531 0.0 -- 12.5 0.0 -- 0.6 -- 0.2 -- 0.3 0.08 --

Monthly total 474 61.0 351 8.29 1,680 3,800 420 70.7 118 342 20.2 133
24 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Table 10. Daily sediment load for West Branch Brandywine Creek at Modena, Pennsylvania, 2009 water year. [Loads given in tons; --, not applicable] Day October November December January February March April May June July August September

1 0.04 0.001 2.2 0.18 0.0 0.09 0.2 0.04 0.1 0.8 6.7 0.22 0.01 0.0002 1.0 0.02 0.3 0.0 0.06 0.4 0.5 0.2 445 0.03 0.005 0.0 0.0 0.009 21.8 0.07 109 1.3 3.1 0.4 67.4 0.04 0.01 0.0 0.0 0.0 2.5 0.03 38.6 71.5 59.7 0.08 5.0 0.05 0.0 0.005 0.0 0.2 0.1 0.002 3.7 84.9 54.9 0.07 0.3 0.06 0.008 0.03 0.005 0.08 0.0 0.05 0.9 10.8 35.5 0.05 0.5 0.07 0.01 0.004 0.08 30.9 0.01 0.2 0.006 234 2.6 0.04 0.4 0.038 0.0004 0.02 0.03 18.9 17.8 2.2 0.0 11.9 1.0 0.04 0.5 0.09 0.0 0.003 0.0 3.3 3.9 0.1 0.0 1.8 203 0.08 127 0.0

10 0.003 0.0002 0.0 1.9 0.2 0.0 0.0 0.5 30.6 0.05 139 0.0

11 0.0 0.009 118 1.5 0.03 0.09 4.6 0.06 5.5 0.2 7.8 72.6

12 0.001 0.01 1,220 1.0 0.4 0.2 0.7 0.1 2.9 21.8 2.8 23.713 0.001 1.8 15.8 1.3 0.4 0.0003 0.07 0.08 79.6 2.8 56.0 1.614 0.0 0.2 1.3 1.5 0.4 0.0006 1.3 5.2 256 1.2 3.1 0.00215 0.01 15.5 0.08 1.2 0.8 0.0 3.2 20.7 9.3 1.3 0.3 0.016 0.0 19.3 1.2 1.0 0.5 0.0 0.8 26.6 2.0 0.3 0.03 0.017 0.07 3.0 7.4 0.4 0.3 0.0008 0.004 9.8 1.1 1.6 0.003 0.018 0.1 0.6 5.9 0.06 0.1 0.007 0.0 0.6 11.0 1.0 0.009 0.019 0.03 0.009 47.9 0.04 0.01 0.2 0.01 0.1 6.3 0.4 0.02 0.00520 0.01 0.0 115 0.0 0.0 0.1 0.7 0.04 49.1 0.5 0.0 0.021 0.02 0.0 3.5 0.0 0.0 0.005 1.6 0.004 33.0 1.1 193 0.022 0.01 0.0 0.2 0.3 0.0 0.006 4.5 0.005 3.3 0.4 57.1 0.00423 0.01 0.001 0.0 0.0 0.0 0.03 0.08 0.02 1.1 5.5 2.8 0.024 0.002 0.04 1.8 0.0 0.0 0.06 0.0004 0.02 0.6 3.9 0.3 13.525 17.2 1.4 16.8 0.0 0.002 0.0 0.001 0.4 0.3 1.2 0.2 0.0226 1.8 0.2 3.3 0.0 0.08 0.8 0.0004 0.09 0.4 0.9 0.0 0.027 0.07 0.2 1.5 0.0 0.2 0.6 0.0 0.01 0.05 3.2 0.1 5.628 10.5 0.3 1.4 1.2 0.06 0.1 0.0 0.03 0.2 96.6 1.0 0.929 3.9 0.4 1.3 0.9 -- 0.3 0.0 40.8 0.004 9.4 83.1 0.0330 0.3 1.2 0.9 0.0 -- 0.1 0.0 4.8 0.04 8.1 5.1 0.031 0.01 -- 0.5 0.0 -- 0.04 -- 0.6 -- 127 0.6 --

Monthly total 34.1 44.2 1,570 65.9 49.9 5.38 170 527 853 290 1,210 118
Estimation of Suspended-Sediment Loads and Yields 25

Suspended-sediment load, in tons

10/1/07 11/1/07 12/1/07 1/1/08 2/1/08 3/1/08 4/1/08 5/1/08 6/1/08 7/1/08 8/1/08 9/1/08 10/1/08

Water year 2008

Water year 2009

Suspended-sediment load, in tons

10/1/08 11/1/08 12/1/08 1/1/09 2/1/09 3/1/09 4/1/09 5/1/09 6/1/09 7/1/09 8/1/09 9/1/09 10/1/09

0

400

800

200

600

1,000

1,200

0

400

800

200

600

1,000

1,200

Figure 15. Suspended-sediment loads for storms at East Branch Brandywine Creek below Downingtown, Pennsylvania, 2008 and 

2009 water years.
26 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

Peak discharge, in cubic feet per second

Suspended-sediment load, in tons

R 2 = 0.75

0

300

600

900

1,200

1,500

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

Figure 16. Relation of storm suspended-sediment load to peak discharge at East Branch Brandywine Creek below 

Downingtown, Pennsylvania, 2008 and 2009 water years.

For the 2008 and 2009 water years, East Branch Brandywine Creek below Downingtown had the highest estimated 

annual SSL, whereas West Branch Brandywine Creek near 

Honey Brook had the highest estimated annual suspendedsediment yield (table 4). West Branch Brandywine Creek at 

Modena had the second highest estimated annual suspendedsediment yield. The high suspended-sediment yield, in part, 

may be the result of the relatively high SSL contributed by 

the upstream Honey Brook subbasin. For the 2008 and 2009 

water years, French Creek near Phoenixville had the lowest 

suspended-sediment yields. For the 2008 water year, French 

Creek had the lowest annual SSL, and for the 2009 water year, 

West Branch Brandywine Creek near Honey Brook had the 

lowest annual SSL. 

For all four basins, storms provided more than 96 percent 

of the annual SSL. Generally, in each basin four storms provided over half the annual SSL each year. The SSLs in 2008, 

in general, were highest in the winter months of February 

and March, whereas in 2009, SSLs were highest in December 

and August.
Estimation of Suspended-Sediment Loads and Yields 27

Table 11. Daily sediment load for East Branch Brandywine Creek below Downingtown, Pennsylvania, 2008 water year.[Loads given in tons; --, not applicable]

Day October November December January February March April May June July August September

1 0.2 1.2 1.2 5.3 731 3.7 3.4 4.4 3.6 0.8 1.0 0.32 0.3 0.7 1.4 1.7 398 3.3 2.7 3.6 1.6 0.7 0.8 0.23 0.3 0.5 33.1 1.1 15.4 1.9 1.6 2.9 1.3 0.7 0.6 0.24 0.2 0.4 4.4 1.0 7.2 2.0 4.6 2.4 16.5 0.9 0.6 0.25 0.2 0.5 1.5 1.0 7.6 77.2 3.1 2.0 5.0 1.0 0.5 0.26 0.3 0.5 1.2 0.7 3.8 13.0 2.0 1.9 1.9 0.8 0.5 2637 0.3 0.4 1.2 0.6 4.1 6.9 1.6 1.9 1.7 8.2 0.3 67.38 0.3 0.3 1.4 0.5 3.1 464 1.6 2.2 1.7 2.0 0.3 2.59 1.0 0.3 1.7 0.5 2.7 96.2 3.3 50.2 1.5 7.2 0.2 10.3

10 4.8 0.3 1.6 0.4 2.9 15.3 1.8 20.9 2.1 9.3 17.9 4.2

11 25.6 0.6 25.6 1.0 2.6 7.7 2.1 5.8 8.0 1.4 7.2 1.0

12 2.9 0.6 5.0 0.6 2.7 5.5 413 4.0 2.0 1.0 1.0 4.913 0.7 1.5 6.6 0.5 1,030 4.7 10.9 3.3 1.4 0.9 0.6 28.314 0.6 1.0 10.2 0.7 146 3.8 3.3 2.3 1.8 6.9 5.4 1.815 0.5 7.5 2.6 0.4 14.1 3.7 2.3 2.6 359 2.0 0.6 0.916 0.4 3.2 336 0.7 8.1 3.6 1.8 54.6 7.1 0.9 0.3 0.417 0.3 1.8 39.7 1.4 6.0 2.6 1.7 18.0 3.2 0.8 0.3 0.718 0.3 1.0 7.0 4.4 39.5 2.1 1.9 12.7 2.1 0.7 0.3 0.319 3.9 8.5 3.7 5.9 21.8 53.5 1.8 23.8 2.0 0.6 0.3 0.320 25.8 4.4 3.3 2.4 9.0 871 3.3 36.9 1.6 0.7 0.3 0.321 1.5 2.1 1.7 1.5 3.6 83.0 3.5 22.6 1.7 4.0 0.3 0.322 0.7 1.9 1.5 1.4 4.2 13.5 2.0 5.7 1.6 0.8 0.4 0.323 0.5 1.6 11.9 1.2 4.4 7.9 2.0 3.4 1.5 102 0.5 0.224 0.5 1.0 80.4 0.7 4.1 6.3 2.1 2.9 1.2 815 0.8 0.225 0.8 1.1 9.7 0.7 4.1 5.0 2.0 2.7 1.5 10.3 0.5 0.226 5.3 14.4 7.1 0.7 5.9 4.2 1.9 2.2 1.4 4.3 0.4 0.327 686 26.7 17.1 0.9 23.4 3.7 4.5 4.9 5.0 3.3 0.4 0.328 23.6 2.7 4.4 0.7 6.2 3.6 232 11.3 2.7 5.3 0.2 0.629 4.9 1.6 17.0 0.6 3.8 3.9 140 1.6 1.3 2.8 0.4 0.630 3.2 1.2 7.0 0.9 -- 3.1 7.8 1.5 34.2 1.5 0.5 0.331 2.1 -- 24.8 0.8 -- 2.7 -- 5.9 -- 1.5 0.4 --

Monthly total 798 89.5 671 40.9 2,520 1,780 866 321 477 998 43.8 391
28 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09 Table 12. Daily sediment load for East Branch Brandywine Creek below Downingtown, Pennsylvania, 2009 water year. [Loads given in tons; --, not applicable] Day October November December January February March April May June July August September

1 0.3 0.3 8.6 1.5 0.8 0.5 0.5 0.9 0.5 2.9 47.5 1.52 0.5 0.3 2.3 1.6 2.0 0.6 0.6 1.7 0.5 1.5 797 1.13 0.2 0.3 1.3 2.0 4.8 0.7 117 2.0 4.0 0.7 60.2 0.94 0.2 0.3 1.1 1.3 1.7 1.1 26.8 77.9 42.2 0.8 7.4 0.95 0.2 7.9 0.9 0.8 0.8 1.1 2.5 74.5 44.9 0.6 4.4 0.86 0.2 0.3 0.3 0.8 0.8 1.5 1.4 7.8 31.4 0.6 19.8 0.77 0.2 0.3 0.3 57.9 0.8 2.1 1.1 407 4.2 0.8 3.1 0.88 0.5 0.2 0.5 15.7 7.1 1.3 0.7 14.6 3.2 0.9 1.3 0.79 0.5 0.3 0.5 2.5 4.8 1.8 0.6 5.6 617 1.2 272 0.8

10 0.3 0.3 0.8 1.1 1.3 0.8 0.6 3.7 180 2.8 29.5 0.9

11 0.2 0.2 51.1 1.2 1.1 1.1 4.9 2.6 16.4 1.4 2.5 152

12 0.3 0.2 1,030 1.4 1.3 0.8 2.5 2.3 16.1 295 1.4 28.913 0.3 1.0 18.1 1.3 1.1 0.7 0.8 2.1 152 5.9 980 5.414 0.3 0.7 4.9 1.4 0.9 1.0 2.1 2.4 315 2.6 9.7 3.015 0.2 22.2 3.2 1.4 0.8 0.8 6.1 42.8 22.5 1.9 5.1 2.416 0.2 8.7 3.2 1.6 0.7 0.2 3.6 3.5 13.8 1.3 3.3 1.817 0.2 1.9 15.0 1.9 0.9 0.2 1.6 8.8 11.9 6.2 2.2 1.618 0.2 0.8 8.3 1.2 1.4 0.4 1.3 2.1 18.1 5.5 1.7 1.819 0.2 0.7 104 1.3 1.5 0.6 1.1 1.4 12.6 1.7 2.2 2.520 0.2 0.8 67.5 1.3 0.9 1.1 1.6 1.2 33.0 2.3 0.7 2.021 0.2 1.4 6.0 0.8 0.9 0.9 2.3 3.9 33.4 2.0 28.2 1.622 0.2 1.2 3.8 0.7 0.9 0.8 2.3 0.5 8.7 1.9 207 1.623 0.2 1.2 2.8 0.7 1.1 0.9 1.2 0.4 7.1 16.0 3.6 0.624 0.2 1.1 4.5 0.6 1.0 0.7 0.8 0.4 5.6 6.4 1.3 17.025 3.9 1.6 13.0 0.6 1.1 0.5 0.7 0.8 3.3 3.1 0.9 1.126 4.6 1.2 3.4 0.5 1.0 0.4 0.7 0.5 2.6 11.4 0.8 0.527 0.4 1.1 2.5 0.5 0.6 1.1 0.6 0.5 2.7 37.5 1.6 12.328 16.5 1.1 2.4 1.7 0.5 0.5 0.6 0.3 1.9 9.4 4.8 2.329 5.0 1.0 1.7 1.8 -- 0.8 0.6 26.8 1.8 8.5 15.9 0.730 0.9 2.6 1.5 0.8 -- 1.0 0.6 2.5 1.7 13.2 15.3 0.431 0.4 -- 1.4 0.7 -- 0.5 -- 0.7 -- 219 3.0 --

Monthly total 37.9 61.2 1,360 109 42.6 26.5 188 702 1,610 665 2,540 249
Summary and Conclusions 29

Limitations of the Investigation

Although the use of turbidity data to estimate SSC

provides better estimates than the traditional discharge-based 

approach, the method is not without limitations. The limitations range from data-collection challenges to limitations in 

data analysis. Consideration must be given to site-specific 

conditions to ensure the turbidity sensors are adequately 

protected from high water, high velocities, and any associated 

debris, while maintaining acceptable in-stream performance. 

Optical turbidity sensors are susceptible to fouling by biological growth, siltation, and entangled debris. Turbidity sensors 

need to be calibrated frequently to prevent equipment drift, 

and equipment fouling can result in the loss of data. Turbidity 

values for periods of lost record were estimated.

The relations of SSC to turbidity described in this report 

are site specific and may be applicable only to the 2008–09 

water years. Differences in basin and land-use characteristics 

produce different SSC to turbidity relations, and, therefore, the 

relations are different for each station. The regression equations describe the SSC to turbidity relations for water years 

2008–09. Additional SSC and turbidity data collected for a 

particular station may change the relations. 

Summary and Conclusions

In-stream continuous turbidity, which was used as a 

surrogate for SSC, and discharge data, in conjunction with 

measured SSC data, were used to compute a time-series of 

SSC and SSL at four stream stations in Chester County, Pa. 

Turbidity and SSC data were collected by the USGS at the four 

stream stations in cooperation with the Chester County Water 

Resources Authority and the Chester County Health Department with the intent of developing time-series SSC estimates. 

Turbidity and SSC data were collected at French Creek near 

Phoenixville, West Branch Brandywine Creek near Honey 

Brook, West Branch Brandywine Creek at Modena, and East 

Branch Brandywine Creek below Downingtown. Data collection began in April 2007.

Turbidity sensors were deployed at the four stations, and 

data were recorded at 15- or 30-minute intervals by a data 

logger and uploaded every 1 to 4 hours to the USGS database. 

Data from the in-situ sensors were compared with annual 

cross-sectional data to verify that the in-situ sensors were 

installed in representative locations in the stream. No bias was 

noted for any of the stations. Missing data occurred because 

of spikes, servicing of the sensor, or fouling. If field measured 

values were available, they were used to replace missing 

data. When the period of time between missing values was 

small, interpolation between measured data points was used 

to estimate missing values. When there were large gaps in the 

record, data from nearby turbidity sensors and discharge and 

rainfall data were used to estimate missing values. 

Most of the suspended-sediment samples were collected 

using automated samplers. Of the 24 samples collected for a 

storm, 1 to 6 samples were selected for laboratory analysis. 

These selections were made to minimize potential bias from 

serial correlation and reduce the effect of any one event on the 

overall relation between turbidity and SSC. 

Regression models were developed between SSC and 

turbidity (T) for each station using SSC sample data collected 

from the automated samplers and turbidity data collected at 

each station. Six regression models were initially developed 

for each station: (1) SSC in relation to T, (2) SSC to Q, (3) 

SSC to T and Q, (4) log10SSC to log10T, (5) log10SSC to log10Q, 

and (6) log10SSC to log10T and log10Q. Statistics, including 

coefficient of determination (R2), adjusted R2, standard error, 

prediction error sum of squares, Mallow’s Cp, variance inflation factor, probability plot correlation coefficient, and model 

standard percentage error, were used to evaluate the models. 

Uncertainty of regression estimates were determined by 

90-percent prediction intervals. 

Instantaneous SSL was computed from time-series turbidity and discharge data for the 2008 and 2009 water years 

using the regression equations developed for each station. The 

instantaneous computations of SSL were summed to provide 

daily, storm, and annual loads. The annual SSL contributed 

from each subbasin was divided by the upstream drainage area 

to estimate the annual sediment yield. 

Storms provided more than 96 percent of the annual 

SSL for all four basins. In each basin, four storms generally 

provided more than one-half the annual SSL each year. Storm 

flows with the highest peak discharges generally carried the 

highest SSLs. For all stations the greatest SSL occurred during 

the late winter in February and March during the 2008 water 

year. During the 2009 water year, the greatest SSL occurred 

during December and August.

For French Creek near Phoenixville, a linear regression model using turbidity as the explanatory variable was 

selected. Turbidity explained 96 percent of the variability in 

SSC values. The estimated annual SSL was 3,500 tons, and the 

estimated yields were 59.1 ton/mi2 and 20 tons per stream mile 

for the 2008 water year. The storm beginning March 19, 2008, 

provided 20.4 percent of the annual SSL. The four storms that 

yielded the greatest SSLs provided 54 percent of the annual 

SSL. For the 2009 water year, the estimated annual SSL was 

4,390 tons, and the estimated yields were 74.3 ton/mi2 and 

25.1 tons per stream mile. The storm beginning December 

10, 2008, provided 23.2 percent of the annual SSL. The four 

storms that yielded the greatest SSLs provided 56.1 percent of 

the annual SSL. 

For West Branch Brandywine Creek near Honey Brook, 

the correlations among SSC, T, and discharge were lower than 

for the other stations. A linear regression model using turbidity 

as the explanatory variable was selected. Turbidity explained 

74 percent of the variability in SSC values. The estimated 

annual SSL was 4,580 tons, and the estimated yields were 

245 ton/mi2 and 93.5 tons per stream mile for the 2008 water 
30 Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09

year. The storm beginning March 19, 2008, provided 42.4 percent of the annual SSL. The four storms that yielded the greatest SSLs provided 61.6 percent of the annual SSL. For the 2009 

water year, the estimated annual SSL was 2,300 tons, and the 

estimated yields were 123 ton/mi2 and 46.9 tons per stream 

mile. The storm beginning December 10, 2008, provided 

17.1 percent of the annual SSL. The four storms that yielded 

the greatest SSLs provided 43.6 percent of the annual SSL.

For West Branch Brandywine Creek at Modena, a linear 

regression model using turbidity as the explanatory variable 

was selected. Turbidity explained 94 percent of the variability 

in SSC values. The estimated annual SSL was 7,480 tons, and 

the estimated yields were 136 ton/mi2 and 51.2 tons per stream 

mile for the 2008 water year. The storm beginning March 19, 

2008, provided 25.5 percent of the annual SSL. The four 

storms that yielded the greatest SSLs provided 65.6 percent of 

the annual SSL. For the 2009 water year, the estimated annual 

SSL was 4,930 tons, and the estimated yields were 90 ton/mi2

and 33.7 tons per stream mile. The storm beginning December 10, 2008, provided 27.5 percent of the annual SSL. The 

four storms that yielded the greatest SSLs provided 50.8 percent of the annual SSL. 

For East Branch Brandywine Creek below Downingtown, 

a linear regression model using a logarithmic base-10 transformation of both the response and explanatory variables was 

selected. Turbidity explained 94 percent of the variability in 

SSC values. The estimated annual SSL was 8,990 tons, and the 

estimated yields were 100 ton/mi2 and 34.5 tons per stream 

mile for the 2008 water year. The storm beginning February 13, 2008, provided 13.4 percent of the annual SSL. The 

four storms that yielded the greatest SSLs provided 48 percent 

of the annual SSL. For the 2009 water year, the estimated 

annual SSL was 7,590 tons, and the estimated yields were 

84 ton/mi2 and 29.1 tons per stream mile. The storm beginning December 11, 2008, provided 14.5 percent of the annual 

SSL. The four storms that yielded the greatest SSLs provided 

50.1 percent of the annual SSL. 

For the 2008 and 2009 water years, East Branch Brandywine Creek below Downingtown had the highest estimated 

annual SSL, and West Branch Brandywine Creek near Honey 

Brook had the highest estimated annual suspended-sediment 

yield. West Branch Brandywine Creek at Modena had the second highest estimated annual suspended-sediment yield. The 

high suspended-sediment yield could be partly the result of the 

high SSL contributed by the upstream Honey Brook subbasin. 

French Creek had the lowest suspended-sediment yields for 

the 2008 and 2009 water years. French Creek had the lowest 

annual SSL for the 2008 water year, and West Branch Brandywine Creek near Honey Brook had the lowest annual SSL for 

the 2009 water year. 

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For more information about this publication, contact:

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

http://pa.water.usgs.gov/

Document prepared by the West Trenton Publishing Service Center
Sloto and Olson—Estimated Suspended-Sediment Loads and Yields in Chester County, Pennsylvania, Water Years 2008–09—Scientific Investigations Report 2011–5109