U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5141

Watershed Characteristics and Water-Quality Trends and 

Loads in 12 Watersheds in Gwinnett County, Georgia
Cover photographs. Foreground left, staff gage and pipe. The staff gage is an outside measuring point used to confirm/compare

with the electronic recording equipment. The water-quality monitor and tubing for the automatic sampler are inside the pipe 

(by Jonathan Evans, USGS).

Foreground right, gage house and shelter. Inside the metal gage house (left) are the DCP, bubbler, and wiring and recording devices for 

measuring stage, rainfall, water temperature, specific conductance, and turbidity. The blue shelter (right) is the housing for the electrical 

components for the gaging station. The refrigerated automatic sampler and controller are inside the shelter (by Jonathan Evans, USGS). 

Background, banks and channel of Crooked Creek near Norcross, Georgia (background and sign photographs by Kerry Caslow, USGS).
Watershed Characteristics and Water-Quality 

Trends and Loads in 12 Watersheds in 

Gwinnett County, Georgia

By John K. Joiner, Brent T. Aulenbach, and Mark N. Landers

Prepared in cooperation with Gwinnett County Department of Water Resources

Scientific Investigations Report 2014–5141

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Joiner, J.K., Aulenbach, B.T., and Landers, M.N., 2014, Watershed characteristics and water-quality trends and loads 

in 12 watersheds in Gwinnett County, Georgia: U.S. Geological Survey Scientific Investigations Report 2014–5141, 

79 p., http://dx.doi.org/10.3133/sir20145141.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3815-9
iii

Contents

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

Introduction.....................................................................................................................................................2

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

Methods...........................................................................................................................................................4

Surface-Water Monitoring..................................................................................................................4

Surface-Water Constituent Concentration Seasonality and Trends............................................7

Stream Water Constituent Load Estimation .....................................................................................7

Total Suspended Solids Load Estimation Error................................................................................9

Watershed Characteristics ..........................................................................................................................9

Watershed Basin Slope .......................................................................................................................9

Population ............................................................................................................................................12

Land Use and Changes in Impervious Area ...................................................................................16

Hydrologic Inputs and Outputs..................................................................................................................18

Climate and Precipitation ..................................................................................................................18

Annual Runoff and Water Yields ......................................................................................................21

Surface-Water Quality ................................................................................................................................23

Continuous Water-Quality Monitoring.............................................................................................23

Base-flow and Stormflow Water Quality ........................................................................................25

Water-Quality Seasonality and Long-Term Trends ................................................................................29

Seasonality in Water Quality.............................................................................................................29

Long-Term Trends in Water Quality .................................................................................................41

Stream Water Loads and Yields ................................................................................................................52

Nutrients...............................................................................................................................................61

Trace Elements....................................................................................................................................62

Total Dissolved Solids ........................................................................................................................64

Suspended Sediment .........................................................................................................................64

Interpreting Aggregate Effects on Water Quality...................................................................................67

Summary........................................................................................................................................................67

Acknowledgments.......................................................................................................................................69

References Cited..........................................................................................................................................69

Appendix........................................................................................................................................................73

Figures

1. Map showing location of the study area showing the 12 monitored watersheds 

and the monitoring stations, Gwinnett County, Georgia.........................................................3

2. Photographs showing multiparameter monitoring station at North Fork Peachtree 

Creek at Graves Road, near Doraville, Georgia.......................................................................5

3. Map showing land-surface altitude for Gwinnett County, Georgia ...................................10

4. Map showing land-surface slope for Gwinnett County, Georgia .......................................11

5. Graph showing average basin slope of the 12 monitored watersheds in Gwinnett 

County, Georgia...........................................................................................................................12

6. Map showing population density for Gwinnett County, Georgia, 2009..............................13
iv

7–12. Graphs showing—

7. Population density for the 12 monitored watersheds in Gwinnett County, 

Georgia, 2009...................................................................................................................14

8. Total population by year and population growth for Gwinnett County, 

Georgia, 1996–2013 ........................................................................................................15

9. Percentage of watershed impervious area from transportation and 

building land cover for 12 monitored watersheds in Gwinnett County, 

Georgia, 2009...................................................................................................................17

10. Watershed impervious area as related to high-density land use and 

population density for 12 monitored watersheds in Gwinnett County, 

Georgia, 2009...................................................................................................................17

11. Percentage of impervious area in 12 monitored watersheds in Gwinnett 

County, Georgia ..............................................................................................................18

12. Spatially averaged annual precipitation totals for each of the 12 monitored 

watersheds in Gwinnett County, Georgia, and the countywide average. ............19

13. Contour map showing the average annual precipitation for Gwinnett County, 

Georgia, and the 12 monitored watersheds. ..........................................................................20

14–16. Graphs showing—

14. Annual runoff totals and annual water yields for each of the 12 monitoring 

stations in Gwinnett County, Georgia .........................................................................22

15. Dilution effects of specific conductance and discharge at station 02336030, 

North Fork Peachtree Creek near Doraville, Georgia, January 5–7, 2009............23

16. Turbidity in relation to suspended-sediment concentration, total zinc, 

and total suspended solids at station 02207120 Yellow River near 

Lithonia, Georgia. ...........................................................................................................24

17. Boxplots of concentrations for specific conductance, turbidity, total nitrite plus 

nitrate, total ammonia plus organic nitrogen, total nitrogen, total phosphorus, 

dissolved phosphorus, total organic carbon, total lead, total zinc, total dissolved 

solids, total suspended solids, and, suspended sediment at 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................26

18–38. Graphs showing—

18. Concentrations of total suspended solids and suspended-sediment 

concentrations for 380 storm samples collected in watersheds in 

Gwinnett County, Georgia.............................................................................................29

19. Seasonal patterns in stormflow total nitrogen flow-adjusted 

residual concentrations for the 12 monitored watersheds 

in Gwinnett County, Georgia.........................................................................................30

20. Seasonal patterns in stormflow total phosphorus flow-adjusted 

residual concentrations for the 12 monitored watersheds 

in Gwinnett County, Georgia.........................................................................................32

21. Seasonal patterns in stormflow total dissolved solids flow-adjusted 

residual concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................35

22. Seasonal patterns in stormflow total suspended solids flow-adjusted 

residual concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................37

23. Seasonal patterns in stormflow total zinc flow-adjusted residual concentrations for the 12 monitored watersheds in Gwinnett County, Georgia...............39
v

24. Trends in stormflow total nitrogen flow- and seasonally adjusted residual 

concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................42

25. Trends in stormflow total phosphorus flow- and seasonally adjusted residual 

concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................44

26. Trends in stormflow total dissolved solids flow- and seasonally adjusted 

residual concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................46

27. Trends in stormflow total suspended solids flow- and seasonally adjusted 

residual concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................48

28. Trends in stormflow total zinc flow- and seasonally adjusted residual 

concentrations for the 12 monitored watersheds in 

Gwinnett County, Georgia.............................................................................................50

29. Total suspended solids in relation to discharge for base-flow and stormflow 

conditions for samples collected at station 02208150, Alcovy River near 

Grayson, Georgia, 1997–2009 .......................................................................................54

30. Average annual yields for total nitrite plus nitrate and total nitrogen, and 

annual runoff in Gwinnett County, Georgia, water years 2004–09.........................54

31. Average annual total nitrogen and total nitrite plus nitrate yields for 12 

monitored watersheds in Gwinnett County, Georgia, water years 2004–09 ........61

32. Average annual total and dissolved phosphorus yields for 12 monitored 

watersheds in Gwinnett County, Georgia, water years 2004–09............................62

33. Average annual total organic carbon yields for 12 monitored watersheds 

in Gwinnett County, Georgia, water years 2004–09..................................................63

34. Average annual total lead and zinc yields for 12 monitored watersheds in 

Gwinnett County, Georgia, water years 2004–09 ......................................................63

35. Average annual total dissolved solids yields for 12 monitored watersheds 

in Gwinnett County, Georgia, water years 2004–09..................................................64

36. Annual total suspended solids yield and runoff at the Alcovy River, 

Gwinnett County, Georgia, water years 2004–09 ......................................................65

37. Average annual total suspended solids yields for 12 monitored watersheds 

in Gwinnett County, Georgia, water years 2004–09..................................................65

38. Average annual total suspended solids and suspended-sediment concentration yields for 12 monitored watersheds in Gwinnett County, Georgia, 

water years 2004–09......................................................................................................66

Tables

1. Twelve U.S. Geological Survey water-quantity and water-quality monitoring 

stations included in the watershed characteristics and water-quality trends study 

in Gwinnett County, Georgia, including dates established and drainage areas for 

each station ...................................................................................................................................4

2. Water-quality constituents analyzed for samples collected in streams in 

Gwinnett County, Georgia, units of measures, and detection limit ......................................6

3. Land use and watershed characteristics for 12 watersheds in Gwinnett County, 

Georgia, 2009 ...............................................................................................................................16
vi

4. Water-quality constituent trends and statistical significance for stormflow samples 

in 12 monitored watersheds in Gwinnett County, Georgia...................................................41

5. Examples of characteristics of regression models used to compute loads of 

total zinc for the 12 monitored watersheds in Gwinnett County, Georgia.........................53

6. Water year 2004 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................55

7. Water year 2005 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................56

8. Water year 2006 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................57

9. Water year 2007 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................58

10. Water year 2008 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................59

11. Water year 2009 runoff and constituent yields for each of the 12 monitored 

watersheds in Gwinnett County, Georgia...............................................................................60

1–1. Statistical summary for selected constituents, Yellow River at Georgia 124 

near Lithonia, Georgia, station number 02207120..................................................................74

1–2. Statistical summary for selected constituents, No Business Creek at Lee Road, 

below Snellville, Georgia, station number 02207185.............................................................74

1–3. Statistical summary for selected constituents, Big Haynes Creek at Lenora Road, 

near Loganville, Georgia, station number 02207385..............................................................75

1–4. Statistical summary for selected constituents, Brushy Fork Creek at Beaver Road, 

near Loganville, Georgia, station number 02207400..............................................................75

1–5. Statistical summary for selected constituents, Alcovy River at New Hope Road, 

near Grayson, Georgia, station number 02208150.................................................................76

1–6. Statistical summary for selected constituents, Wheeler Creek at Bill Cheek Road, 

near Auburn, Georgia, station number 02217274...................................................................76

1–7. Statistical summary for selected constituents, Apalachee River at Fence Road, 

near Dacula, Georgia, station number 02218565 ...................................................................77

1–8. Statistical summary for selected constituents, Richland Creek at 

Suwanee Dam Road, near Buford, Georgia, station number 02334480 .............................77

1–9. Statistical summary for selected constituents, Level Creek at Suwanee Dam Road, 

near Suwanee, Georgia, station number 02334578...............................................................78

1–10. Statistical summary for selected constituents, Suwanee Creek at Suwanee, 

Georgia, station number 02334885 ...........................................................................................78

1–11. Statistical summary for selected constituents, Crooked Creek near Norcross, 

Georgia, station number 02335350 ...........................................................................................79

1–12. Statistical summary for selected constituents, North Fork Peachtree Creek 

at Graves Road, near Doraville, Georgia, station number 02336030 ..................................79
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

inch per hour (in/h) 0.0254 meter per hour (m/h)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
Abbreviations

AMLE adjusted maximum likelihood estimates

BMP best management practices

C carbon

DP dissolved phosphorus

EWI equal-width increment

TKN total Kjeldahl nitrogen

TP total phosphorus

LOWESS locally weighted regression and smoothing scatterplots

LTTM Long-Term Trend Monitoring

MNGWPD Metropolitan North Georgia Water Planning District

N nitrogen

NO2 + NO3 nitrite plus nitrate

NPDES National Pollutant Discharge Elimination System

P phosphorus

Pb total lead

SC specific conductance

SE standard error

SEP standard error of the prediction

SSC suspended-sediment concentration

TDS total dissolved solids

TMDL total maximum daily load

TN total nitrogen

TOC total organic carbon

TSS total suspended solids

USGS U.S. Geological Survey

WY water year

Zn total zinc

viii
Watershed Characteristics and Water-Quality Trends and 

Loads in 12 Watersheds in Gwinnett County, Georgia

By John K. Joiner, Brent T. Aulenbach, and Mark N. Landers 

Abstract

The U.S. Geological Survey, in cooperation with Gwinnett County Department of Water Resources, established a 

Long-Term Trend Monitoring (LTTM) program in 1996. The 

LTTM program is a comprehensive, long-term, water-quantity 

and water-quality monitoring program designed to document 

and analyze the hydrologic and water-quality conditions of 

selected watersheds of Gwinnett County, Georgia. Waterquality monitoring initially began in six watersheds and was 

expanded to another six watersheds in 2001. 

As part of the LTTM program, streamflow, precipitation, 

water temperature, specific conductance, and turbidity were 

measured continuously at the 12 watershed monitoring 

stations for water years1

 2004–09. In addition, discrete waterquality samples were collected seasonally from May through 

October (summer) and November through April (winter), 

including one base-flow and three stormflow event composite 

samples, during the study period. Samples were analyzed for 

nutrients (nitrogen and phosphorus), total organic carbon, 

trace elements (total lead and total zinc), total dissolved solids, 

and total suspended sediment (total suspended solids and 

suspended-sediment concentrations). The sampling scheme 

was designed to identify variations in water quality both 

hydrologically and seasonally.

The 12 watersheds were characterized for basin slope, 

population density, land use for 2009, and the percentage of 

impervious area from 2000 to 2009. Precipitation in water 

years 2004–09 was about 18 percent below average, and the 

county experienced exceptional drought conditions and below 

average runoff in water years 2007 and 2008. Watershed 

water yields, the percentage of precipitation that results in 

runoff, typically are lower in low precipitation years and 

are higher for watersheds with the highest percentages of 

impervious areas. 

A comparison of base-flow and stormflow water-quality 

samples indicates that turbidity and concentrations of total 

ammonia plus organic nitrogen, total nitrogen, total phosphorus, total organic carbon, total lead, total zinc, total suspended 

1

A water year is the period from October 1 to September 30 and is 

designated by the year in which the period ends.

solids, and suspended-sediment concentrations increased with 

increasing discharge at all watersheds. Specific conductance, 

however, decreased during stormflow at all watersheds, 

and total dissolved solids concentrations decreased during 

stormflow at a few of the watersheds. Total suspended solids 

and suspended-sediment concentrations typically were two 

orders of magnitude higher in stormflow samples, turbidities 

were about 1.5 orders of magnitude higher, total phosphorus 

and total zinc were about one order of magnitude higher, 

and total ammonia plus organic nitrogen, total nitrogen, total 

organic carbon, and total lead were about twofold higher than 

in base-flow samples.

Seasonal patterns and long-term trends in flow-adjusted 

water-quality concentrations were identified for five representative constituents—total nitrogen, total phosphorus, total zinc, 

total dissolved solids, and total suspended solids. Seasonal 

patterns for all five constituents were fairly similar, with 

higher concentrations in the summer and lower concentrations 

in the winter. Significant linear long-term trends in stormflow 

composite concentrations were identified for 36 of the 60 

constituent-watershed combinations (5 constituents multiplied 

by 12 watersheds) for the period of record through water year 

2011. Significant trends typically were decreasing for total 

nitrogen, total phosphorus, total suspended solids, and total 

zinc and increasing for total dissolved solids. Total dissolved 

solids and total suspended solids trends had the largest 

magnitude changes per year. 

Stream water loads were estimated for 10 water-quality 

constituents. These estimates represent the cumulative effects 

of watershed characteristics, hydrologic processes, biogeochemical processes, climatic variability, and human influences 

on watershed water quality. Yields, in load per unit area, were 

used to compare loads from watersheds with different sizes. A 

load estimation approach developed for the Gwinnett County 

LTTM program that incorporates storm-event composited 

samples was used with some minor modifications. This 

approach employs the commonly used regression-model 

method. Concentrations were modeled as a function of 

discharge, time, season, and turbidity to improve model predictions and reduce errors in load estimates. Total suspended 

solids annual loads have been identified in Gwinnett County’s 

Watershed Protection Plan for target performance criterion.
2 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

The amount of annual runoff is the primary factor in 

determining the amount of annual constituent loads. Below 

average runoff during water years 2004–09, especially during 

water years 2006–08, resulted in corresponding below average 

loads. Variations in constituent yields between watersheds 

appeared to be related to various watershed characteristics. 

Suspended sediment (total suspended solids and suspendedsediment concentrations) along with constituents transported 

predominately in solid phase (total phosphorus, total organic 

carbon, total lead, and total zinc) and total dissolved solids 

typically had higher yields from watersheds that had high 

percentages of impervious areas or high basin slope. High 

total nitrogen yields were also associated with watersheds 

with high percentages of impervious areas. Low total nitrogen, 

total suspended solids, total lead, and total zinc yields appear 

to be associated with watersheds that have a low percentage 

of high-density development. Total suspended solids yields 

were lower in drought years, water years 2007–08, from the 

combined effects of less runoff and the result of fewer, lower 

magnitude storms, which likely resulted in less surface erosion 

and lower stream sediment transport. 

Introduction

Watershed surface-water quantity and water quality reflect 

and integrate the effects of watershed characteristics, inputs 

(for example, precipitation and dry deposition), hydrologic 

and biogeochemical processes, climatic variability (such as 

droughts and floods), and human influences. Changes in land 

use alter complex interactions that affect many processes 

within a watershed (MacDonald, 2000). Urbanization, with its 

associated increases in impervious area, can have a great effect 

on stream hydrology by (1) decreasing rainfall infiltration and 

groundwater recharge rates, resulting in lower stream base 

flow, and (2) increasing storm runoff, peak discharges, and 

flood flows (Leopold, 1968). Urbanization and the resulting 

hydrologic response can affect surface-water quality by 

increasing surface erosion, sediment transport, and pollutant 

loadings and by altering stream-channel stability. Best management practices (BMPs; for example, stormwater detention 

ponds, diversion structures, and stream buffers) and other erosion controls frequently are implemented to mitigate the effects 

of urbanization on stream hydrology and minimize sediment 

transport in urbanized areas. Monitoring specific water-quality 

constituents allows for the identification of trends and the 

computation of loads and yields to help attribute changes in 

water quality to natural variations as opposed to those related 

to changes in land use and watershed management strategies. 

The U.S. Geological Survey (USGS), in cooperation 

with Gwinnett County Department of Water Resources, 

established a comprehensive Long-Term Trend Monitoring 

(LTTM) program in 1996 to monitor, analyze, and quantify 

the magnitudes of pollutants and the effects of urbanization 

on six watersheds. In 2001, six additional watersheds were 

added to the LTTM program (fig. 1). These 12 watersheds 

were continuously monitored during water years (WYs) 

2004–09 for water level (stage), discharge, precipitation, and 

water-quality properties (water temperature, specific conductance [SC], and turbidity). Three stormflow-composited 

samples and one base-flow sample were collected in both 

summer (May–October) and winter (November–April) of 

each year; samples were analyzed for nutrients, trace metals, 

total dissolved solids, and total suspended sediment (total 

suspended solids and suspended-sediment concentration). The 

sampling scheme was designed to identify variations in water 

quality both hydrologically and seasonally. 

The primary purpose of the monitoring program is to 

collect and interpret consistent, high-quality water-quantity 

and water-quality data. Watershed managers can use these data 

to make informed management decisions in order to maintain 

the designated uses of streams, protect aquatic habitats, and 

optimize the effectiveness of water-management practices 

(BMPs). Intensive, long-term monitoring of watershed 

characteristics, streamflow, and stream quality are essential 

to quantifying the impacts of specific land uses, point-source 

and nonpoint-source discharges, and management processes 

on surface water (water quantity, flow characteristics, and 

water quality). The LTTM program also fulfills requirements 

outlined in the Gwinnett County Watershed Protection Plan 

as well as requirements for the National Pollutant Discharge 

Elimination System (NPDES) permit and the Metropolitan 

North Georgia Water Planning District (MNGWPD). 

Purpose and Scope

The purpose of this report is to summarize and analyze 

the hydrologic, water-quality, and land-use data collected as 

part of Gwinnett County’s LTTM program for 12 watersheds 

for the 6-year period WYs 2004–09. This report is an update 

to, and continuation of, the report by Landers and others 

(2007) that presented and analyzed data for the original six 

watersheds for WYs 1996–2003. The specific goals of this 

report are to:

• Expand the watershed characterization reported in Landers 

and others (2007) to include the additional six watersheds 

added to the LTTM program;

• Discuss changes and trends in population, land use, and 

impervious area;

• Report annual hydrologic inputs and outputs (precipitation 

and runoff);

• Summarize water-quality data through WY 2009;

• Determine trends for five representative water-quality constituents (total nitrogen, total phosphorus, total dissolved 

solids, total suspended solids, and total zinc) for the period 

of record through WY 2011, and;

• Provide annual load and yield estimates for 10 constituents 

(total nitrite plus nitrate, total nitrogen, total phosphorus, 

dissolved phosphorus, total organic carbon, total lead, total 

zinc, total dissolved solids, total suspended solids, and 

suspended sediment) for WYs 2004–09.
Introduction 3

Chattahoochee–

Flint River Basin

Ocmulgee–

Oconee–Altamaha

River Basin

GEORGIA

Ocean Atlantic

Atlanta

GWINNETT

COUNTY

Lake Sidney Lanier

Buford

Dam

Piedmont Physiographic Province

Monitored watershed boundary

Eastern Continental Divide

USGS water-quality monitoring station,

 identifier, and watershed

0 21 3 4 5 MILES

0

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

21 3 4 5 KILOMETERS

Chattahooche e River

EXPLANATION

Original Gwinnett County LTTM program

 watersheds, 1996–97

Watersheds added in 2001

Figure 1.

N

Wheeler Creek

watershed

Suwanee Creek 02217274

watershed

02334885

Richland Creek

watershed

02334480

Level Creek

watershed

02334578

Crooked Creek

watershed

02335350

North Fork

Peachtree Creek

watershed

02336030

Yellow River

watershed

02207120

No Business Creek

watershed

02207185

 

Big Haynes Creek

watershed

02207385

Brushy Fork Creek

watershed

02207400

Alcovy River

watershed

02208150

Apalachee River

watershed

02218565

02207185

Figure 1. Location of the study area showing the 12 monitored watersheds and the monitoring stations, Gwinnett County, Georgia.
4 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Methods

Twelve watersheds in Gwinnett County, Georgia, were 

monitored for stage, discharge, precipitation, and continuous 

water-quality properties as part of the LTTM program. 

Samples were collected during base-flow and stormflow 

events and were analyzed for 12 water-quality constituents. 

Seasonal patterns and long-term trends were identified in a 

representative subset of water-quality constituents in both 

base-flow and stormflow samples. Stream water loads were 

estimated for 10 water-quality constituents.

Surface-Water Monitoring

The Gwinnett County LTTM program consists of 12 

watersheds monitored at their outlets for stage (water level), 

discharge, precipitation, and continuous water-quality 

constituents. Surface water-quality samples were collected 

seasonally during base-flow conditions and stormflow events 

at each watershed gaging station. The samples were analyzed 

for 12 selected constituents, including field properties, 

nutrients, trace metals, suspended sediment, and total dissolved solids. Initially, six watersheds were monitored when 

the LTTM program started in 1996: (1) Brushy Fork Creek, 

(2) Alcovy River, (3) Big Haynes Creek, (4) Suwanee Creek, 

(5) Yellow River, and (6) Crooked Creek Basins. In 2001, 

six additional watersheds were added to the LTTM program: 

(7) No Business Creek, (8) Wheeler Creek, (9) Apalachee 

River, (10) Richland Creek, (11) Level Creek, and (12) North 

Fork Peachtree Creek Basins (table 1). Program watersheds 

were selected to ensure diverse basin characteristics and land 

use for evaluating streamflow quantity and quality characteristics, while ensuring an appropriate spatial coverage of the 

county. The selected watersheds have variable water-quality 

attainment status (whether or not water-quality standards are 

met) and contain a diverse amount of point-source discharges. 

The specific monitoring station locations were determined on 

the basis of the suitability for hydrologic instrumentation and 

personnel safety.

The LTTM program follows standard USGS protocols 

for measuring stage, making streamflow measurements, 

and computing discharge (Rantz and others, 1982a, 1982b). 

Stage is recorded every 15-minutes to the nearest 0.01 foot 

and is routinely verified to an outside reference gage. This 

reference gage is periodically checked with surveying levels 

and other established reference marks to verify the gage 

datum. Discharge measurements are made over a wide range 

of hydrologic conditions to develop a stage-discharge relation 

at each monitoring station used to compute discharge from the 

stage measurements. Discharge measurements were regularly 

made in order to continually refine the stage-discharge relation 

and to account for any temporal changes in the relation due to 

changes in the shape of the streambed. Discharge for periods 

of missing or unreliable stage data were estimated from relations in hydrograph data between the station with the missing 

data and from nearby basins having similar characteristics 

(Rantz, 1982b).

Precipitation was measured at each station and was 

recorded at 15-minute intervals, using self-calibrating tipping 

bucket rain gages that measure precipitation in 0.01-inch increments. The rain gages were routinely cleaned and calibrated as 

outlined in the Surface-Water Quality-Assurance Plan for the 

USGS Georgia Water Science Center (Gotvald, 2010).

Table 1. Twelve U.S. Geological Survey (USGS) water-quantity and water-quality monitoring stations included in the 

watershed characteristics and water-quality trends study in Gwinnett County, Georgia, including dates established and 

drainage areas for each station.

USGS station 

number Station name Date established Drainage area(square miles)

02207120 Yellow River at GA Hwy 124 near Lithonia, GA April 1996 162

02207185 No Business Creek at Lee Road, below Snellville, GA March 2001 10.1

02207385 Big Haynes Creek at Lenora Road, near Snellville, GA June 1996 17.3

02207400 Brushy Fork Creek at Beaver Road near Loganville, GA June 1996 8.15

02208150 Alcovy River at New Hope Road, near Grayson, GA June 1997 30.8

02217274 Wheeler Creek at Bill Cheek Road, near Auburn, GA June 2001 1.31

02218565 Apalachee River at Fence Road, near Dacula, GA July 2001 5.68

02334480 Richland Creek at Suwanee Dam Road, near Buford, GA May 2001 9.34

02334578 Level Creek at Suwanee Dam Road, near Suwanee, GA May 2001 5.04

02334885 Suwanee Creek at Suwanee, GA September 1996 47

02335350 Crooked Creek near Norcross, GA March 1996 8.89

02336030 North Fork Peachtree Creek at Graves Road, near Doraville, GA June 2001 1.42
Methods 5

Continuous water-quality monitors were deployed at 

each monitoring station to measure water temperature, SC, 

and turbidity at 15-minute intervals. These water-quality 

monitors typically are cleaned and their calibration checked 

every 2 weeks and more frequently following hydrologic 

events or after observing abnormal readings, which could 

be associated with point sources or nonpoint sources of 

pollution. These water-quality monitors are maintained, and 

their corresponding sensor records are checked using the 

quality-assurance and quality-control procedures outlined in 

Wagner and others (2006).

All continuously monitored data (stage, precipitation, 

and water quality) are transmitted hourly by way of satellite 

communication and are made available to the public at 

the USGS Web site as values and time-series plots, which 

can be accessed at http://waterdata.usgs.gov/ga/nwis/

current/?type=flow&group_key=basin_cd. Due to public 

interest in receiving the data in near real-time during periods 

of extreme runoff, each station is designed to send emergency 

transmissions every 15 minutes during these periods. These 

transmissions occur when rainfall intensities exceed 2 inches 

per hour or when there are large rates of change in stage, as 

determined from thresholds defined for each station. The 

real-time data combined with the USGS WaterAlert tool 

(http://maps.waterdata.usgs.gov/mapper/wateralert/) can be 

used as a flood warning system for emergency managers and 

the public. A typical multiparameter stream monitoring station 

is shown in figure 2.

Two discrete base-flow samples and six stormflow 

samples were collected each year at each of the 12 stations for 

water-quality analyses. The year is divided into two seasons, 

summer (May through October) and winter (November 

through April), with one base-flow sample and three stormflow 

samples collected during each season. Base-flow samples 

were collected using a USGS DH-81 manual sampler, using 

depth integrated, equal-width-increment (EWI) integrating 

techniques to ensure a representative sample as outlined in 

the National Field Manual for the Collection of Water-Quality 

Data (U.S. Geological Survey, variously dated). Base-flow 

samples were collected after no more than 0.1 inch of 

precipitation had fallen during the previous 72 hours. During 

base-flow sampling, a water-quality “sonde” was used to 

concurrently measure the field properties of pH, turbidity, SC, 

water temperature, and dissolved oxygen in accordance with 

the Watershed Protection Plan for Gwinnett County (Gwinnett 

County Department of Public Utilities, 2000).

Stormflow samples were collected using automatic 

samplers that pump water from a designated point in the 

stream. The sampler is programmed to begin sampling when 

precipitation and (or) stage thresholds are reached, and a 

sample is collected each time a specified volume of water 

flows by the station. These samples are composited into a 

Figure 2.

Figure 2. Multiparameter monitoring station at North Fork 

Peachtree Creek at Graves Road, near Doraville, Georgia. 

(USGS station number 02336030; photographs by Jonathan 

Evans, USGS)
6 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

single discharge-weighted event sample for water-quality 

analysis, which is assumed to be representative of the 

constituent concentration variations that occur during an 

event. All stormflow samples are collected in accordance with 

the applicable wastewater permits and Watershed Protection 

Plan for Gwinnett County (Gwinnett County Department of 

Public Utilities, 2000). Stormflow samples are required to be 

collected during an event in which a minimum of 0.3 inch of 

precipitation occurs. Additionally, a minimum of 72 hours 

is required between each event to ensure that the events are 

discrete and that the measured water-quality properties are 

associated with the sampled event.

The automatic sampler tubing and intake are cleaned 

and prepared prior to each sampling event. The programmed 

volume at which each sample is collected is set in accordance 

with the anticipated magnitude of the approaching storm. 

The samples are refrigerated in the automatic sampler at 

about 4 degrees Celsius and are retrieved from the sampler 

within 24 hours of the end of an event. Sampler cleaning 

and maintenance procedures are further documented in the 

USGS Georgia Water Science Center quality-assurance plan 

(S.J. Lawrence, U.S. Geological Survey, written commun., 

April 6, 2010). Because automatic samplers collect samples 

from a single point in the stream cross section, periodic 

concurrent EWI and automatic samples are collected. These 

samples are independently analyzed and compared to ensure 

that the automatic point sample is representative of the entire 

stream cross section.

Base-flow and stormflow samples were processed and 

preserved following USGS field methods (U.S. Geological 

Survey, variously dated) and were analyzed for 10 constituents: nutrients (total nitrite plus nitrate [NO2

 + NO3], total 

ammonia plus organic nitrogen [total Kjeldahl nitrogen, TKN], 

total phosphorus [TP], dissolved phosphorus [DP], and total 

organic carbon [TOC]); trace metals (total lead [Pb] and total 

zinc [Zn]); total dissolved solids (TDS); and sediment concentrations (total suspended solids [TSS] and suspended sediment 

[SSC]) in USGS laboratories in Lakewood, Colorado; Atlanta, 

Georgia; and other USGS-approved laboratories. Total 

nitrogen (TN) was calculated as the sum of NO2 + NO3 and 

TKN. Units of measurement and laboratory detection limits 

for each constituent are listed in table 2. The detection limits 

for some constituents have changed over time due to changes 

in approved laboratory methods. The detection limits listed in 

table 2 reflect the analytical methods used during the 2004–09 

water years discussed in this report. Several constituents that 

were included in the 1996–2003 period of this study have 

been discontinued as of 2004: total cadmium, total chromium, 

total copper, biological oxygen demand, and chemical oxygen 

demand. Field water-quality blank and replicate samples were 

collected in compliance with USGS protocols (U.S. Geological Survey, 2006); laboratory analyses include an extensive 

quality-control and quality-assurance program.

The amount of suspended sediment in water was determined in this study using two different laboratory analytical 

methods—SSC and TSS. The SSC analytical method 

Table 2. Water-quality constituents analyzed for samples collected in streams in Gwinnett County, 

Georgia, units of measures, and detection limit.

[μS/cm, microsiemens per centimeter; °C, degrees Celsius; FNU, formazin nephelometric units; mg/L, milligrams per 

liter; N, nitrogen; P, phosphorus; C, carbon; μg/L, micrograms per liter; na, not applicable] 

Constituent Constituent

abbreviation Units 

Laboratory

detection

limit

Specific conductance SC μS/cm at 25 °C na

Turbidity1 na FNU na

Total nitriteplusnitrate NO2 +NO3 mg/L as N 0.019

Total ammonia plus organic nitrogen TKN mg/L as N 0.25

Total nitrogen TN mg/L as N Calculated

Total phosphorus TP mg/L as P 0.02

Dissolved phosphorus DP mg/L as P 0.02

Total organic carbon TOC mg/L as C 1.0

Total lead Pb μg/L 1.0

Total zinc Zn μg/L 2.0

Total dissolved solids TDS mg/L 1.0

Total suspended solids TSS mg/L 1.0

Suspended sediment SSC mg/L 1.0

1

Turbidity measured by monochrome near infrared light emitting diode (LED), 780 – 900 nanometers, 

detection angle 90 ± 2.5 degrees, in formazin nephelometric units (FNU).
Methods 7

measures the dry weight of the sediment in an entire sample of 

a known volume, and the TSS method involves measuring the 

dry weight of sediment in a subsample of the available sample 

volume, rather than the entire sample (Landers, 2013). Annual 

yield of TSS is the primary performance criterion for suspended sediment in Gwinnett County’s Watershed Protection 

Plan (Gwinnett County Department of Public Utilities, 2000). 

Surface-Water Constituent Concentration 

Seasonality and Trends

Analysis of long-term trends often requires a decade 

or more of monitoring because natural variability can 

obscure trends, there can be a lag time between watershed 

changes and water-quality effects, and effects from multiple 

activities within a watershed can offset each other (Landers 

and others, 2007). Water quality varies in response to many 

natural processes, such as changes in discharge, seasonal 

biogeochemical processes, and climatic variations, which 

makes it difficult to attribute cause to changes in water quality. 

It is therefore imperative to remove the effects of natural 

variation from water- quality data in order to determine trends 

that may be the result of human influences; for example, many 

water-quality constituents vary strongly with discharge, which 

can easily obscure smaller, underlying variations, making it 

important to remove the effects of discharge on water quality. 

For long-term trends, the effects of seasonal variability on 

water quality are less of an issue because of the relatively 

short-term, recurring nature. 

Trend analysis is performed on water-quality concentrations rather than load estimates because it is easier to remove 

the effects of climatic variability on concentrations and 

because predefined formulations for seasonality and long-term 

trend terms in the load estimates may alter the conceptualization of any patterns and trends. To remove the effects of 

discharge on water-quality concentrations, the evaluations of 

trend and seasonality are performed on the residual concentrations to a linear regression of log-transformed concentration 

as a function of log-transformed discharge (Hirsch and others, 

1991) for each constituent/station combination. Residuals are 

defined as the observed concentration minus the predicted 

concentration. For the long-term trend analysis, concentrations 

are also seasonally adjusted. Although the effects of discharge 

are removed, climatic effects may not necessarily be fully 

accounted. For example, in a drought, the flow-adjusted 

residual concentrations would compensate for the systematically lower discharges, but the concentration-discharge 

relation may also be altered by drought conditions, resulting 

in additional unexplained variance in water quality. It can also 

be difficult to parse out whether a change in the concentrationdischarge relation over time is the result of climatic variability 

or human influences.

Seasonal patterns and long-term trends in water quality 

were analyzed for five representative constituents—TN and 

TP (representing nitrogen and phosphorus nutrients), TDS 

(representing dissolved constituents), TSS (representing 

suspended sediment), and Zn (representing trace elements). 

Although most results presented in this report are for the 

period of record through WY 2009, the seasonal pattern and 

long-term trend analysis was for the period of record through 

WY 2011. Extending this analysis minimizes the effects of 

an unusual year on seasonality and improves the chances of 

detecting long-term trends.

Seasonal variations in water-quality concentrations can 

be the result of both natural variations—such as seasonal 

variability in base flow, the types of weather systems 

(storms), and biogeochemical processes—and human influences attributed to seasonal practices, such as construction, 

agriculture, and fertilizer application. To examine seasonal 

variability, flow-adjusted residual concentrations were plotted 

in logarithmic space in relation to month and then a smoothed 

line known as locally weighted regression and smoothing 

scatterplots (LOWESS; Cleveland, 1979) was fitted to the 

residual concentrations so that the seasonal pattern could 

easily be discerned.

Long-term trends were determined separately for 

base-flow and stormflow samples. The significance of a 

linear fit of the flow-adjusted and seasonally adjusted residual 

concentrations with time is used to determine the presence of a 

trend (p-value less than or equal to 0.10). If a significant trend 

is determined, a linear trend line was fitted to the residual 

concentrations in logarithmic space to better illustrate the 

trend in water quality. Logarithmic space was used because the 

regression model was based on log-transformed concentration 

and using logarithmic space made it easier to compare the 

slope of the trend of different water-quality constituents.

Stream Water Constituent Load Estimation

Stream water constituent load, often referred to as mass 

flux, is the mass of chemical solutes or sediment transported at 

a point in a stream during a specific period. Load serves as an 

integrated measure of all processes within the watershed that 

affect water quality (for example, Semkin and others, 1994). 

With increased emphasis on watershed-based strategies for 

the control of nonpoint-source pollutants, reliable, temporal 

measures of loads are needed to address whether water quality 

is improving or degrading within a reasonably short period 

of time. In the United States, stream reaches that do not 

meet U.S. Environmental Protection Agency water-quality 

standards (U.S. Environmental Protection Agency, 2000a) are 

subject to waste-load allocation schemes that are based on the 

total maximum daily load (TMDL). A TMDL is defined as the 

maximum amount of a pollutant that a water body can receive 

and still meet water-quality standards (U.S. Environmental 

Protection Agency, 2000b). Load is highly dependent on 

the amount of runoff; hence, large watersheds typically will 

transport high loads. To better compare load from different 

sized watersheds, load is divided by the watershed area to 

determine yield, which is the load per unit area.
8 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Stream water constituent load (L) is the product of 

constituent concentration (C) and discharge (Q) integrated over 

time (t):

L = ∫ C(t) Q(t) dt (1)

Load estimation using the integral in equation 1 requires a continuous record of both concentration and discharge. Although 

discharge can readily be measured in a nearly continuous 

manner, constituent concentration typically is measured less 

frequently because of the effort and expense of collecting and 

analyzing samples for water quality. Therefore, various techniques have been developed to estimate loads using discrete 

concentration observations. These methods can be categorized 

into four classes: (1) averaging methods, (2) period-weighted 

approaches (for example, Likens and others, 1977; Larson 

and others, 1995), (3) regression-model methods, and (4) ratio 

estimators (for example, Dann and others, 1986; Preston and 

others, 1989). In the current study, a regression-model (or rating-curve) method is used to estimate loads. In this approach, 

C(t) is estimated continuously using a regression model that 

relates concentration to a continuously measured variable, such 

as discharge and day of year (Johnson, 1979; Crawford, 1991; 

Cohn and others, 1992), thus enabling a direct calculation 

of equation 1. The concentration model predicts the average 

concentration response for the set conditions, such as discharge 

and season.

Regression-models and load estimates were developed 

and estimated using the USGS LOAD ESTimator software 

(LOADEST; Runkel and others, 2004), using the adjusted 

maximum likelihood estimates (AMLE) algorithm (Cohn and 

others, 1989, 1992). This algorithm applies a correction factor 

to account for retransformation bias of a logarithmic model 

transformed back to linear space (Ferguson, 1986) and can 

appropriately handle censored water-quality data—concentrations that are below the analytical detection limit. The TIBCO 

Spotfire S+ statistical software version of LOADEST software 

was used in this analysis.

Stormflow samples for the Gwinnett LTTM program 

generally are collected as storm composites. Composite 

samples are hydrodynamically different than discrete samples; 

hence, Landers and others (2007) developed an alternative 

regression method that can be used to estimate loads separately 

for base-flow and stormflow conditions in order to apply the 

different modeled concentration discharge relations to the 

appropriate flow regime. In this method, separate concentration models are fit between concentration and instantaneous 

discharge for base-flow samples and between average event 

concentration and average event discharge for the stormflow 

samples. An analysis was done for each watershed to determine the optimal time-step to use for stormflow load estimation. LOADEST can estimate loads at time intervals as short as 

1 hour. The fitted time-step is related to the average duration of 

storms sampled for each watershed. The time-step is longer for 

larger watersheds because of the integration of runoff over a 

larger area with varied and longer travel times to the watershed 

outlet as well as the attenuation of the storm hydrograph as 

the runoff travels downstream, which reduces peak flows and 

spreads out the hydrograph over time. A graphical hydrograph 

separation was then performed using the HYdrograph 

SEParation (HYSEP) software developed by the USGS (Sloto 

and Crouse, 1996) to determine whether each time interval 

represented base-flow or stormflow conditions. The loads 

were then estimated using both the base-flow and stormflow 

concentration models, and the loads were compiled for each 

time interval, depending on the hydrograph separation.

For this update of the Gwinnett County LTTM program 

watershed loads for WYs 2004–09, two changes in approach 

have been made to the Landers and others (2007) method. 

First, instead of building two separate base-flow and stormflow 

concentration models, a single model was fit with a base-flow/

stormflow variable to indicate whether sampling conditions 

were base flow (=0) or stormflow (=1). This approach forced 

the model parameters to be the same for both flow regimes (for 

example, same concentration-discharge slope, same seasonality 

variations) with the exception of an offset as determined by 

the multiplier of the base-flow/stormflow parameter. This 

combining of models expedited the model-building step while 

still fitting the base-flow and stormflow samples. Second, the 

hydrograph separation was based on levels of turbidity or 

discharge instead of the dynamics of the hydrograph, which 

requires a hydrograph separation. For the streams in this study, 

turbidities greater than 20 formazin nephelometric units (FNU) 

were determined to be indicative of stormflow; therefore, 

conditions were determined as stormflow when turbidity from 

the continuous water-quality sensor was greater than or equal 

to this 20 FNU threshold. If turbidity data were missing, then 

discharge was used to determine stormflow conditions—stormflow occurred when discharge was greater than the average 

of the 50th and the 75th percentiles of discharge for that 

watershed. Both of these changes appear to have little effect on 

the estimated loads, because a comparison of loads made using 

both approaches for WY 2003 indicated that the values were 

similar.

The regression models used for load estimation had the 

form of logarithm of concentration as a function of logarithm 

of discharge and discharge-squared, seasonal sine and cosine 

functions, long-term time and time-squared terms, and the 

logarithm of turbidity (from the continuous water-quality 

sensors). If a model parameter was not significant (p-value 

>0.05), the parameter was excluded from the regression model 

to avoid overparameterization by excluding variables that did 

not explain much variation in the concentrations. Models were 

calibrated for the entire period of record for each watershed 

through WY 2009. The addition of the parameter logarithm of 

turbidity to the concentration models was started in WY 2004 

and was not included in the WY 1998–2003 load presented in 

Landers and others (2007). The inclusion of turbidity improved 

the models by increasing the amount of variation in concentrations that the models explained, likely because turbidity can be 

a surrogate for suspended sediment and other constituents in 

the water. Because of maintenance and difficulties in measuring water quality using continuous sensors, there are inevitable 
Watershed Characteristics 9

gaps in the data record. Hence, a second concentration relation 

without turbidity was modeled to use as a fallback for periods 

where the turbidity record was missing. A residual analysis was 

performed to check for outlier concentrations that would affect 

the fit of the concentration relation, and these outliers were 

removed so they would not affect model predictions. Loads 

were estimated for all of the constituents in table 2 except for 

SC and turbidity, which are not direct measures of concentration, and TKN, which is used to calculate TN. TSS loads for 

2004–09 are updated from those reported in Landers (2013).

Total Suspended Solids Load Estimation Error

Errors in the estimates for TSS annual loads are calculated 

because of the importance of TSS as a target performance 

criterion in the Gwinnett County Watershed Protection Plan. 

There are many sources of errors, including streamflow 

measurements, water-quality sample representativeness, 

laboratory analytical measurements, and load estimation 

modeling. Horowitz (2003) indicated that suspended-sediment 

load errors of ≤ ±15–20 percent should be considered relatively 

accurate for small to large rivers and for quarterly to greater 

timeframes.

LOADEST calculates both the standard error (SE) 

and the standard error of the prediction (SEP) of the mean 

load estimate. The SE represents the variability attributed to 

parameter uncertainty of the model calibration, while the SEP 

also includes the effects of random error. Therefore, SEP tends 

to be larger than SE, but SEP provides a better estimate of the 

errors between the load estimates and the actual loads. SEP 

calculations assume that the errors are independent in time 

(no serial correlation), as serial correlation can result in SEPs 

underestimating the actual uncertainty in the load estimates 

(Aulenbach, 2013). The size of error for any given year 

depends on the actual streamflow and turbidity observed for 

that year.

In this report, errors are based on the SEP and are 

expressed as 95th percent lower and upper confidence 

intervals. The percentage of the confidence interval represents 

the percentage of load estimates that are expected to reliably 

be within that confidence interval. The errors are assumed to 

follow a normal probability distribution, and as load models 

were created in logarithmic space, the upper 95th percent 

confidence interval will be larger than the lower 95th percent 

confidence interval. Landers and others (2007) previously 

reported LTTM program watershed load errors as SEs, which 

represent the error within ± one standard deviation of the 

mean and a confidence interval of about 68.3 percent; whereas 

the 95th percent confidence interval represents the error 

within ± 1.96 standard deviations. Hence, the reported range 

of 95th percent confidence intervals will be almost twice as 

large as the reported range of SEs. This difference needs to be 

accounted for when comparing the errors between this report 

and Landers and others (2007).

LOADEST provides estimates of the 95th percent 

confidence intervals on an annual basis. Error estimates are 

complicated by the fact that loads are estimated using two 

models—one that includes turbidity and one that does not. The 

annual error in loads of the combined models was estimated 

by weighting the annual errors of the two models by the 

fraction of annual runoff each of the models contribute to 

the annual load. The lower (CI95%Lower) and upper (CI95%Upper) 

95th percent confidence intervals are estimated using equations 2 and 3, respectively.

CI95%Lower = LTotal – [(LTQ – CI95%Lower(TQ)) * RFTQ

+ (LQ – CI95%Lower(Q)) * RFQ] (2)

CI95%Upper = LTotal + (CI95%Upper(TQ) – LTQ) * RFTQ

+ (CIQ95%Upper(Q) – LQ) * RFQ (3)

where LTotal is the total annual load; L is annual load; RF is the 

annual runoff fraction for that model’s load; and TQ and Q represent the turbidity-flow and flow-only models, respectively. 

Confidence intervals for yields were then calculated from the 

load estimate confidence intervals.

Watershed Characteristics

Gwinnett County is located in north-central Georgia, 

about 15 miles northeast of Atlanta (fig. 1). The county, 

which encompasses about 436 square miles, is located in 

the Piedmont physiographic province. The geology of the 

county is a mixture of complex and varied metamorphic rocks 

(U.S. Geological Survey, 2014). 

Gwinnett County is composed predominantly of 

headwater streams that drain into one of three major rivers: the 

Chattahoochee, the Ocmulgee, and the Oconee. The Eastern 

Continental Divide, which separates drainages that flow into 

the Gulf of Mexico from those that flow into the Atlantic 

Ocean, runs approximately northeast-southwest across the 

northwestern portion of the county. Five study watersheds 

(Richland Creek, Level Creek, Suwanee Creek, Crooked Creek 

and North Fork Peachtree Creek) are northwest of the divide 

and lie within the Apalachicola-Chattahoochee-Flint River 

Basin, which flows into the Gulf of Mexico. The remaining 

seven study watersheds (Yellow River, Alcovy River, Big 

Haynes Creek, Brushy Fork Creek, No Business Creek, 

Wheeler Creek, and Apalachee River) lie within the AltamahaOcmulgee-Oconee River Basin, which flows into the Atlantic 

Ocean (fig. 3).

Watershed Basin Slope

Landers and others (2007) showed that basin slope 

has a direct effect on stream hydrology and water quality in 

Gwinnett County. Altitudes in Gwinnett County range from 

720 to 1,290 feet above North American Vertical Datum of 

1988 (NAVD 88; fig. 4). Land-surface slope across Gwinnett 
10 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

0 1 2 3 4 5 MILES

0

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

1 2 3 4 5 KILOMETERS

Chattahooche e River

Figure 3. 

EXPLANATION

Monitored watershed boundary

Eastern Continental Divide

USGS water-quality monitoring station, 

 identifier, and watershed

02207185

Land-surface altitude,

 in feet above NAVD 88

High: 1,293

Low: 281

N

Wheeler Creek

watershed

Suwanee Creek 02217274

watershed

02334885

Richland Creek

watershed

02334480

Level Creek

watershed

02334578

Crooked Creek

watershed

02335350

North Fork

Peachtree Creek

watershed

02336030

Yellow River

watershed

02207120

No Business Creek

watershed

02207185

Big Haynes Creek

watershed

02207385

Brushy Fork Creek

watershed

02207400

Alcovy River

watershed

02208150

Apalachee River

watershed

02218565

Figure 3. Land-surface altitude for Gwinnett County, Georgia. [Altitude data from aerial light detection and ranging (lidar; laser-radar) 

survey from Gwinnett County Department of Public Utilities, unpub. data, 2009]
Watershed Characteristics 11

Sweetwater Creek

watershed

Pew Creek

watershed

Shoal Creek

watershed

0 1 2 3 4 5 MILES

0

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

1 2 3 4 5 KILOMETERS

Chattahooche e River

Figure 4.

0 to 6

>6 to 12

>12 to 18

>18

Slope, in percent

EXPLANATION

Monitored watershed boundary

Eastern Continental Divide

USGS water-quality monitoring station, 

 identifier, and watershed

02207185

N

Wheeler Creek

watershed

Suwanee Creek 02217274

watershed

02334885

Richland Creek

watershed

02334480

Level Creek

watershed

02334578

Crooked Creek

watershed

02335350

North Fork

Peachtree Creek

watershed

02336030

Yellow River

watershed

02207120

No Business Creek

watershed

02207185

Big Haynes Creek

watershed

02207385

Brushy Fork Creek

watershed

02207400

Alcovy River

watershed

02208150

Apalachee River

watershed

02218565

Figure 4. Land-surface slope for Gwinnett County, Georgia. [Derived from altitude data from Gwinnett County Department of Public 

Utilities, unpub. data, 2009]
12 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

County ranges to greater than 18 percent; the higher slopes 

generally are in the northern portion of the county within the 

Apalachicola-Chattahoochee-Flint River Basin (fig. 4). The 

average basin slope of the 12 LTTM program watersheds is 

9.0 percent, with the Richland Creek watershed having the 

steepest slope of 13.8 percent and the Brushy Fork Creek 

watershed having the shallowest slope of 5.3 percent (fig. 5). 

The assessment of basin slope for the 12 watersheds is consistent with the initial determination from Landers and others 

(2007) for the initial six watersheds in the LTTM program.

Population

Gwinnett County is a densely populated, suburban 

county of the Atlanta metropolitan area. Gwinnett County has 

undergone rapid population growth from about 1980 to 2010, 

and land use has changed from what was once predominantly 

agriculture and forest to a highly developed area. Both 

land-use changes and population growth can make watershed 

management more challenging, because increases in impervious areas typically are associated with increased storm runoff 

and decreased base flow, and population growth increases the 

demands on the water supply and for wastewater treatment. 

The population density in Gwinnett County in 2009 was 

about 789,000, about 1,840 people per square mile (fig. 6); 

whereas, the national average in 2010 was about 87.4 people 

per square mile (U.S. Census Bureau, 2011). The population 

density is greater in the western portion of the county and 

along the major roadways. Average watershed population 

densities in 2009 ranged from about 880 people per square 

mile in the Wheeler Creek watershed to about 4,200 people 

per square mile in the North Fork Peachtree Creek watershed 

(fig. 7).

Population in Gwinnett County increased by about 

195,000 people from 1996 to 2003 (the 7-year period 

reported by Landers and others [2007]), a 41 percent increase 

(fig. 8). From 2003 to 2009 (the 6-year period summarized 

in this report), the population increased by about 113,000 (a 

17 percent increase from 2003). The rate of population growth 

decreased from about 27,900 people per year from 1996 to 

2003 to about 18,900 people per year from 2003 to 2009. 

Annual county population estimates are from a combination of 

data from the U.S. Census Bureau (U.S. Census Bureau, 2004, 

2011) and Gwinnett County State of the County annual reports 

(Gwinnett County, Georgia, 2004, 2005, 2006, 2007, 2008, 

2009, 2010, 2012, 2013, 2014). 

Altamaha River watershed Chattahoochee River watershed

0

2

4

6

8

10

12

14

16

Average basin slope, in percent

Monitored watersheds, Gwinnett County

Figure 5. 

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Figure 5. Average basin slope of the 12 monitored watersheds in Gwinnett County, Georgia. [Derived from altitude data 

from Gwinnett County Department of Public Utilities, unpub. data, 2009]
Watershed Characteristics 13

N

Wheeler Creek

watershed

Suwanee Creek 02217274

watershed

02334885

Richland Creek

watershed

02334480

Level Creek

watershed

02334578

Crooked Creek

watershed

02335350

North Fork

Peachtree Creek

watershed

02336030

Yellow River

watershed

02207120

No Business Creek

watershed

02207185

Big Haynes Creek

watershed

02207385

Brushy Fork Creek

watershed

02207400

Alcovy River

watershed

02208150

Apalachee River

watershed

02218565

Buford

Dacula

Duluth

Grayson

Lawrenceville

Lilburn

Norcross

Suwanee

Sugar

Hill

Snellville

316

141

140

317

324

124

120

20

13

23

29

78

0 1 2 3 4 5 MILES

0

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

1 2 3 4 5 KILOMETERS

Chattahooche e River

EXPLANATION

Monitored watershed boundary

Eastern Continental Divide

USGS water-quality monitoring station, 

 identifier, and watershed

02207185

Population density during

 2009, per square mile

379 to 1,000

1,001 to 3,000

3,001 to 5,000

5,001 to 6,000

6,001 to 14,265

85

985

Figure 6. Figure 6. Population density for Gwinnett County, Georgia, 2009. [Data from American community survey 5-year estimates of 2006–10 

block group data, U.S. Census Bureau, 2012]
14 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

4,000 

4,500 

Figure 7. 

0 

Altamaha River watershed Chattahoochee River watershed

2009 population density, per square mile

Monitored watersheds, Gwinnett County

Yellow

River

No Business Creek

Big

Haynes Creek

Brushy

Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Figure 7. Population density for the 12 monitored watersheds in Gwinnett County, Georgia, 2009. [Derived from American 

community survey 5-year estimates of 2006–10 block group data, U.S. Census Bureau, 2012]
Watershed Characteristics 15

Figure 8. 

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

Year

0

10

20

30

40

50

60

70

80

90

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Cumulative population growth, in percent Total population

B

A

Figure 8. A, total population by year and B, population growth for Gwinnett County, Georgia, 1996–2013. [Data from 

U.S. Census Bureau (2004, 2011) and Gwinnett County, Georgia (2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013, 2014)]
16 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Land Use and Changes in Impervious Area

Land use and impervious area are summarized for the 

12 LTTM program watersheds in table 3; data are based on 

the Atlanta Regional Commission 2009 land use geographic 

information system (GIS) dataset (Atlanta Regional Commission, 2010). Individual land-use categories have been grouped 

into six categories: high density, low density, estate/park, 

transportation/utilities, undeveloped/water, and other. Land 

use varies greatly among watersheds. For example, North 

Fork Peachtree Creek and Crooked Creek watersheds have 

much higher high-density land use, 63.74 and 44.79 percent 

of the watershed, respectively, relative to the other watersheds, which have high-density land-use areas of less than 

21 percent.

Impervious area has been determined to be an important 

factor affecting the rainfall-runoff relations in many studies 

(for example, Leopold, 1968; Hollis, 1975; Ogden and others, 

2011), including areas in Gwinnett County (Landers and 

others, 2007). Impervious areas reduce infiltration and, hence, 

groundwater recharge, resulting in lower base flows. Impervious areas also increase runoff rates, resulting in a “flashy” 

hydrologic response along with higher peak storm discharges 

that may increase both erosion and transport of land surface, 

tributary and stream sediments. The percentage of impervious 

area in the 12 study watersheds ranged from 12.21 percent in 

the Richland Creek watershed to 52.79 percent in the North 

Fork Peachtree Creek watershed (table 3; fig. 9). Watershed 

impervious area is categorized into transportation elements, 

which include land uses such as roads, parking lots, and 

driveways, and building elements (fig. 9). On average, about 

two-thirds of the total impervious area in the watersheds is 

from transportation elements. High-density land use typically contains a large proportion of impervious area, hence, 

the strong association between these characteristics. The 

percentage of impervious area in the watersheds is associated 

more with high-density land use than with population density 

(fig. 10).

Table 3. Land use and watershed characteristics for 12 watersheds in Gwinnett County, Georgia, 2009.

[Land-use data from Atlanta Regional Commission, 2010]

Station 

number

Watershed 

name

Drainage 

area1

( square

 miles)

Land use (percent) Watershed impervious area 

(percent)

Slope 

(percent)

High density 2

Low density 3Estate/park4Transportation/ utilities 5Undeveloped/  water 6OtherTransportationBuildingTotal

02207120 Yellow River 161.95 20.75 61.61 11.48 1.31 2.43 1.90 16.67 6.96 23.63 9

02207185 No Business Creek 10.17 13.42 64.36 18.38 0.10 3.00 0.74 12.58 5.99 18.57 9

02207385 Big Haynes Creek 17.34 7.46 69.71 19.37 0.05 2.07 1.35 10.24 6.40 16.64 7

02207400 Brushy Fork Creek 8.17 9.25 47.14 30.84 0.07 7.59 4.14 9.36 5.21 14.57 5

02208150 Alcovy River 30.76 13.71 44.29 30.95 2.68 7.06 1.04 11.01 4.19 15.21 10

02217274 Wheeler Creek 1.31 20.24 49.29 19.71 4.75 5.98 0.00 10.05 5.88 15.92 10

02218565 Apalachee River 5.63 7.16 65.53 25.01 0.01 2.23 0.00 9.95 7.02 16.97 10

02334480 Richland Creek 9.37 13.16 40.71 31.99 2.17 10.68 0.09 8.04 4.17 12.21 14

02334578 Level Creek 5.08 4.90 60.57 31.16 0.00 2.84 0.00 10.31 6.33 16.63 10

02334885 Suwanee Creek 47.26 20.54 37.18 30.02 3.36 6.96 1.07 12.53 6.38 18.91 11

02335350 Crooked Creek 8.91 44.79 39.67 7.46 4.78 3.00 0.31 24.18 12.63 36.81 9

02336030 North Fork 

Peachtree Creek

1.42 63.74 29.93 0.51 5.09 0.77 0.00 32.81 19.98 52.79 7

1

Drainage areas calculated from Atlanta Regional Commission land-use dataset and vary slightly from drainage areas shown in table 1. 

2

High-density land use includes commercial, industrial, schools, religious, quarries, and residential lots with less than 1/4 acre.

3

Low-density land use includes residential lots sizes greater than or equal to 1/4 acre and less than 5 acres.

4

Estate/park use includes public and private parks, golf courses, and agricultural lands, and forests.

5

Transportation/utilities land use includes pervious and impervious areas within the right-of-way for roads, railroads, communications, and utilities.

6

Undeveloped land use includes landfills, waste dumps, water control structures, and recently cleared transitional land; water land use includes reservoirs 

and wetlands.
Watershed Characteristics 17

Figure 9. 

Building

Transportation

0

10

20

30

40

50

60

Watershed i

mpervious area, in percent

Monitored watersheds, Gwinnett County

Yellow

River

No Business Creek

Big

Haynes Creek

Brushy

Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

EXPLANATION

Figure 9. Percentage of watershed impervious area from transportation and building land cover for 12 

monitored watersheds in Gwinnett County, Georgia, 2009. [Data from Gwinnett County land coverage data, 

Gwinnett County Department of Public Utilities, unpub. data, 2009]

0 

10 

20 

30 

40 

50 

60 

0 10 20 30 40 50 60 70 

Watershed impervious area, in percent 

High-density land use, in percent 

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

Population density, per square mile 

A B

Figure 10. Figure 10. Watershed impervious area as related to A, high-density land use and B, population density for 12 

monitored watersheds in Gwinnett County, Georgia, 2009. [Impervious area and land-use data from Gwinnett County 

land coverage data, Gwinnett County Department of Public Utilities, unpub. data, 2009; Population data derived from 

American community survey 5-year estimates of 2006–10 block group data, U.S. Census Bureau, 2012]
18 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 11 illustrates how the percentage of impervious 

area has changed over time in the 12 watersheds for the year 

2000 and annually from 2005 to 2009 (Gwinnett County 

Department of Public Utilities, unpub. data, 2009). All 12 

watersheds show increases in impervious area over the period. 

The increases in impervious area from 2000 to 2005 are 

greater than the increases from 2005 to 2009. These increases 

are likely related to development within the watersheds 

associated with population growth (fig. 8). The two watersheds 

with the highest percentage of impervious areas, North 

Fork Peachtree Creek and Crooked Creek, did not change 

much over the later years, 2005 to 2009. The Wheeler Creek 

watershed, which had the lowest percentage of impervious 

area of the 12 watersheds in 2000, has shown the largest 

increases in the percentage of impervious area over the 2000 

to 2009 period. Yellow River, Alcovy River, Apalachee Creek, 

and Suwanee Creek watersheds also exhibited large increases 

in percentage of impervious areas over this same period.

Year

0

10

20

30

40

50

60

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Percent of impervious area

Figure 11. 

EXPLANATION

Yellow River

Monitored watershed

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Figure 11. Percentage of impervious area in 12 monitored 

watersheds in Gwinnett County, Georgia. [Data from 

Gwinnett County land cover data, Gwinnett County 

Department of Public Utilities, unpub. data, 2009]

Hydrologic Inputs and Outputs

The main components of the hydrologic budget for the 

LTTM program watersheds are precipitation, stream runoff, 

evapotranspiration, and groundwater storage. Climatic patterns 

in precipitation, runoff, and evapotranspiration are discussed. 

Annual precipitation and runoff for the 12 watersheds are 

presented, and water yields, the percentage of precipitation 

that results in runoff, are calculated. The effects that the 

WY 2007 and 2008 drought had on runoff and groundwater 

storage are discussed.

Climate and Precipitation

Gwinnett County has a humid, subtropical climate 

characterized by warm, humid summers and cool, wet winters. 

Monthly average air temperatures range from 42.2 degrees 

Fahrenheit (°F) in January to 79.8 °F in July (1981–2010 

thirty-year average for Atlanta Dekalb Peachtree Airport station 

USW00053863, National Oceanic and Atmospheric Administration, 2014). Mean annual precipitation, is 54.7 inches 

(1981–2010 thirty-year average for Norcross station 

USC00096407, National Oceanic and Atmospheric Administration, 2014). Precipitation is fairly evenly distributed throughout 

the year with the fall having the lowest amount of precipitation. 

Winter rainstorms are characterized by long duration, evenly 

distributed, and typically low-intensity frontal systems. In 

contrast, spring and summer rainstorms are characterized 

by short duration, unevenly distributed, intense convective 

thunderstorms. Despite their short duration, the intensity of 

these rainstorms often result in higher peak streamflows in 

headwater watersheds than those observed for longer-duration, 

low-intensity storms, and may result in increased erosion and 

wash off of constituents. Mean annual evapotranspiration is 

estimated to account for about 50 to 60 percent of the annual 

precipitation in the study area (Sanford and Selnick, 2013). The 

seasonal pattern of evapotranspiration results in declining base 

flows throughout the growing season (April–September) and 

progressively increasing base flows from event recharge during 

the dormant season.

The annual precipitation totals for WYs 2004–09 for 

the 12 study watersheds are shown in figure 12 and are based 

on USGS rain gages, co-located with the stream gages at the 

watershed outlet, and four additional gages within the study 

area. The precipitation totals represent a spatial average 

and are not just the precipitation recorded at the gage at 

each watershed outlet. All precipitation gages were used to 

interpolate the point precipitation data and create a “surface” 

with a 50-meter cell size and then the cells within each 

watershed shape were averaged to estimate the precipitation 

within the watershed for each year. The average shown for 

Gwinnett County represents the spatial average of the area 

within the 12 LTTM program watersheds. The highest annual 

average precipitation (56.4 inches) was in WY 2009 and the 

lowest (33.0 inches) was in WY 2007. The variability between 
Hydrologic Inputs and Outputs 19

0 

10 

20 

30 

40 

50 

60 

70 

2004 2005 2006 2007 2008 2009 Average

2004–09 Water year 

Figure 12

Precipitation, in inches

EXPLANATION

Monitoring stations

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Gwinnett

County

precipitation

average

Figure 12. Spatially averaged annual precipitation totals for each of the 12 monitored watersheds in Gwinnett 

County, Georgia, and the countywide (LTTM program watersheds) average.

watersheds is much smaller than the annual variations. Average annual precipitation for the county was average to slightly 

above average for two of the years, WYs 2005 (54.7 inches) 

and 2009 (56.4 inches), and was below average for the other 

four years when compared to the 1981–2010 thirty-year 

average for the Norcross weather station (54.7 inches). During 

the two-year drought, WYs 2007 and 2008, average annual 

precipitation was 21.7 and 18.5 inches below normal, respectively. The U.S. Drought Monitor publishes weekly maps 

classifying the severity of droughts across the United States 

(U.S. Drought Monitor, 2014a and 2014b). On the basis of 

these maps, the drought conditions in Gwinnett County were 

classified as (1) severe, (2) extreme, and (3) exceptional for 

5, 12, and 5 weeks, respectively, in WY 2007, and for 5, 30, 

and 12 weeks, respectively, in WY 2008. Although WY 2009 

precipitation totals were somewhat above normal, an extreme 

rainfall-runoff event in September 2009 resulted in historic 

high flows at a majority of the 12 LTTM program stations. 

Overall, precipitation for WYs 2004–09 was about 18 percent 

lower than the 30-year average for Norcross, Georgia.

The average annual precipitation contour map graphically 

depicts how precipitation varies with geographical location 

within Gwinnett County (fig. 13). The precipitation contours 

were generated from data collected at LTTM program 

monitoring stations for the period of record through 2009. 

Average precipitation is highest in the northern part of the 

county where altitudes are the highest (fig. 3) and are lowest 

in the center of the county. Variability in precipitation is due 

to factors such as topography (McCrary, 2011), the variable 

distribution of precipitation from conductive thunderstorms, 

and proximity to urban heat islands (Bornstein and Lin, 2000). 

It is important to consider the variations in precipitation 

among the 12 LTTM program watersheds when comparing 

discharge and water quality.
20 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

45.17

46.94

02205865

43.02

42.52

45.71

02205000

40.13

48.09

49.46

48.29

49.38

02208130

42.61

46.58

46.58

55.12

49.08

49.25

02205522

45.67

45

46

45

46

47

48

45

46

47

49

48

47

50

50

51

52

53

54

55

48

49

44

43

42

41

47

48

49

44

43

0 1 2 3 4 5 MILES

0

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

1 2 3 4 5 KILOMETERS

Chattahooche e River

EXPLANATION

Monitored watershed boundary

Eastern Continental Divide

USGS water-quality monitoring station, identifier, and watershed—

 Value shown is precipitation, in inches per year

46 Line of equal mean annual precipitation, 1996–2012—Interval 1 inch

Figure 13

N

Wheeler Creek

watershed

Suwanee Creek 02217274

watershed

02334885

Richland Creek

watershed

02334480

Level Creek

watershed

02334578

Crooked Creek

watershed

02335350

North Fork

Peachtree Creek

watershed

02336030

Yellow River

watershed

02207120

No Business Creek

watershed

02207185

 

Big Haynes Creek

watershed

02207385

Brushy Fork Creek

watershed

02207400

Alcovy River

watershed

02208150

Apalachee River

watershed

02218565

Figure 13. Average annual precipitation for Gwinnett County, Georgia, and the 12 monitored watersheds. 
Hydrologic Inputs and Outputs 21

Annual Runoff and Water Yields

Annual runoff is the volume of streamflow from the 

watershed for a given year. Annual runoff is expressed as 

depth in inches (the volume of runoff divided by the area of 

the watershed) to allow for comparison between watersheds 

with different sized drainage areas and to make comparisons 

with annual precipitation easier. The overall pattern in 

annual runoff (fig. 14A) follows the general pattern in annual 

precipitation (fig. 12), but the ongoing effects of the drought 

can be observed. In the second year of the drought, WY 2008, 

annual precipitation was slightly higher than the previous 

year, but annual runoff continued to decline. Although WYs 

2005 and 2009 had similar amounts of precipitation, WY 2009 

had much lower runoff because a larger portion of precipitation recharged (replenished) groundwater storage instead 

of resulting in runoff. The variations in annual runoff are 

pronounced, with annual runoffs about half as much during the 

two drought years, WYs 2007 and 2008, as during WY 2005, 

an average precipitation year. In most years, runoff was higher 

in the Yellow River, Crooked Creek, North Fork Peachtree 

Creek, and Richland Creek watersheds and does not appear to 

be associated with higher precipitation for those watersheds 

(figs. 12 and 13).

Water yield is the portion of precipitation that results in 

runoff as opposed to being evapotranspired or resulting in 

changes in groundwater storage within the watershed. Annual 

water yield is calculated as annual runoff divided by annual 

precipitation, expressed as a percentage. Water yields are 

higher in wetter years, when a greater percentage of precipitation results in runoff as opposed to being evapotranspired and 

recharging groundwater storage. Water yields decreased in 

successive years of the drought, WYs 2007 and 2008, even 

though precipitation was fairly similar during those 2 years. 

Declining groundwater storage during the drought resulted in 

decreasing stream base flows, and a larger portion of precipitation in the second year of the drought was evapotranspirated 

instead of recharging the groundwater.

Water yields for the WY 2004–09 period are highest for 

the North Fork Peachtree Creek (62.1 percent), Crooked Creek 

(51.9 percent), Richland Creek (48.8 percent), and Yellow 

River (48.5 percent) watersheds and is lowest in the Level 

Creek watershed (36.4 percent; fig. 14B). High water yields in 

the North Fork Peachtree Creek, Crooked Creek, and Yellow 

River watersheds are likely related to impervious areas (fig. 9; 

table 3), because these three watersheds have the three highest 

percentages of impervious areas of the 12 watersheds, 52.79, 

36.81, and 23.63 percent of their watershed areas, respectively. 

Richland Creek high water yields are likely the result of 

having the highest basin slope of any of the 12 watersheds 

(13.8 percent; fig. 5; table 3). Interestingly, annual water yields 

for the Richland Creek watershed during the 3 years with 

the lowest precipitation, WYs 2006–08, were higher than all 

but one watershed, while in more average years, water yields 

were not notable relative to the other watersheds. This finding 

appears to indicate that the effect of high basin slope in this 

watershed is more pronounced on water yield during dry 

conditions when runoff is reduced.
22 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

0 

5 

10 

15 

20 

25 

30 

Gwinnett

County

runoff

average

Gwinnett

County

water yield

35 

40 

45 

Runoff, in inches per year 

EXPLANATION

Monitoring stations

Yellow

River

No Business Creek

Big

Haynes Creek

Brushy

Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Figure 14.

0

10

20

30

40

50

60

70

80

90

2004 2005 2006 2007 2008 2009

Water yield, in percent

Water year

A. Runoff

B. Water yield

2004–2009 

Figure 14. A, annual runoff totals and B, annual water yields for each of the 12 monitoring stations in 

Gwinnett County, Georgia. Gwinnett County average represents the spatial average of the area of the 12 

monitored watersheds. 
Surface-Water Quality 23

Surface-Water Quality

Patterns in continuous water-quality monitoring data for 

specific conductance (SC) and turbidity and their relation to 

other water-quality constituents were assessed. Statistical summaries of 13 measured or calculated constituents are provided 

for each of the 12 watersheds. Comparisons are made between 

base-flow and storm-composited concentrations.

Continuous Water-Quality Monitoring

Water-quality monitors were installed at all gages to continuously measure water temperature, SC, and turbidity. These 

continuous water-quality data are used to assess relations with 

constituents analyzed from base-flow and stormflow samples. 

For example, SC is related to the type and concentration of 

dissolved constituents in water and, thus, is a useful surrogate 

for total dissolved solids or other dissolved constituents. 

Specific conductance typically varies inversely with 

discharge—a process referred to as dilution—at the 12 LTTM 

program stations (fig. 15). Dilution is a commonly observed 

relation that occurs for many dissolved constituents when 

during stormflow there is a larger proportion of dilute water 

from precipitation runoff and shallow flow paths, compared 

to base-flow concentrations, contributing to discharge. This is 

not always the case, because some dissolved constituents are 

mobilized and transported during higher discharges.

Specific conductance varied by watershed area, with 

larger watersheds having higher mean daily values typically 

between 100–300 microsiemens per centimeter (μS/cm) at 

25 degrees Celsius, and the smaller watersheds having lower 

values typically between 50–100 μS/cm. Mean daily SC 

ranged from 9 μS/cm at Brushy Fork Creek watershed to 

651 μS/cm at North Fork Peachtree Creek watershed. The high 

mean daily SC at North Fork Peachtree Creek was observed 

after a snowstorm and was attributed to snowmelt containing 

road salts. 

Turbidity is an optical water-quality property related 

to the amount of light scattered or absorbed by suspended 

and dissolved matter and is a good surrogate for suspended 

sediment. Turbidity can also be a surrogate for a wide range 

of other constituents, because certain pollutants, such as heavy 

metals, pesticides, and fecal coliform, adhere to suspendedsediment particles. The U.S. Environmental Protection Agency 

(2010) lists sediment and other solid-phase constituents as 

some of the most common sources of impairment in Georgia 

streams. Good relations were observed during this study 

between turbidity and many of the water-quality constituents 

measured during base-flow and stormflow sampling. For 

example, relations for suspended sediment, total zinc, and total 

suspended solids are shown for the Yellow River Watershed 

(fig. 16). Because turbidity is continuously monitored and 

is highly related to many of the water-quality constituents 

analyzed as part of this program, turbidity was included as a 

possible explanatory variable for the concentration models 

used to estimate loads.

Figure 15.

Specific conductance

Discharge

0

20

40

60

80

100

120

140

160

0

5 6

January 2009

7

10

20

30

40

50

60

70

80

90

100

Specific conductance, in microsiemens per centimeter

Discharge, in cubic feet per second

Figure 15. Dilution effects of specific conductance and discharge at station 02336030, North Fork Peachtree Creek near 

Doraville, Georgia, January 5–7, 2009.
24 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Turbidity is the expression of the optical properties of liquid that causes light rays to be 

scattered rather than transmitted in straight lines through a sample. Turbidity, which can 

make water appear cloudy or muddy, is caused by the presence of suspended and dissolved matter and microscopic organisms (ASTM International, 2003). 

Turbidity, measured in formazin nephelometric units (FNU), is continuously monitored at 

15-minute intervals and reported in near real-time for all 12 stations. 

The direct correlation between turbidity and many of the analyzed constituents is primarily 

due to the analyte’s adsorption to the sediment particles, or because the constituent 

concentrations vary similarly to variations in sediment concentrations, by similar sources as 

the sediment being transported. All regression equations used in the estimation of annual 

loads were evaluated for relations between the continuously monitored water-quality 

constituents, specific conductance and turbidity, discharge, season, and time. Turbidity was 

consistently found to be a strong explanatory variable and was included whenever statistically significant. 

Figure 16. Turbidity in relation to suspended-sediment 

concentration, total zinc, and total suspended solids at 

station 02207120 Yellow River near Lithonia, Georgia.

Figure 16.

1

10

5 10

100

100 600 5 10 100 600

1,000

10 100

1

1

10

100

1,000

1,000

Turbidity, in formazin nephelometric units

Turbidity, in formazin nephelometric units

Total suspended solids, in milligrams per liter

Total zinc, in micrograms per liter

Suspended-sediment concentration, 

in milligrams per liter

10

4

100
Surface-Water Quality 25

Base-flow and Stormflow Water Quality

Statistical summaries of the water-quality sample concentrations for 13 field and laboratory constituents (table 2) are 

provided by watershed in appendix tables 1–1 through 1–12. 

Graphical summaries for the 13 constituents in table 2 for both 

base-flow and stormflow conditions are shown in figure 17.

Specific conductance typically varied between 50 and 

200 μS/cm (fig. 17A). Specific conductance was lower during 

stormflow than base flow at all 12 watersheds, indicating 

that dissolved constituents generally were diluted during 

stormflow conditions. Specific conductance typically was 

highest for Yellow River and Suwannee Creek, the two largest 

watersheds in the study area, with drainage areas of 162 and 

47 square miles, respectively (table 1). Turbidity typically 

varied between 2 and 20 FNU for base-flow samples and 

between 20 and 2,000 FNU for stormflow samples (fig. 17B). 

Turbidity values generally are about 1.5 orders of magnitude 

higher in stormflow, reflecting the transport of sediment 

during these conditions.

Nitrogen concentrations typically varied between 0.2 

and 4 milligrams per liter as nitrogen (mg/L as N) for total 

nitrite plus nitrate (NO2 + NO3; fig. 17C), between 0.15 and 

4 mg/L as N for total ammonia plus organic nitrogen (TKN; 

fig. 17D), and between 0.5 and 4 mg/L as N for total nitrogen 

(TN; fig. 17E). Differences between base flow and stormflow 

NO2 + NO3 concentrations varied across the watersheds, with 

lower concentrations observed during stormflow at Yellow 

River, Big Haynes Creek, Wheeler Creek, and Suwanee Creek 

watersheds, higher concentrations observed during stormflow 

at Level Creek watershed, and similar ranges in concentrations 

observed at the other watersheds. Total ammonia plus organic 

nitrogen concentrations in stormflow samples were higher 

than concentrations in base-flow samples at all the watersheds, 

and TN concentrations in stormflow samples were higher than 

concentrations in base-flow samples at most of the watersheds. 

Stormflow sample concentrations were typically about twice 

as high as base-flow sample concentrations. Both Yellow 

River and No Business Creek watersheds have similar TN 

concentrations for base-flow and stormflow samples, and the 

increase in TN concentrations during stormflow observed for 

Suwannee Creek watershed was negligible. 

Total phosphorus concentrations typically varied 

between 0.005 and 1 mg/L as phosphorus (P) (fig. 17F). 

Total phosphorus concentrations typically were greater than 

0.1 mg/L, which is the threshold at which TP concentrations 

are considered able to support nuisance levels of algal 

production in flowing waters (U.S. Environmental Protection 

Agency, 2000a). Stormflow TP concentrations were about an 

order of magnitude higher than during base flow, indicating 

that the majority of transport occurred during storm events. 

Dissolved phosphorus concentrations typically were less than 

0.1 mg/L as P and exhibited no particular pattern of differences between base-flow and stormflow conditions (fig. 17G). 

A comparison of mean DP and TP concentrations (tables 1–1 

to 1–12) indicates that about two-thirds to three-quarters of the 

phosphorus exists in solid form; therefore, it is not unexpected 

that TP concentrations are higher when discharge and 

suspended-sediment concentrations are higher. Richland Creek 

watershed had the highest mean stormflow TP concentration, 

0.267 mg/L as P, which may be the result of high runoffs 

(fig. 14A) and water yields (fig. 14B) associated with the 

watershed having the highest basin slope of the 12 watersheds, 

(fig. 5; table 3). No Business Creek had the lowest mean 

stormflow TP concentration of 0.066 mg/L as P.

Total organic carbon (TOC) typically varied between 0.5 

and 4 mg/L as carbon in base-flow samples and between 2 

and 20 mg/L as C in stormflow samples (fig. 17H). Stormflow 

TOC concentrations are about half an order of magnitude 

higher than during base flow, but comparisons varied by 

watershed. Stormflow TOC concentrations were fairly similar 

among all 12 watersheds, whereas individual watersheds had 

more distinct base-flow concentrations.

Total lead (Pb) concentrations typically varied between 

0.1 and 10 μg/L in base-flow samples and between 0.5 and 

100 μg/L in stormflow samples (fig. 17I). Base-flow Pb 

concentrations had a large range at 6 of the 12 watersheds, 

with about a 1.5 order of magnitude range between the 25th 

and 75th percentiles of concentrations. Stormflow Pb concentrations were about a half an order of magnitude higher than 

base-flow concentrations.

Total zinc (Zn) concentrations typically varied between 

2 and 500 μg/L (fig. 17J). Stormflow Zn concentrations 

were about one order of magnitude higher than base-flow 

concentrations, but comparisons varied by watershed. 

Richland Creek and Crooked Creek watersheds exhibited the 

largest order of magnitude increases between base-flow and 

stormflow Zn concentrations. Alcovy River, Richland Creek, 

Crooked Creek, and North Fork Peachtree Creek watersheds 

had the highest stormflow Zn and Pb concentrations. These 

high concentrations may be related to the high basin slope at 

Richland Creek (fig. 5; table 3) and the high percentage of 

impervious areas for Crooked Creek and North Fork Peachtree 

Creek (fig. 9; table 3).

Total dissolved solids (TDS) concentrations represent 

the amount of inorganic and organic constituents that are 

dissolved in water; concentrations typically varied between 20 

and 200 mg/L (fig. 17K). The U.S. Environmental Protection 

Agency has established TDS as a secondary drinking water 

standard with a maximum contaminant level of 500 mg/L, 

because high concentrations of TDS can be an indicator 

of elevated levels of other pollutants (U.S. Environmental 

Protection Agency, 2011). Five of the watersheds, Yellow 

River, Level Creek, Suwanee Creek, Crooked Creek, and 

North Fork Peachtree Creek, exhibited dilution in TDS 

concentrations during storm events. Four of these watersheds, 

Yellow River, Suwanee Creek, Crooked Creek, and North 

Fork Peachtree Creek, have the four highest percentages of 

impervious area (fig. 9; table 3). Yellow River watershed had 

the highest mean TDS concentrations of 95 mg/L (table 1–1) 

of the 12 watersheds. Richland Creek watershed had higher 

stormflow TDS and also has the highest basin slope of any of 
26 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

E. Total nitrogen

Concentration, in milligrams per liter as N

F. Total phosphorus

A. Specific conductivity B. Turbidity

C. Total nitrite plus nitrate D. Total ammonia plus organic nitrogen

8

7

6

5

4

3

2

1

0

Figure 17.

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Level Creek

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Suwanee Creek

Crooked Creek

North Fork 

 Peachtree Creek

Level Creek

0.001

0.01

0.1

1

10

Concentration, in milligrams per liter as P

x

x

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x

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69 77 49 59 88 79 89 75 70 75 50 58 47 53 49 56 49 57 72 84 70 82 50 58 69 76 49 58 88 78 89 75 70 74 50 57 47 53 50 55 48 57 85 84 82 81 50 57

x

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10

100

1,000

10,000 63 82 47 67103 8286 84 68 75 47 59 72 80 67 76 93 99 97 89 97 95 46 64

Conductance, in microsiemens per centimeter

at 25 degrees Celsius

x

x

x

x

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o

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x

x

x

x

0.1

1

10

100

1,000

10,000 62 73 48 60 88 78 79 81 65 73 48 63 73 77 50 98 88 94 81 98 94 86 44 58

Turbidity, in formazin nephelometric units

x

x

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x x

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oo

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x x

1

10

0.1

60 68 43 49 85 64 81 63 63 65 47 51 42 43 40 48 44 47 82 69 73 69 44 48

Concentration, in milligrams per liter as N 

x

x

x x

x

x

x

x

x

x

x

x

x

x

0.1

1

10 64 69 46 49 84 67 85 66 65 65 47 52 44 43 44 48 46 49 71 71 65 71 46 49

Concentratoin, in milligrams per liter as N 

Figure 17. Boxplots of concentrations for A, specific conductance, B, turbidity, C, total nitrite plus nitrate, D, total 

ammonia plus organic nitrogen, E, total nitrogen, F, total phosphorus, G, dissolved phosphorus, H, total organic 

carbon, I, total lead, J, total zinc, K, total dissolved solids, L, total suspended solids, and, M, suspended sediment at 12 

monitored watersheds in Gwinnett County, Georgia. Data collection period ranges from 1996 to 2009.
Surface-Water Quality 27

Concentration, in milligrams per liter

100

10

1

1,000

10,000

L. Total suspended solids

Figure 17. —Continued

68 77 49 59 87 79 88 75 67 74 50 58 47 53 50

56

49 57 79 85 81 82 50 58

x

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G. Dissolved phosphorus H. Total organic carbon

Concentration, in milligrams per liter Concentration, in micrograms per liter

Concentration, in micrograms per liter

10

5

100

1,000

K. Total dissolved solids

10

1.0

100

10

1

100

1,000

I. Total lead J. Total zinc

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Level Creek

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Suwanee Creek

Crooked Creek

North Fork 

 Peachtree Creek

Level Creek

x

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69 77 49 59 88 78 89 76 68 75 50 58 47 53 50 57 48 58 68 85 71 82 50 58

69 77 32 39 87 79 88 75 66 74 31 41 30 36 31

40 30 42 69

85

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82

31 39

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70 81 33 64 88 79 89 75 67 74 31 47 29 36 32 45 32 45 71 89 72 82 32 45

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29 38 46 61 43 37 27 37 27 34 47 55 45 52 49 58 48 58 39 38 37 38 49 60

Concentration, in milligrams per liter

x

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0.2

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0.4 65 65 43 48 85 63 85 65 64 63 47 52 44 41 42 47 45 47 80 71 74 68 47 48

Concentratoin, in milligrams per liter as P 

Figure 17. (Continued) Boxplots of concentrations for A, specific conductance, B, turbidity, C, total nitrite plus nitrate, 

D, total ammonia plus organic nitrogen, E, total nitrogen, F, total phosphorus, G, dissolved phosphorus, H, total organic 

carbon, I, total lead, J, total zinc, K, total dissolved solids, L, total suspended solids, and, M, suspended sediment at 12 

monitored watersheds in Gwinnett County, Georgia. Data collection period ranges from 1996 to 2009.
28 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

M. Suspended sediment

Figure 17. —Continued

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Suwanee Creek

Crooked Creek

North Fork 

 Peachtree Creek

Level Creek

EXPLANATION

x

x

o

o

30

75th percentile

90th percentile

10th percentile

Upper outside

Lower outside

Upper detached

Lower detached

Number of samples 

25th percentile

Median

Base flow

Stormflow

x

x

x

x

x

x

x

x

x

0.1

1

10

100

1,000

10,000

100,000 12 35 21 38 15 35 14 37 19 33 19 42 34 51 33 59 35 58 22 42 17 36 24 36

Concentration, in milligrams per liter

Figure 17. (Continued) Boxplots of concentrations for A, specific conductance, B, 

turbidity, C, total nitrite plus nitrate, D, total ammonia plus organic nitrogen, E, total 

nitrogen, F, total phosphorus, G, dissolved phosphorus, H, total organic carbon, I, total 

lead, J, total zinc, K, total dissolved solids, L, total suspended solids, and, M, suspended 

sediment at 12 monitored watersheds in Gwinnett County, Georgia. Data collection period 

ranges from 1996 to 2009.

One of the leading causes of stream impairment in Georgia is sediment-associated constituents 

(U.S. Environmental Protection Agency, 2010). Annual yield of TSS is the primary performance 

criterion for suspended sediment in Gwinnett County’s Watershed Protection Plan (Gwinnett 

County Department of Public Utilities, 2000). Sources of in-stream sediment can be highly variable. Sediment transport can occur from natural processes such as resuspension of streambed 

sediment or from land disturbance from development and urbanization. Land disturbances and 

extreme hydrologic events can cause streambanks to collapse and fail, adding to the alluvial 

sediment being transported. Runoff from impervious surfaces also contributes to the fluvial 

sediments and contaminant attributed in the annual sediment yields. Certain contaminants, such 

as trace elements and pesticides, sometimes preferentially partition onto sediment; thus, their 

distribution throughout the watersheds is controlled primarily by sediment transport (Hardy and 

others, 2005).
Water-Quality Seasonality and Long-Term Trends 29

the 12 watersheds (fig. 5; table 3). For the remaining seven 

watersheds, base-flow and stormflow TDS concentrations 

varied in similar, overlapping ranges. Variations in TDS are 

difficult to interpret because TDS comprises all the dissolved 

constituents, each with their own particular relations, into a 

single measure.

Total suspended solids (TSS) and suspended-sediment 

concentrations (SSC) typically varied between 1 and 

2,000 mg/L (fig. 17L and 17M). Total suspended solids and 

SSC concentrations are similar, with the same pattern of 

relative difference between watersheds observed for both 

constituents that is especially apparent in the stormflow 

concentrations. Suspended-sediment concentrations typically 

were slightly higher than TSS concentrations during both 

base-flow and stormflow conditions. Stormflow TSS and SSC 

concentrations are about two orders of magnitude higher than 

baseflow concentrations, emphasizing the relation between 

discharge and sediment mobilization and transport. Richland 

Creek watershed, which has the highest basin slope of any of 

the 12 watersheds (fig. 5; table 3), had the highest mean TSS 

concentration, 592 mg/L (appendix table 1–8). The two orders 

of magnitude increase of TSS concentrations from base flow 

to stormflow compared to the one order of magnitude increase 

for TP concentrations suggests that the suspended sediment 

transported during storm events has a disproportionately 

smaller ratio of TP compared to base-flow conditions. Total 

suspended solids concentrations are about three orders of 

magnitude greater than TP in storm event samples, indicating 

that TP is only a small portion of TSS.

In a previous analysis of the Gwinnett County LTTM program watersheds for six watersheds for the period 1998–2003, 

(Landers and others, 2007), concentrations (and loads) of 

TSS typically were less than concentrations (and loads) of 

SSC. The comparison of TSS to SSC was reevaluated using 

data from all 12 watersheds for the period 2005–09 (fig. 18; 

Landers, 2013). Measured TSS concentrations continue to 

be less than SSC concentrations. This difference is more 

pronounced for samples in which the sediment contains more 

than 25 percent sand (the coarser portion of the sediment).

Water-Quality Seasonality and LongTerm Trends

Both seasonal patterns and long-term trends for five 

representative water-quality constituents—TN, TP, TDS, TSS, 

and Zn—were examined in base-flow and stormflow samples. 

These patterns and trends can be the result of natural variability, such as seasonal biogeochemical processes, climatic 

variations, and human influences. The effects of variations in 

discharge, for which a strong relation is observed for most of 

the water-quality constituents in this study, have been removed 

by using flow-adjusted residual concentrations to model-fitted 

logarithmic concentration-discharge relations.

Suspended-sediment concentration, 

in milligrams per liter

10

10

100

100

1,000

1,000

10,000

10,000

Equal-value line

EXPLANATION

Total suspended solids, in milligrams per liter

Sand less than 25 percent

Sand greater than 25 percent

Figure 18. Figure 18. Concentrations of total suspended solids 

and suspended-sediment concentrations for 380 storm 

samples collected in watersheds in Gwinnett County, 

Georgia (modified from Landers, 2013).

Seasonality in Water Quality

An analysis of seasonal patterns in base-flow and 

stormflow samples was performed for five representative 

water-quality constituents—TN, TP, TDS, TSS, and Zn—on 

flow-adjusted residual concentrations. Base-flow residuals 

typically did not exhibit any significant seasonal variations at 

any of the 12 watersheds. Seasonal terms were significant in 

all the stormflow concentration regression models (for all five 

constituents and all 12 watershed combinations). Seasonality 

is considered significant when the seasonal sine and cosine 

explanatory variables in the regression equations used for load 

estimation are significant for p-values less than or equal to 0.1. 

Total nitrogen and TP seasonal patterns were fairly 

similar at all 12 watersheds, with the highest concentrations 

typically between May through September and the lowest 

concentrations between November and January (figs. 19–20). 

The only obvious exception to this was for the Suwanee Creek 

watershed, which had high TN and TP concentrations in 

January that were similar to summer peak concentrations. The 

magnitude of the seasonal variations ranged from about 0.1 to 

0.3 logarithmic unit for TN and about 0.2 to 0.4 logarithmic 

unit for TP.
30 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 19. 

02207120 Yellow River 02207185 No Business Creek

–1.0

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

Month

–1.0

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total nitrite plus nitrate residuals, in milligrams per liter as nitrogen

02207400 Brushy Fork Creek

02208150 Alcovy River 02217274 Wheeler Creek

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

–1.0 –1.0

–0.5

0

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1.0

–1.0

–0.5

0

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1.0

02207385 Big Haynes Creek

Figure 19. Seasonal patterns in stormflow total nitrogen flow-adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots 

(LOWESS) curve is shown to graphically identify the seasonal pattern.
Water-Quality Seasonality and Long-Term Trends 31

02218565 Apalachee River

02334885 Suwanee Creek

02335350 Crooked Creek 02336030 North Fork Peachtree Creek

–1.0

–0.5

0

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1.0

–1.0

–0.5

0

0.5

1.0

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

Month

Logarithm of total nitrite plus nitrate residuals, in milligrams per liter as nitrogen

02334480 Richland Creek

–1.0

–0.5

0

0.5

1.0

–0.5

0

0.5

1.0

02334578 Level Creek

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

–1.0

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Figure 19.—Continued

Figure 19. (Continued) Seasonal patterns in stormflow total nitrogen flow-adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing 

scatterplots (LOWESS) curve is shown to graphically identify the seasonal pattern.
32 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 20. 

Logarithm of total phosphorus residuals, in milligrams per liter as phosphorus

02207400 Brushy Fork Creek

02208150 Alcovy River 02217274 Wheeler Creek

02207385 Big Haynes Creek

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

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1.50

–1.50

–0.75

0

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–0.75

0

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1.50

–1.50

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

02207120 Yellow River 02207185 No Business Creek

–0.75

0

0.75

1.50

–0.75

0

0.75

1.50

–1.50 –1.50

Figure 20. Seasonal patterns in stormflow total phosphorus flow-adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots 

(LOWESS) curve is shown to graphically identify the seasonal pattern.
Water-Quality Seasonality and Long-Term Trends 33

02334480 Richland Creek

02334578 Level Creek

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

02218565 Apalachee River

02334885 Suwanee Creek

02335350 Crooked Creek 02336030 North Fork Peachtree Creek

Figure 20.—Continued

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

Logarithm of total phosphorus residuals, in milligrams per liter as phosphorus

Figure 20. (Continued) Seasonal patterns in stormflow total phosphorus flow-adjusted residual concentrations for the 

12 monitored watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing 

scatterplots (LOWESS) curve is shown to graphically identify the seasonal pattern.
34 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Seasonal patterns in TDS were small, typically less than 

0.1 logarithmic unit, but getting as high as 0.2 logarithmic 

unit (fig. 21). For the few watersheds that had large seasonal 

variations (Apalachee River, Richland Creek, and North 

Fork Peachtree Creek), peak concentrations were observed 

in July and August and the lowest TDS concentrations were 

in December.

Stormflow TSS flow-adjusted residual concentrations 

are plotted by month for each watershed (fig. 22). LOWESS 

curves indicate that the timing of the seasonal patterns in 

TSS concentrations were fairly similar at all 12 watersheds, 

typically highest in June through September and lowest in 

December through March. These seasonal patterns may be 

related to seasonal variations in rainfall intensities; winter 

rainstorms are characterized by typically low-intensity frontal 

systems, whereas spring and summer rainstorms are characterized by intense convective thunderstorms. Intense rainfall 

can result in greater storm-related erosion due to higher peak 

streamflows and the higher occurrence of overland flow. 

Increased summer TSS concentrations may also be related to 

seasonal construction, which can result in increased wash-off 

of sediments. On the basis of the LOWESS curves, seasonal 

TSS flow-adjusted residual concentrations ranged from about 

0.3 to about 0.6 logarithmic unit in the 12 watersheds. Yellow 

River, No Business Creek, Wheeler Creek, and Suwanee 

Creek had the largest seasonal variations. Yellow River and 

Suwanee Creek watersheds are the two largest watersheds 

in the study, with drainage areas of 162 and 47 square miles, 

respectively, and Wheeler Creek is the smallest watershed in 

the study with a drainage area of 1.31 square miles (table 1).

Seasonal patterns in total Zn concentrations were fairly 

similar at all 12 watersheds, with the highest concentrations 

typically between June through September and the lowest 

concentrations between November and February (fig. 23). The 

magnitude of the seasonal variations ranged from about 0.2 to 

0.5 logarithmic unit.
Water-Quality Seasonality and Long-Term Trends 35

02207120 Yellow River 02207185 No Business Creek

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

–1.0

Figure 21.

02208150 Alcovy River 02217274 Wheeler Creek

02207385 Big Haynes Creek 02207400 Brushy Fork Creek

–1.0

–0.5

0

0.5

1.0

–1.0

–0.5

0

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1.0

–0.5

0

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–0.5

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–1.0

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total dissolved solids residuals, in milligrams per liter

Figure 21. Seasonal patterns in stormflow total dissolved solids flow-adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots 

(LOWESS) curve is shown to graphically identify the seasonal pattern.
36 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 21.—Continued

02334885 Suwanee Creek

02335350 Crooked Creek 02336030 North Fork Peachtree Creek

02218565 Apalachee River

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

–0.5

0

0.5

1.0

–0.5

0

0.5

1.0

–1.0 –1.0

–1.0

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total dissolved solids residuals, in milligrams per liter

02334578 Level Creek

02334480 Richland Creek

–1.0

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

Figure 21. (Continued) Seasonal patterns in stormflow total dissolved solids flow-adjusted residual concentrations for the 

12 monitored watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing 

scatterplots (LOWESS) curve is shown to graphically identify the seasonal pattern.
Water-Quality Seasonality and Long-Term Trends 37

–1.50

–0.75

0

0.75

1.50

02207185 No Business Creek 

–1.50

–0.75

0

0.75

1.50

02207120 Yellow River 

Figure 22.

–0.75

0

0.75

1.50

02208150 Alcovy River 

–1.50

–0.75

0

0.75

1.50

02207385 Big Haynes Creek 

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

–1.50 –2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

02217274 Wheeler Creek 

02207400 Brushy Fork Creek 

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total suspended solids residuals, in milligrams per liter

Figure 22. Seasonal patterns in stormflow total suspended solids flow-adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots (LOWESS) 

curve is shown to graphically identify the seasonal pattern.
38 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

–1.50

–0.75

0

0.75

1.50

02334578 Level Creek 

–2

–1

0

1

2

02334480 Richland Creek 

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

02218565 Apalachee River 

–1.50

–0.75

0

0.75

1.50

02335350 Crooked Creek 

–1.50

–0.75

0

0.75

1.50

02336030 North Fork Peachtree Creek 

–1.50

–0.75

0

0.75

1.50

02334885 Suwanee Creek 

Figure 22.—Continued

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total suspended solids residuals, in milligrams per liter

Figure 22. (Continued) Seasonal patterns in stormflow total suspended solids flow-adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots 

(LOWESS) curve is shown to graphically identify the seasonal pattern.
Water-Quality Seasonality and Long-Term Trends 39

02207120 Yellow River 02207185 No Business Creek

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

Figure 23.

02207400 Brushy Fork Creek

02208150 Alcovy River 02217274 Wheeler Creek

02207385 Big Haynes Creek

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total zinc residuals, in micrograms per liter

Figure 23. Seasonal patterns in stormflow total zinc flow-adjusted residual concentrations for the 12 monitored watersheds 

in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots (LOWESS) curve is 

shown to graphically identify the seasonal pattern.
40 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 23.—Continued

02218565 Apalachee River

02334885 Suwanee Creek

02335350 Crooked Creek 02336030 North Fork Peachtree Creek

–0.75

0

0.75

1.50

–1.50

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

–0.75

0

0.75

1.50

–1.50

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Logarithm of total zinc residuals, in micrograms per liter

02334480 Richland Creek

02334578 Level Creek

–1.50

–0.75

0

0.75

1.50

–1.50

–0.75

0

0.75

1.50

Figure 23. (Continued) Seasonal patterns in stormflow total zinc flow-adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. A line representing a locally weighted regression and smoothing scatterplots 

(LOWESS) curve is shown to graphically identify the seasonal pattern.
Water-Quality Seasonality and Long-Term Trends 41

Long-Term Trends in Water Quality

Long-term trends in TN, TP, TDS, TSS and Zn were 

assessed for both base-flow and storm-event flow- and seasonally adjusted residual concentrations. Long-term trends were 

considered to be significant when the linear time explanatory 

variable in the regression equations used for load estimation 

was significant for a p-value of less than or equal to 0.1. A 

trend analysis of annual runoff for the same periods as the 

water-quality trend analysis indicated no significant long-term 

trends in runoff. Trends in runoff may result in trends in waterquality constituents that are the result of climatic variability.

Base-flow residuals did not exhibit any significant 

long-term trends for any of the five constituents or for any of 

the 12 watersheds; however, long-term trends in storm-event 

flow- and seasonally adjusted residual concentrations occurred 

frequently. For the 12 watersheds and five constituent combinations (60 “cases”), no trend (NT) occurred in 24 cases, 

an increasing trend (+) occurred in 9 cases, and a decreasing 

trend (–) occurred in 27 cases (table 4). Trends were detected 

in almost equal frequency whether the trends represented a 

10- to 11-year period (11 of 30 cases had trends) or a 15-year 

period (13 of 30 cases had trends), indicating that the period of 

record may not have been an important factor in determining 

whether or not a trend was detected. 

Stormflow TN, TP, TDS, TSS, and Zn flow- and seasonally adjusted residual concentrations are plotted in relation to 

time for each watershed (figs. 24–28). A linear least-squares 

trend line is shown on plots that had significant trends. For 

TN, Suwanee Creek has an increasing trend whereas four 

of the watersheds—No Business Creek, Big Haynes Creek, 

Apalachee River, and North Fork Peachtree Creek—have 

a decreasing trend. The slopes of the trend lines are small, 

indicating that the changes are minimal.

Trends in storm-event TP residuals were consistent across 

the county, with all but one of the watersheds having decreasing trends (fig. 25; table 4). No Business Creek watershed 

showed no significant trend. The slopes of the trend lines are 

small, indicating that the changes are minimal.

Seven of 12 watersheds exhibited increasing trends in 

stormflow TDS residuals (Yellow River, Brushy Fork Creek, 

Alcovy River, Wheeler Creek, Apalachee River, Level Creek, 

and Suwanee Creek), whereas one watershed (No Business 

Creek) had a decreasing trend (fig. 26; table 4). Unlike TN and 

TP, the TDS trends typically were increasing and the slopes of 

the trend lines were greater, indicating that TDS trends were 

likely the result of trends in dissolved constituents rather than 

the dissolved components of TN and TP.

Table 4. Water-quality constituent trends and statistical significance for stormflow samples in 12 monitored watersheds in 

Gwinnett County, Georgia. 

[p-value, probability of statistical significance; NT, no significant linear trend, p-value greater than 0.10; +, positive or upward trend; –, negative or downward 

trend; <, less than]

USGS 

station 

number

Basin Record 

(years)

Total nitrogen Total phosphorus Total

dissolved solids

Total

 suspended solids Total zinc

Trend p-value Trend p-value Trend p-value Trend p-value Trend p-value

02207120 Yellow River 15 NT 0.94 – < 0.01 + < 0.01 NT 0.16 NT 0.67

02207185 No Business Creek 11 – < 0.01 NT 0.49 – < 0.01 – 0.02 – 0.03

02207385 Big Haynes Creek 15 – < 0.01 – < 0.01 NT 0.32 – < 0.01 NT 0.32

02207400 Brushy Fork Creek 15 NT 0.17 – < 0.01 + 0.02 – < 0.01 NT 0.68

02208150 Alcovy River 15 NT 0.61 – < 0.01 + < 0.01 – 0.06 NT 0.52

02217274 Wheeler Creek 10 NT 0.79 – < 0.01 + 0.04 – < 0.01 – 0.02

02218565 Apalachee River 10 – 0.03 – < 0.01 + < 0.01 NT 0.2 – 0.05

02334480 Richland Creek 10 NT 0.44 – < 0.01 NT 0.12 – 0.06 NT 0.21

02334578 Level Creek 10 NT 0.32 – < 0.01 + < 0.01 NT 0.29 NT 1.00

02334885 Suwanee Creek 15 + < 0.01 – < 0.01 + < 0.01 – < 0.01 – 0.10

02335350 Crooked Creek 15 NT 0.19 – < 0.01 NT 0.26 NT 0.62 NT 0.92

02336030 North Fork 

Peachtree Creek

10 – 0.07 – < 0.01 NT 0.92 + 0.03 NT 0.61
42 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

–1.0

–0.5

0

0.5

1.0

02207185 No Business Creek

10/1/199210/1/199510/1/199810/1/200110/1/200410/1/200710/1/201010/1/201310/1/201410/1/199810/1/200010/1/200210/1/200410/1/200610/1/200810/1/201010/1/2011

–1.0

–0.5

0

0.5

1.0

02207120 Yellow River

Figure 24.

10/1/199210/1/199510/1/1998

10/1/1998

10/1/2001

10/1/2001

10/1/2004

10/1/2004

10/1/2007

10/1/2007

10/1/2010

10/1/2010

10/1/2013

10/1/2013

10/1/2014

–1.0

–0.5

0

0.5

1.0 02207400 Brushy Fork Creek

10/1/19923/24/19989/14/2003

3/6/2009

8/27/2014

–1.0

–0.5

0

0.5

1.0 02208150 Alcovy River

–1.0

–0.5

0

0.5

1.0

02217274 Wheeler Creek

10/1/19923/24/19989/14/2003

3/6/2009

8/27/2014

–1.0

–0.5

0

0.5

1.0 02207385 Big Haynes Creek

Date Date

Logarithm of total nitrogen residuals, in milligrams per liter as nitrogen

Figure 24. Trends in stormflow total nitrogen flow- and seasonally adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a least-squares 

trend line to graphically identify the trend.
Water-Quality Seasonality and Long-Term Trends 43

Figure 24.—Continued

5/1/1998

1/25/2001

10/22/2003

7/18/20064/13/2009

1/8/2012

10/4/2014

–1.0

–0.5

0

0.5

1.0 02218565 Apalachee River

10/1/19923/24/19989/14/2003

3/6/2009

8/27/2014

–1.0

–0.5

0

0.5

1.0

02334885 Suwanee Creek

–1.0

–0.5

0

0.5

1.0 02335350 Crooked Creek

10/1/199810/1/200110/1/200410/1/200710/1/201010/1/2013

–1.0

–0.5

0

0.5

1.0

02336030 North Fork Peachtree Creek

Date Date

Logarithm of total nitrogen residuals, in milligrams per liter as nitrogen

5/1/1998

1/25/2001

10/22/2003

7/18/20064/13/2009

1/8/2012

10/4/2014

–1.0

–0.5

0

0.5

1.0

02334480 Richland Creek

10/1/199810/1/200010/1/200210/1/200410/1/200610/1/200810/1/201010/1/2012

10/1/199210/1/199510/1/199810/1/200110/1/200410/1/200710/1/201010/1/2013

–1.0

–0.5

0

0.5

1.0

02334578 Level Creek

Figure 24. (Continued) Trends in stormflow total nitrogen flow- and seasonally adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a leastsquares trend line to graphically identify the trend.
44 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

10/1/19922/17/1997

7/6/2001

11/22/2005

4/10/20108/27/2014

–2

–1

0

1

2

02207185 No Business Creek

10/1/1992

10/1/199510/1/199810/1/200110/1/200410/1/200710/1/201010/1/201310/1/2014

10/1/1992

10/1/199510/1/199810/1/200110/1/200410/1/200710/1/201010/1/201310/1/2014

–2

–1

0

1

2

02207120 Yellow River

Figure 25.

10/1/19922/17/1997

7/6/2001

11/22/2005

4/10/2010

8/27/2014

–2

–1

0

1

2

02207400 Brushy Fork Creek 

–2

–1

0

1

2

02208150 Alcovy River 

10/1/199810/1/200110/1/200410/1/200710/1/201010/1/201310/1/2014

–3

–2

–1

0

1

2

02217274 Wheeler Creek

–2

–1

0

1

2 02207385 Big Haynes Creek 

Date Date

Logarithm of total phosphorus residuals, in milligrams per liter as phosphorus

10/1/1992

10/1/199510/1/199810/1/200110/1/200410/1/2007

10/1/201010/1/201310/1/2014

Figure 25. Trends in stormflow total phosphorus flow- and seasonally adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a least-squares 

trend line to graphically identify the trend.
Water-Quality Seasonality and Long-Term Trends 45

10/1/1992

10/1/199510/1/1998

10/1/200110/1/2004

10/1/200710/1/2010

10/1/201310/1/2014

10/1/199210/1/199510/1/199810/1/200110/1/2004

10/1/2007

10/1/2010

10/1/201310/1/2014

10/1/199810/1/200110/1/200410/1/200710/1/2010

10/1/201310/1/2014

Figure 25.—Continued

–2

–1

0

1

2

02218565 Apalachee River

–2

–1

0

1

2

02334885 Suwanee Creek

–2

–1

0

1

2

02335350 Crooked Creek

–2

–1

0

1

2

02336030 North Fork Peachtree Creek

Date Date

Logarithm of total phosphorus residuals, in milligrams per liter as phosphorus

10/1/19922/17/1997

7/6/2001

11/22/2005

4/10/20108/27/2014

–2

–1

0

1

2

02334480 Richland Creek

10/1/19922/17/1997

7/6/2001

11/22/2005

4/10/20108/27/2014

–2

–1

0

1

2

02334578 Level Creek

10/1/199810/1/200110/1/200410/1/200710/1/201010/1/201310/1/2014

Figure 25. (Continued) Trends in stormflow total phosphorus flow- and seasonally adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a leastsquares trend line to graphically identify the trend.
46 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/200910/1/2011

10/1/2002

10/1/200510/1/2008

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/2009

10/1/201110/1/2012

–1.0

–0.5

0

0.5

1.0

02207120 Yellow River

02217274 Wheeler Creek 

10/1/1999

–1.0

–0.5

0

0.5

1.0

10/1/199510/1/199910/1/2003

10/1/200710/1/2011

–1.0

–0.5

0

0.5

1.0

–1.0

–0.5

0

0.5

1.0

10/1/199510/1/199910/1/200310/1/200710/1/2011

–1.0

–0.5

0

0.5

1.0

10/1/199910/1/200210/1/200510/1/200810/1/2011

–1.0

–0.5

0

0.5

1.0

10/1/2011

Figure 26.

Date Date

02207400 Brushy Fork Creek

02208150 Alcovy River 

02207385 Big Haynes Creek 

02207185 No Business Creek

Logarithm of total dissolved solids residuals, in milligrams per liter

Figure 26. Trends in stormflow total dissolved solids flow- and seasonally adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a leastsquares trend line to graphically identify the trend.
Water-Quality Seasonality and Long-Term Trends 47

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/200910/1/2011

–1.0

–0.5

0.0

0.5

1.0

10/1/2011

Figure 26.—Continued

Date Date

02334480 Richland Creek

02334885 Suwanee Creek

02335350 Crooked Creek

02218565 Apalachee River

10/1/200010/1/200310/1/200610/1/200910/1/2011

–0.7

–0.5

–0.3

–0.1

0.1

0.3

0.5

10/1/1999

–1.0

–0.5

0

0.5

1.0

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/2009

–1.0

–0.5

0

0.5

1.0

10/1/200210/1/200510/1/200810/1/2011

Logarithm of total dissolved solids residuals, in milligrams per liter

10/1/200010/1/200310/1/200610/1/200910/1/2011

–1.0

–0.5

0

0.5

1.0

10/1/199910/1/200210/1/200510/1/200810/1/2011

–1.0

–0.5

0

0.5

1.0

02206030 North Fork Peachtree Creek

02334578 Level Creek

Figure 26. (Continued) Trends in stormflow total dissolved solids flow- and seasonally adjusted residual concentrations for 

the 12 monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a 

least-squares trend line to graphically identify the trend.
48 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia Logarithm of total suspended solids residuals, in milligrams per liter

10/1/199410/1/2013

–2

–1

0

1

2 02207400 Brushy Fork Creek

10/1/199410/1/199710/1/2013

–2

–1

0

1

2 02208150 Alcovy River 

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/200910/1/2012

10/1/200010/1/200310/1/200610/1/2009

10/1/2012

10/1/1997

10/1/2000

10/1/2003

10/1/2006

10/1/2009

10/1/2012

10/1/201310/1/199710/1/200010/1/200310/1/200610/1/200910/1/2012

–2

–1

0

1

2 02207385 Big Haynes Creek 

10/1/199810/1/200310/1/2008

–2

–1

0

1

2

02207185 No Business Creek

Figure 27.

Date Date

10/1/2013

–2

–1

0

1

2

02207120 Yellow River

10/1/199410/1/2013

–2

–1

0

1

2 02217274 Wheeler Creek 

10/1/199410/1/199710/1/200010/1/200310/1/200610/1/2009

10/1/2012

10/1/2013

Figure 27. Trends in stormflow total suspended solids flow- and seasonally adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a leastsquares trend line to graphically identify the trend.
Water-Quality Seasonality and Long-Term Trends 49

10/1/200310/1/2008

10/1/200310/1/2008

10/1/200310/1/2008

10/1/200310/1/2008

Figure 27.—Continued

Date Date

10/1/1998

–2

–1

0

1

2

02334578 Level Creek

10/1/1998

–2

–1

0

1

2

02206030 North Fork Peachtree Creek

Logarithm of total suspended solids residuals, in milligrams per liter

10/1/1998

–2

–1

0

1

2

02334480 Richland Creek

10/1/199410/1/199910/1/200410/1/2009

10/1/1999

10/1/2004

10/1/2009

10/1/2013

–2

–1

0

1

2

02334885 Suwanee Creek

10/1/199410/1/2013

–2

–1

0

1

2

02335350 Crooked Creek

10/1/1998

–2

–1

0

1

2 02218565 Apalachee River

10/1/201310/1/2013

10/1/2013

10/1/2013

Figure 27. (Continued) Trends in stormflow total suspended solids flow- and seasonally adjusted residual concentrations for 

the 12 monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a 

least-squares trend line to graphically identify the trend.
50 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

10/1/199210/1/199810/1/200310/1/200810/1/2014

–2

–1

0

1

2 02208150 Alcovy River

–2

–1

0

1

2 02217274 Wheeler Creek

11/8/1992

8/5/19955/1/1998

1/25/2001

10/22/2003

7/18/20064/13/2009

1/8/2012

–2

–1

0

1

2

02207120 Yellow River

Logarithm of total zinc residuals, in milligrams per liter

Figure 28.

Date Date

–2

–1

0

1

2 02207400 Brushy Fork Creek 

10/1/198710/1/199010/1/199310/1/199610/1/199910/1/200210/1/200510/1/200810/1/201110/1/201410/1/198710/1/199010/1/199310/1/199610/1/199910/1/200210/1/200510/1/200810/1/201110/1/2014

–2

–1

0

1

2 02207385 Big Haynes Creek 

8/5/19955/1/1998

1/25/2001

10/22/2003

7/18/20064/13/2009

1/8/2012

10/4/2014

–2

–1

0

1

2

02207185 No Business Creek 

10/1/200110/1/200410/1/200710/1/201010/1/2013

10/1/2014

Figure 28. Trends in stormflow total zinc flow- and seasonally adjusted residual concentrations for the 12 monitored 

watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a least-squares 

trend line to graphically identify the trend.
Water-Quality Seasonality and Long-Term Trends 51

10/1/2001

10/1/2001

10/1/200110/1/200410/1/200710/1/201010/1/201310/1/200410/1/200710/1/201010/1/2013

10/1/200410/1/200710/1/201010/1/2013

10/1/2014

Figure 28.—Continued

Date Date

10/1/2014

–2

–1

0

1

2 02218565 Apalachee River

10/1/19923/24/19989/14/2003

3/6/2009

8/27/2014

–2

–1

0

1

2 02335350 Crooked Creek

–2

–1

0

1

2 02336030 North Fork Peachtree Creek 

Logarithm of total zinc residuals, in milligrams per liter

–2

–1

0

1

2

02334480 Richland Creek 

10/1/2014

10/1/200110/1/200410/1/200710/1/201010/1/2013

10/1/2014

–2

–1

0

1

2 02334578 Level Creek

10/1/19923/24/19989/14/2003

3/6/2009

8/27/2014

–2

–1

0

1

2 02334885 Suwanee Creek

Figure 28. (Continued) Trends in stormflow total zinc flow- and seasonally adjusted residual concentrations for the 12 

monitored watersheds in Gwinnett County, Georgia. Stations with statistically significant trends (p-value >= 0.10) have a leastsquares trend line to graphically identify the trend.
52 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

For TSS, seven of the watersheds show decreasing 

trends—No Business Creek, Big Haynes Creek, Brushy Fork 

Creek, Alcovy River, Wheeler Creek, Richland Creek, and 

Suwanee Creek—and one watershed shows an increasing 

trend—North Fork Peachtree Creek (fig. 27; table 4). All 

trend slopes for TSS were much greater than those observed 

for TN and TP, with Wheeler Creek and Suwanee Creek 

watersheds having the highest rate of change in logarithmic 

residual concentrations per year (both decreasing trends). The 

decreasing TSS trends observed for seven of the watersheds is 

consistent with corresponding decreasing trends observed for 

TP, but the rates of change were much higher for TSS.

The large decreasing trends in TSS at Wheeler Creek 

and Suwanee Creek are in contrast with large increases in 

impervious areas observed at these watersheds during the 

timeframe of the trend analysis (fig. 11). The other three 

watersheds that exhibited large increases in impervious areas 

have either no trends (Yellow River and Apalachee River) or 

decreasing trends (Alcovy Creek) in TSS. Gwinnett County 

has long implemented BMPs to reduce and mitigate the effects 

of impervious areas on sediment transport, and this practice 

may be affecting TSS concentrations over time. It is also still 

possible that climatic patterns may be playing a role in the 

TSS trends, even though no corresponding significant trends 

were found for annual runoff. Suspended-sediment concentrations and transport is controlled particularly by the magnitude 

of storm runoff, rather than the amount of annual runoff. 

Any trends in the variations of the frequency, magnitude, and 

duration of storm runoff have not been assessed in this study. 

Trends in total suspended solids could be the result of climatic 

patterns (changes in the magnitude, intensity, and duration 

of storms), changes in impervious areas, and the continued 

implementation of Gwinnett County’s BMPs used to counteract the effects of impervious areas.

For total Zn, four of the watersheds have decreasing 

trends—No Business Creek, Wheeler Creek, Apalachee River, 

and Suwanee Creek (fig. 28; table 4). Trends in TSS and Zn 

are consistent for No Business Creek, Wheeler Creek, and 

Suwanee Creek watersheds, with significant decreases for 

both TSS and Zn; however, the slopes of the Zn trend lines are 

small compared to the slopes of the TSS trend lines, indicating 

that the cause of changes in Zn and TSS may be different.

Stream Water Loads and Yields

Stream water constituent load is the mass of chemical 

solutes or sediment transported at a point in a stream during a 

set period of time. Load calculations essentially flow weight 

concentration variations throughout the measurement period, 

thereby integrating the effects of all processes within the 

watershed that temporally affect water quality. Yields are 

defined as loads per unit area and are used to allow easier 

comparison of the magnitude of loads among watersheds with 

different drainage areas.

Table 5 is a summary of the concentration regression 

models for Zn for all 12 watersheds. A pair of models is 

shown for each station—the model that includes turbidity 

is used unless turbidity data are missing. Although each 

model includes discharge, only some of the models include 

a discharge-squared term, seasonal terms, or time and timesquared long-term trend terms. The fitted model time step 

to address composite stormflow samples varied from 4 to 

24 hours, depending on the watershed.

Figure 29 illustrates how the model was applied for 

base-flow and stormflow samples. Concentrations are plotted 

in relation to discharge, and regression lines show the fitted 

concentration-discharge relation. For the updated methodology 

used in this report, the slopes of the concentration-discharge 

relation for base-flow and stormflow samples are the same, 

but the relations still fit both types of samples. Tables 6 

through 11 summarize the yields for WYs 2004 through 2009, 

respectively, for each of the 12 monitored watersheds and 

10 water-quality constituents.

The biggest factor related to the variability in annual 

loads and yields is the amount of annual runoff. For example, 

the variation shown in figure 30 for NO2+NO3 and TN is 

similar to the variation in annual runoff. For this same reason, 

loads and yields for WYs 2004–09 will likely be below 

average due to lower-than-average precipitation during this 

period (18-percent below average), which resulted in belowaverage runoff.
Table 5. Examples of characteristics of regression models used to compute loads of total zinc (Zn) for the 12 monitored watersheds 

in Gwinnett County, Georgia.

[Discharge 2, discharge-squared; Time 2, time-squared. Explanatory variables: X, variable used; —, variable not used]

USGS

station 

number

Number

of

samples

Type and number 

of samples Regression

R2

 Record 

(years)

Model

time step 

(hours)

Explanatory variables

Base 

flow

Stormflow Discharge Turbidity Discharge2 Season Time Time2

02207120 131 65 66 0.963 14 24 X — X X X —

02207120 131 65 66 0.969 14 24 X X — X X —

02207185 56 29 27 0.941 6 6 X — — X X —

02207185 56 29 27 0.95 6 6 X X — X X —

02207385 146 84 62 0.874 14 12 X — — X X —

02207385 146 84 62 0.965 14 12 X X — X X X

02207400 147 85 62 0.93 12 12 X — — X — —

02207400 145 85 60 0.956 12 12 X X — — X X

02208150 125 59 66 0.948 14 8 X — X X X X

02208150 123 59 64 0.956 14 8 X X X X X X

02217274 61 29 32 0.943 6 8 X — X X — —

02217274 61 29 32 0.962 6 8 X X — X X X

02218565 52 27 25 0.976 6 4 X — — X X —

02218565 52 27 25 0.979 6 4 X X — X X —

02334480 59 27 32 0.936 6 6 X — X — — —

02334480 59 27 32 0.948 6 6 X X — — — —

02334578 58 28 30 0.953 6 6 X — — X X —

02334578 58 28 30 0.964 6 6 X X X — — —

02334885 137 69 68 0.916 14 12 X — — X X X

02334885 133 69 64 0.953 14 12 X X X — X X

02335350 128 66 62 0.961 14 6 X — — — X X

02335350 128 66 62 0.972 14 6 X X — — X X

02336030 57 28 29 0.975 6 4 X — — — X —

02336030 57 28 29 0.981 6 4 X X — — X —

Water-Quality Seasonality and Long-Term Trends 53
54 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Discharge, in cubic feet per second

1

10

10

100

100

1,000

1,000

Totoal suspended solids, in milligrams per liter

Stormflow

Base flow

Figure 29. Figure 29. Total suspended solids in relation to discharge for base-flow and stormflow conditions 

for samples collected at station 02208150, Alcovy River near Grayson, Georgia, 1997–2009. Lines 

through data show general concentration discharge relation.

Figure 30.

Total nitrogen

Total nitrite 

 plus nitrate

Runoff

EXPLANATION

0 

5 

10 

15 

20 

25 

30 

35 

0 

2 

4 

6 

8 

10 

12 

14 

2004 2005 2006 2007 2008 2009 

Annual runoff, in inches 

Yield, in pounds per acre per year as N 

Water year 

Figure 30. Average annual yields for total nitrite plus nitrate 

(NO2+NO3) and total nitrogen, and annual runoff in Gwinnett 

County, Georgia, water years 2004–09.
Water-Quality Seasonality and Long-Term Trends 55

Table 6. Water year 2004 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia.

USGS

station number

and name

Annual 

runoff

(inches 

per

year)

Annual constituent yields (pound per acre per year)

Total

nitrite

plus

nitrate

as

nitrogen

Total

nitrogen

as

nitrogen

Total

phosphorus

as

phosphorus

Dissolved 

phosphorus

as

phosphorus

Total 

organic 

carbon

as

carbon

Total

lead

Total

zinc

Total 

dissolved 

solids

Total suspended solids

Suspended 

sediment Yield

Lower

95th 

percentile 

confidence 

interval

Upper

95th 

percentile 

confidence 

interval

02207120 Yellow River at GA 124, 

near Lithonia, GA

23.4 7.47 11.6 0.419 0.0890 19.1 0.0331 0.220 460 874 513 1,390 960

02207185 No Business Creek at 

Lee Road, below Snellville,GA

21.2 5.64 7.91 0.326 0.0665 14.7 0.0234 0.139 300 465 361 606 620

02207385 Big Haynes Creek at 

Lenora Road, near Snellville,GA

21.2 3.87 7.04 0.468 0.0998 19.5 0.0273 0.0935 220 722 441 1,120 1,200

02207400 Brushy Fork Creek at 

Beaver Road, near Loganville,GA

19.6 1.43 4.90 0.468 0.0670 14.0 0.0164 0.0563 170 697 505 938 2,230

02208150 Alcovy River at New 

Hope Road,near Grayson, GA

23.0 2.29 6.42 0.803 0.0775 17.4 0.0632 0.192 270 1,740 1,100 2,670 1,570

02217274 Wheeler Creek at Bill 

Cheek Road, near Auburn, GA

20.0 2.97 6.30 0.870 0.185 21.0 0.184 0.224 210 2,420 926 5,180 4,040

02218565 Apalachee River at 

Fence Road, near Dacula, GA

23.1 2.63 7.88 1.25 0.0444 18.0 0.101 0.249 220 2,470 1,580 3,760 4,770

02334480 Richland Creek at Suwanee

Dam Road, near Buford, GA

23.0 2.95 7.96 2.06 0.0825 17.0 0.119 0.408 400 3,670 2,590 5,310 5,700

02334578 Level Creek at Suwanee 

Dam Road, near Suwanee, GA

22.0 1.83 6.76 1.21 0.129 21.6 0.0608 0.220 270 1,090 692 1,640 2,080

02334885 Suwanee Creek at

Suwanee, GA

22.5 3.17 6.63 0.714 0.0696 17.1 0.0354 0.138 310 895 626 1,240 610

02335350 Crooked Creek near

Norcross, GA

26.1 2.18 7.52 1.03 0.0934 24.8 0.0700 0.387 340 1,780 1,220 2,560 4,490

02336030 North Fork Peachtree Creek

at Graves Road, near Doraville, GA

41.5 4.56 14.5 1.84 0.290 37.0 0.174 1.09 420 3,170 2,140 4,710 5,260
56 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia Table 7. Water year 2005 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia. USGS station number and name Annual runoff (inches per year) Annual constituent yields (pound per acre per year) Total nitrite plus nitrate as nitrogen Total nitrogen as nitrogen Total phosphorus as phosphorus Dissolved phosphorus as phosphorus Total organic carbon as carbon Total lead Total zinc Total dissolved solids Total suspended solids Suspended sediment Yield Lower 95th percentile confidence interval Upper 95th percentile confidence interval 02207120 Yellow River at GA 124, near Lithonia, GA 33.7 9.16 16.3 0.648 0.120 33.0 0.0579 0.384 590 1,590 848 2,690 1,850 02207185 No Business Creek at Lee Road, below Snellville,GA 31.8 6.35 11.5 0.635 0.109 24.4 0.0584 0.347 400 1,430 876 2,270 1,220 02207385 Big Haynes Creek at Lenora Road, near Snellville,GA 33.5 5.16 10.8 0.623 0.121 31.8 0.0393 0.173 330 883 633 1,200 2,430 02207400 Brushy Fork Creek at Beaver Road, near Loganville,GA 34.5 2.64 9.69 0.889 0.123 29.3 0.0385 0.131 300 1,580 1,080 2,230 10,100 02208150 Alcovy River at New Hope Road,near Grayson, GA 30.4 3.04 8.70 0.830 0.0912 24.7 0.0755 0.249 350 2,410 1,630 3,460 1,900 02217274 Wheeler Creek at Bill Cheek Road, near Auburn, GA 24.3 3.92 7.11 0.475 0.117 17.6 0.0477 0.149 280 734 499 1,040 740 02218565 Apalachee River at Fence Road, near Dacula, GA 32.1 3.76 10.4 1.02 0.0584 22.6 0.0626 0.244 320 1,960 1,180 3,090 3,510 02334480 Richland Creek at Suwanee Dam Road, near Buford, GA 34.0 3.98 12.9 4.57 0.123 35.1 0.350 1.19 770 6,590 4,800 9,540 16,000 02334578 Level Creek at Suwanee Dam Road, near Suwanee, GA 28.7 2.55 9.33 1.45 0.170 28.5 0.0596 0.284 360 1,380 975 1,910 2,350 02334885 Suwanee Creek at Suwanee, GA 31.3 4.21 9.82 1.11 0.0923 24.8 0.0528 0.237 420 1,610 1,210 2,110 1,080 02335350 Crooked Creek near Norcross, GA 35.6 3.65 12.6 1.73 0.154 39.7 0.113 0.703 520 2,940 2,160 3,950 7,430 02336030 North Fork Peachtree Creek at Graves Road, near Doraville, GA 31.9 3.36 9.86 1.10 0.222 25.0 0.0695 0.674 350 1,340 908 1,950 2,120
Water-Quality Seasonality and Long-Term Trends 57

Table 8. Water year 2006 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia.

USGS

station number

and name

Annual 

runoff

(inches 

per

year)

Annual constituent yields (pound per acre per year)

Total

nitrite

plus

nitrate

as

nitrogen

Total

nitrogen

as

nitrogen

Total

phosphorus

as

phosphorus

Dissolved 

phosphorus

as

phosphorus

Total 

organic 

carbon

as

carbon

Total

lead

Total

zinc

Total 

dissolved 

solids

Total suspended solids

Suspended 

sediment Yield

Lower

95th 

percentile 

confidence 

interval

Upper

95th 

percentile 

confidence 

interval

02207120 Yellow River at GA 124, 

near Lithonia, GA

19.8 7.08 9.78 0.216 0.0655 14.2 0.0117 0.102 420 376 261 532 370

02207185 No Business Creek at 

Lee Road, below Snellville,GA

17.9 3.46 5.38 0.185 0.0515 11.7 0.0120 0.0853 230 250 184 331 350

02207385 Big Haynes Creek at 

Lenora Road, near Snellville,GA

18.1 3.11 5.05 0.175 0.0632 11.0 0.0105 0.0600 190 211 142 301 340

02207400 Brushy Fork Creek at 

Beaver Road, near Loganville,GA

17.6 1.22 3.58 0.236 0.0737 12.5 0.00909 0.0429 160 171 134 214 580

02208150 Alcovy River at New 

Hope Road,near Grayson, GA

16.7 1.71 3.69 0.210 0.0423 9.23 0.0204 0.0843 210 416 295 574 390

02217274 Wheeler Creek at Bill 

Cheek Road, near Auburn, GA

21.5 3.29 6.10 0.443 0.141 14.9 0.0412 0.126 240 582 415 814 650

02218565 Apalachee River at 

Fence Road, near Dacula, GA

19.5 2.12 4.79 0.299 0.0356 11.7 0.0184 0.0968 210 536 307 871 630

02334480 Richland Creek at Suwanee

Dam Road, near Buford, GA

25.9 3.17 7.82 1.23 0.105 15.1 0.0606 0.326 430 2,520 1,670 3,710 2,910

02334578 Level Creek at Suwanee 

Dam Road, near Suwanee, GA

17.5 1.48 4.19 0.401 0.0797 13.0 0.0140 0.106 240 465 314 671 660

02334885 Suwanee Creek at

Suwanee, GA

19.3 3.04 5.62 0.353 0.0468 11.8 0.0157 0.0978 280 378 294 479 420

02335350 Crooked Creek near

Norcross, GA

20.0 1.57 4.87 0.668 0.0663 15.4 0.0311 0.288 260 913 715 1,180 4,260

02336030 North Fork Peachtree Creek

at Graves Road, near Doraville, GA

20.8 2.12 6.03 0.614 0.136 15.3 0.0378 0.415 240 612 415 878 920
58 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia Table 9. Water year 2007 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia. USGS station number and name Annual runoff (inches per year) Annual constituent yields (pound per acre per year) Total nitrite plus nitrate as nitrogen Total nitrogen as nitrogen Total phosphorus as phosphorus Dissolved phosphorus as phosphorus Total organic carbon as carbon Total lead Total zinc Total dissolved solids Total suspended solids Suspended sediment Yield Lower 95th percentile confidence interval Upper 95th percentile confidence interval 02207120 Yellow River at GA 124, near Lithonia, GA 13.8 5.89 7.47 0.125 0.0431 9.41 0.00755 0.0706 330 205 134 301 230 02207185 No Business Creek at Lee Road, below Snellville,GA 11.2 1.76 3.03 0.123 0.0336 7.25 0.00984 0.0619 140 215 152 304 320 02207385 Big Haynes Creek at Lenora Road, near Snellville,GA 12.1 2.05 3.17 0.0875 0.0449 6.23 0.00580 0.0371 130 130 83.7 191 200 02207400 Brushy Fork Creek at Beaver Road, near Loganville,GA 12.1 0.781 2.46 0.145 0.0590 8.91 0.00575 0.0296 110 166 113 234 480 02208150 Alcovy River at New Hope Road,near Grayson, GA 12.1 1.20 2.70 0.144 0.0295 7.01 0.0160 0.0779 160 312 228 416 320 02217274 Wheeler Creek at Bill Cheek Road, near Auburn, GA 16.0 2.38 4.17 0.220 0.0925 9.21 0.0131 0.0683 180 254 158 386 410 02218565 Apalachee River at Fence Road, near Dacula, GA 13.2 1.34 3.26 0.167 0.0275 8.49 0.00998 0.0623 150 355 242 512 420 02334480 Richland Creek at Suwanee Dam Road, near Buford, GA 19.5 2.49 5.30 0.723 0.0983 10.6 0.0325 0.184 320 1,380 793 2,260 1,860 02334578 Level Creek at Suwanee Dam Road, near Suwanee, GA 12.2 0.897 2.73 0.240 0.0527 8.71 0.00950 0.0742 170 328 186 532 490 02334885 Suwanee Creek at Suwanee, GA 12.3 2.21 3.79 0.181 0.0278 7.48 0.00907 0.0672 190 223 159 304 310 02335350 Crooked Creek near Norcross, GA 16.3 1.28 4.01 0.459 0.0529 13.3 0.0267 0.243 210 700 462 1,020 2,020 02336030 North Fork Peachtree Creek at Graves Road, near Doraville, GA 18.5 1.61 4.62 0.400 0.0900 13.6 0.0323 0.351 200 441 327 595 950
Water-Quality Seasonality and Long-Term Trends 59

Table 10. Water year 2008 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia.

USGS

station number

and name

Annual 

runoff

(inches 

per

year)

Annual constituent yields (pound per acre per year)

Total

nitrite

plus

nitrate

as

nitrogen

Total

nitrogen

as

nitrogen

Total

phosphorus

as

phosphorus

Dissolved 

phosphorus

as

phosphorus

Total 

organic 

carbon

as

carbon

Total

lead

Total

zinc

Total 

dissolved 

solids

Total suspended solids

Suspended 

sediment Yield

Lower

95th 

percentile 

confidence 

interval

Upper

95th 

percentile 

confidence 

interval

02207120 Yellow River at GA 124, 

near Lithonia, GA

12.5 5.36 6.98 0.104 0.0349 8.85 0.00686 0.0656 290 194 136 270 310

02207185 No Business Creek at 

Lee Road, below Snellville,GA

8.35 1.01 1.91 0.0525 0.0220 5.00 0.00411 0.0304 100 81.0 58.0 109 180

02207385 Big Haynes Creek at 

Lenora Road, near Snellville,GA

9.17 1.46 2.36 0.0573 0.0389 4.99 0.00460 0.0312 100 108 82.4 139 180

02207400 Brushy Fork Creek at 

Beaver Road, near Loganville,GA

8.57 0.527 1.78 0.0983 0.0537 7.38 0.00362 0.0218 80 86.1 60.0 120 220

02208150 Alcovy River at New 

Hope Road,near Grayson, GA

11.1 1.09 2.36 0.0956 0.0222 6.12 0.0122 0.0776 160 240 179 315 290

02217274 Wheeler Creek at Bill 

Cheek Road, near Auburn, GA

11.1 1.49 2.75 0.113 0.0556 5.73 0.00539 0.0445 130 127 90.5 173 260

02218565 Apalachee River at 

Fence Road, near Dacula, GA

9.21 0.837 2.59 0.109 0.0256 7.24 0.00685 0.0521 110 263 173 383 510

02334480 Richland Creek at Suwanee

Dam Road, near Buford, GA

17.6 2.29 4.71 0.557 0.109 10.0 0.0225 0.132 260 1,020 626 1,610 1,820

02334578 Level Creek at Suwanee 

Dam Road, near Suwanee, GA

9.85 0.659 2.08 0.140 0.0416 6.78 0.00787 0.0578 140 283 151 480 570

02334885 Suwanee Creek at

Suwanee, GA

12.0 2.25 3.93 0.138 0.0256 8.60 0.00969 0.0791 190 242 169 335 510

02335350 Crooked Creek near

Norcross, GA

14.9 1.18 3.65 0.391 0.0477 12.7 0.0225 0.249 190 593 434 803 1,880

02336030 North Fork Peachtree Creek

at Graves Road, near Doraville, GA

20.6 1.85 5.14 0.339 0.103 15.6 0.0230 0.382 260 329 238 447 920
60 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia Table 11. Water year 2009 runoff and constituent yields for each of the 12 monitored watersheds in Gwinnett County, Georgia. USGS station number and name Annual runoff (inches per year) Annual constituent yields (pound per acre per year) Total nitrite plus nitrate as nitrogen Total nitrogen as nitrogen Total phosphorus as phosphorus Dissolved phosphorus as phosphorus Total organic carbon as carbon Total lead Total zinc Total dissolved solids Total suspended solids Suspended sediment Yield Lower 95th percentile confidence interval Upper 95th percentile confidence interval 02207120 Yellow River at GA 124, near Lithonia, GA 25.4 6.62 14.2 0.641 0.0817 36.2 0.0452 0.492 460 5,290 3,740 8,330 11,800 02207185 No Business Creek at Lee Road, below Snellville,GA 18.8 1.35 4.32 0.307 0.0613 17.3 0.0331 0.249 200 1,030 588 1,740 3,280 02207385 Big Haynes Creek at Lenora Road, near Snellville,GA 17.5 2.17 4.31 0.101 0.0432 17.4 0.0186 0.104 190 351 234 505 3,250 02207400 Brushy Fork Creek at Beaver Road, near Loganville,GA 18.9 1.01 4.38 0.213 0.135 19.9 0.00962 0.0712 190 316 236 414 990 02208150 Alcovy River at New Hope Road,near Grayson, GA 20.4 1.70 5.25 0.418 0.0473 14.6 0.0438 0.426 280 1,720 749 3,250 2,270 02217274 Wheeler Creek at Bill Cheek Road, near Auburn, GA 17.4 1.69 4.82 0.342 0.218 13.7 0.0172 0.109 170 541 257 1,000 2,560 02218565 Apalachee River at Fence Road, near Dacula, GA 20.6 1.94 7.26 0.857 0.0685 24.8 0.0529 0.318 250 2,660 969 5,690 20,400 02334480 Richland Creek at Suwanee Dam Road, near Buford, GA 20.2 2.47 6.04 1.37 0.182 18.7 0.0524 0.340 300 2,470 1,760 3,540 6,480 02334578 Level Creek at Suwanee Dam Road, near Suwanee, GA 16.9 1.03 3.88 0.222 0.0858 14.6 0.0264 0.133 220 668 369 1,110 3,010 02334885 Suwanee Creek at Suwanee, GA 22.5 3.32 6.77 0.205 0.0535 21.5 0.0291 0.260 320 564 357 848 1,670 02335350 Crooked Creek near Norcross, GA 31.6 2.40 9.50 1.49 0.117 39.0 0.102 1.32 360 2,430 1,410 3,940 13,000 02336030 North Fork Peachtree Creek at Graves Road, near Doraville, GA 39.2 4.24 14.9 0.841 0.227 46.3 0.132 1.70 560 3,160 1,430 6,140 13,300
Stream Water Loads and Yields 61

Nutrients

Average annual TN yields for 2004–09 were highest for 

Yellow River and North Fork Peachtree Creek watersheds 

(11.1 and 9.18 pounds per acre per year (lbs/acre/year) as 

N, respectively; fig. 31) compared to TN yields for the 12 

monitored watersheds. Yellow River also had the highest 

annual NO2+NO3 yields (6.93 lbs/acre/year as N). Yellow 

River and North Fork Peachtree Creek have the third and first 

highest percentages of impervious area, respectively (table 3; 

fig. 9). Brushy Fork Creek, Alcovy River, and Level Creek 

watersheds had the three lowest average annual TN (4.47, 

4.85, and 4.83 lbs/acre/year as N, respectively) and NO2+NO3

(1.27, 1.84, and 1.41 lbs/acre/year as N, respectively) yields 

for 2004–09. None of these watersheds have high percentages 

of impervious areas, and Brushy Fork Creek and Level Creek 

watersheds have the fourth lowest and the lowest percentages 

of high-density development, 9.25 and 4.90 percent, respectively (table 3). 

Nitrite plus nitrate loads make up about 57 percent of TN 

loads for the 12 watersheds combined for 2004–09 (fig. 30). 

Treated wastewater contributes much of the nitrogen load 

associated with base flow for Yellow River, Big Haynes Creek, 

and Suwanee Creek watersheds (Landers and others, 2007). 

These three watersheds had the highest, third highest, and 

fifth highest percentage of NO2+NO3 loads of 62.7, 54.5 and 

49.8 percent, respectively, relative to TN loads. 

Average annual TP yields for 2004–09 were highest for 

Richland Creek, Crooked Creek, and North Fork Peachtree 

Creek watersheds (1.75, 0.962, and 0.855 lbs/acre/year as P, 

respectively; fig. 32) compared to TP yields for the 12 monitored watersheds. These three watersheds also had three of the 

highest four average annual TSS yields. Average annual DP 

yields for 2004–09 were highest for Wheeler Creek, Richland 

Creek, and North Fork Peachtree Creek watersheds (0.135, 

0.117, and 0.178 lbs/acre/year as P, respectively). These 

averages indicate that high TP and DP yields may be related 

even though DP makes up a small portion of TP. Richland 

Nitrogen yield, in pounds per acre per year as N 

Yellow

River

No Business Creek

Big

Haynes Creek

Brushy

Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Figure 31.

2

0

4

6

8

10

Total nitrogen

EXPLANATION

Total nitrite plus nitrate

12

Figure 31. Average annual total nitrogen and total nitrite plus nitrate yields for 12 monitored 

watersheds in Gwinnett County, Georgia, water years 2004–09.
62 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Figure 32.

0 

0.5 

1.0 

1.5 

2.0 

Phosphorus yield, in pounds per acre per year as P

Yellow

River

No Business Creek

Big

Haynes Creek

Brushy

Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Total phosphorus

EXPLANATION

Dissolved phosphorus

Figure 32. Average annual total and dissolved phosphorus yields for 12 monitored 

watersheds in Gwinnett County, Georgia, water years 2004–09.

Creek watershed, which has the highest average annual TP 

yield and third highest average annual DP yield (fig. 32), has 

the highest basin slope of any of the 12 watersheds (fig. 5; 

table 3). Crooked Creek, which has the second highest average 

annual TP yield, and North Fork Peachtree Creek, which has 

the third highest average annual TP yield and highest average 

annual DP yield (fig. 32), have the two highest percentages of 

impervious area, 36.81 and 52.79, respectively (fig. 9; table 3). 

No Business Creek and Big Haynes Creek watersheds had 

the two lowest average annual TP yields for WYs 2004–09, 

0.271 and 0.252 lbs/acre/year as P, respectively (fig. 32). Big 

Haynes Creek watershed had the third lowest percentage of 

high-density development, 7.46 percent (table 3). Dissolved 

phosphorus loads make up about 16 percent of TP loads for 

the 12 watersheds combined for WYs 2004–09.

Average annual TOC yields for WYs 2004–09 are highest 

for Yellow River, Crooked Creek, and North Fork Peachtree 

Creek watersheds (20.1, 24.2 and 25.5 lbs/acre/year as C, 

respectively; fig. 33) compared to TOC yields for the 12 

monitored watersheds. These three watersheds have the three 

highest percentages of impervious area (table 3; fig. 9). Alcovy 

River watershed has the lowest average annual TOC yield for 

WY 2004–09, 13.2 lbs/acre/year as C. 

Trace Elements 

Average annual Pb and Zn yields for water years 

2004–09 were highest for Richland Creek, Crooked Creek, 

and North Fork Peachtree Creek watersheds (0.106, 0.061, and 

0.078 lbs/acre/year Pb, and 0.430, 0.531 and 0.769 lbs/acre/

year Zn, respectively, fig. 34) compared to Pb and Zn yields 

for the 12 monitored watersheds. These three watersheds 

also had three of the highest four average annual TSS yields. 

Richland Creek watershed has the highest basin slope of 

any of the 12 watersheds (fig. 5; table 3), whereas Crooked 

Creek and North Fork Peachtree Creek have the two highest 

percentages of impervious area (table 3; fig. 9). Big Haynes 

Creek and Brushy Fork Creek watershed had the two lowest 

average annual Pb and Zn yields for WYs 2004–09, 0.018 and 

0.014 lbs/acre/year Pb, and 0.083 and 0.059 lbs/acre/year Zn, 

respectively. These watersheds had the third and fourth lowest 

percentages of high-density development with 7.46 percent 

for Big Haynes Creek and 9.25 percent for Brushy Fork Creek 

(table 3). Average annual TSS yields were also low for these 

two watersheds.
Stream Water Loads and Yields 63

Ttoal organic carbon yield, in pounds per acre per year as C

0

5

10

15

20

25

30

Figure 33.

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Figure 33. Average annual total organic 

carbon yields for 12 monitored watersheds 

in Gwinnett County, water years 2004–09.

0.0 

0.2 

0.4 

0.6 

0.8 

Trace element yield, in pounds per acre per year 

Total lead

EXPLANATION

Total zinc

Figure 34.

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Figure 34. Average annual 

total lead and zinc yields for 

12 monitored watersheds in 

Gwinnett County, Georgia, 

water years 2004–09.
64 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Total Dissolved Solids

Average annual TDS yields for WYs 2004–09 were 

highest for Yellow River and Richland Creek watersheds 

(425 and 413 lbs/acre/year, respectively; fig. 35). Yellow River 

watershed had the third highest percentage of impervious 

area (fig. 9; table 3), whereas Richland Creek watershed had 

the highest basin slope of any of the 12 watersheds (fig. 5; 

table 3). Brushy Fork Creek watershed had the lowest average 

annual TDS yield for WYs 2004–09, 168 lbs/acre/year, and 

had the fourth lowest percentage of high-density development, 

9.25 percent (table 3).

Suspended Sediment

Total suspended solids and SSC typically increased by 

two orders of magnitude between base-flow and stormflow 

conditions (fig. 17L and 17M). One consequence of this 

dramatic increase in sediment concentration with discharge is 

that the majority of the annual load is transported during the 

few largest events of the year. A second consequence is that 

during drier years, very little sediment may be transported. 

For example, for the Alcovy River watershed, WYs 2004–05 

and 2009 had average precipitation and runoff, whereas 

WYs 2006–08 were very dry (fig. 36). Although runoff was 

about 46 percent lower in the drier years, WYs 2006–08, TSS 

yields were about 84 percent lower. There are fewer storms 

during drier years, and the storms tend to have low peak flows 

because base-flow discharges are also low, which then results 

in less sediment transported than might be expected on the 

basis of annual runoff alone.

Average annual TSS yields for WYs 2004–09 were 

highest for Yellow River, Apalachee River, Richland Creek, 

Crooked Creek, and North Fork Peachtree Creek watersheds 

(1,420, 1,370, 2,940, 1,560, and 1,510 lbs/acre/year, 

respectively; fig. 37). Richland Creek watershed, which had 

the highest average annual TSS yields, also had the highest 

basin slope of any of the 12 watersheds (fig. 5; table 3). Three 

of the other four watersheds, Yellow River, Crooked Creek, 

and North Fork Peachtree Creek had the three highest percentages of impervious area (fig. 9; table 3). Big Haynes Creek 

watershed had the lowest average annual TSS yield for WYs 

2004–09, 401 lbs/acre/year, and had the third lowest percentage of high-density development, 7.46 percent (table 3).

There is much variation in the annual TSS yields among 

the 12 watersheds because the ranks of watershed yield are 

not consistent from year to year. Richland Creek exhibited the 

highest annual yields of 6,590 lbs/acre in WY 2005 (table 7). 

Total dissolved solids yield, in pounds per acre per year

0

100

200

300

400

500

Figure 35.

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Figure 35. Average annual total 

dissolved solids yields for 12 monitored 

watersheds in Gwinnett County, Georgia, 

water years 2004–09.
Stream Water Loads and Yields 65

Water year

Figure 36.

EXPLANATION

Runoff

Total suspended solids

0 

5 

10 

15 

20 

25 

30 

35 

0 

500 

1,000 

1,500 

2,000 

2,500 

2004 2005 2006 2007 2008 2009 

Annual runoff, in inches 

Total suspended solids yield, in pounds per acre per year 

Figure 36. Annual total suspended solids yield and 

runoff at the Alcovy River, Gwinnett County, Georgia, 

water years 2004–09.

EXPLANATION

Monitoring stations

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

2004 2005 2006 2007 

Water year 

2008 2009 2004–2009 

Figure 37.

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

Annual total suspended solids yield, in pounds per acre per year 

Figure 37. Average annual total suspended solids yields for 12 monitored watersheds in Gwinnett County, 

Georgia, water years 2004–09.
66 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

These high yields may have been associated with construction 

of a large Gwinnett County park adjacent to Richland Creek 

during that timeframe. Gwinnett County experienced extreme 

flooding September 20–22, 2009, with the highest periodof-record discharges recorded at the Yellow River gage. The 

flood magnitude had an annual exceedance probability about 

equal to a 200-year flood (Gotvald and McCallum, 2010). 

A 200-year flood is defined as a flood that has a 1 in 200 

chance (or 0.5 percent chance) of being equaled or exceeded 

in any one year (Holmes and Dinicola, 2010). Although the 

flood affected the entire county, the Yellow River watershed 

was particularly affected by the flood, which resulted in the 

Yellow River having the highest WY 2009 TSS yields of 

5,290 lbs/acre (table 11). It is not unusual for a few of the 

largest storms in a year to transport the majority of the annual 

sediment load and is the result of the nearly two orders of 

magnitude increase in sediment concentrations between base 

flow and average stormflow observed at the 12 watersheds 

(fig. 17L and 17M). Annual sediment yields are, therefore, 

sensitive to the occurrence and magnitude of the largest 

storms, and this may help explain the large variability in 

annual yields observed among the watersheds.

Average annual SSC tells a somewhat different story 

for sediment than TSS. Although the watersheds with the 

highest TSS yields also had the highest SSC yields, SSC 

yields were significantly higher than TSS yields (fig. 38). 

For WYs 2004–09, the ratio of SSC to TSS loads for all 

watersheds combined was about 1.9, but this ratio varied 

widely by watershed: Alcovy River had the lowest ratio of 

1.0, and Brushy Fork Creek and Apalachee River had the 

highest ratios of 4.8 and 3.7, respectively. The watersheds with 

higher TSS and SSC yields generally had higher SSC to TSS 

ratios. Presumably, watersheds that have a higher percentage 

of sand tend to have a greater discrepancy between SSC and 

TSS (fig. 18). Despite the differences between SSC and TSS, 

annual yields of TSS are used as the primary performance 

criterion for suspended sediment in the Gwinnett County 

Watershed Protection Plan (Gwinnett County Department of 

Public Utilities, 2000).

0 

1000 

2000 

3000 

4000 

5000 

6000 

Sediment yield, in pounds per acre per year 

Figure 38.

Yellow River

No Business Creek

Big Haynes Creek

Brushy Fork Creek

Alcovy River

Wheeler Creek

Apalachee River

Richland Creek

Level Creek

Suwanee Creek

Crooked Creek

North Fork Peachtree Creek

Monitored watersheds, Gwinnett County

Total suspended solids

EXPLANATION

Suspended-sediment 

 concentration

Figure 38. Average annual total suspended solids and suspended-sediment concentration 

yields for 12 monitored watersheds in Gwinnett County, Georgia, water years 2004–09.
Summary 67

Interpreting Aggregate Effects on 

Water Quality

The water quality of streams in a watershed reflects and 

integrates the effects of watershed characteristics, hydrologic 

processes and variability, biogeochemical processes, and 

human influences. Changes in land use alter complex 

interactions that affect many processes within a watershed 

(MacDonald, 2000).

Although varying watershed characteristics and land 

use can indicate possible explanations for variations in water 

quality among the watersheds; relations rarely reflect straightforward cause and effect. There may be specific, unaccounted 

human activities present within individual watersheds that 

result in point-source pollutants that affect water-quality 

constituents. Several factors can result in similar water-quality 

patterns; for example, high impervious area and steep basin 

slope both appear to be related to greater transport and higher 

yields of TSS and SSC in this study. A specific watershed 

characteristic may not always relate to a particular waterquality pattern in all cases because water quality is integrating 

all the processes within the watershed; for example, in this 

study the watersheds with the lowest Zn yields also had low 

percentages of high-density development, yet watersheds with 

low percentages of high-density development do not all have 

low Zn yields. Many watershed characteristics are also related 

to each other, whereas individual characteristics may be better 

at explaining particular relations in water quality. For example, 

the percentage area of high-density development and impervious area are correlated with each other, but high-density 

development may be better related to specific trace elements 

that might be the result of pollutants, while impervious 

area may be more related to the transport of TSS and SCC. 

Furthermore, Gwinnett County has long required BMPs to 

control and mitigate the effects of increases in impervious 

areas. The effectiveness of these measures to counteract the 

effects of impervious areas is difficult to assess because the 

two variables are correlated with each other: more BMPs are 

implemented in watersheds that have higher percentages of 

impervious area.

The effects of variations in climate can have large effects 

on water-quality concentrations, loads, and yields. These 

effects occur for any water-quality constituent that exhibits 

concentration variations with discharge or season, as variations in the amount and distribution of runoff annually and 

seasonally will affect the concentrations observed along with 

the flow-weight for estimating load. This is especially notable 

for constituents that are transported during storms that exhibit 

a strong relation of increasing concentration with discharge, 

such as sediment (TSS and SSC) and other sediment related 

constituents (TP, Zn, and Pb). Dry years have less runoff 

(lower average annual discharges) with generally smaller 

storm hydrologic responses (due to reduced base flow), which 

result in less opportunity for storm transport. Similarly, wet 

years have more runoff and greater storm hydrologic response, 

resulting in more transport. For constituents that exhibit 

increasing concentrations with discharge, dry years will have 

lower annual average concentrations whereas wet years will 

have higher annual average concentrations. For constituents 

that exhibit dilution at higher discharge, annual average 

concentrations will be higher in dry years and lower in wet 

years. While the amount of annual runoff is often the most 

important controlling factor determining the magnitude of 

annual loads, variations in concentrations related to variations 

in discharge may also have an important effect, as observed 

for TSS yields at the Alcovy River (fig. 36). Although runoff 

was about 46 percent lower in the drier years, WYs 2006–08, 

TSS yields were about 84 percent lower, due to the effects of 

lower TSS concentrations at lower discharges.

Summary

The U.S. Geological Survey (USGS), in cooperation 

with Gwinnett County Department of Water Resources, 

established a comprehensive Long-Term Trend Monitoring 

(LTTM) program in 1996 to monitor, analyze, and quantify the 

magnitudes of pollutants and the effects of urbanization on six 

watersheds. In 2001, six additional watersheds were added to 

the LTTM program. These 12 watersheds were continuously 

monitored for water level (stage), discharge, precipitation, 

and water-quality properties (water temperature, specific 

conductance [SC], and turbidity) for water years (WYs) 

2004–09. For each season, summer (May–October) and winter 

(November–April), three storm-composited samples were 

collected during storm events and one sample was collected 

during base-flow conditions for a total of eight samples per 

year; samples were analyzed for SC, turbidity, nutrients 

(nitrite plus nitrate [NO2 + NO3], total ammonia plus organic 

nitrogen [TKN], total phosphorus [TP], dissolved phosphorus 

[DP], and total organic carbon [TOC]), total dissolved solids 

(TDS), trace metals (total lead [Pb] and total zinc [Zn]), 

and suspended sediment (total suspended solids [TSS] and 

suspended-sediment concentration [SSC]). The sampling 

scheme was designed to identify variations in water quality 

both hydrologically and seasonally. 

Gwinnett County, a suburban county of the Atlanta 

metropolitan area, is composed predominantly of headwater 

watersheds along the Eastern Continental Divide. Watershed 

basin slopes range from 5.3 to 13.8 percent. Gwinnett County 

has undergone rapid population growth from about 1980 to 

2010, though growth has slowed somewhat during the latter 

part of the study period, 2004–09. Land use has become more 

highly developed over time. Land use is summarized for each 

watershed for 2009. The percentage of impervious area has 

a large influence on hydrology and water quality. Changes 

in the percentage of impervious area are documented for 

2000–2009, with increases of varying extent observed within 

most watersheds.
68 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Gwinnett County has a humid, subtropical climate with a 

mean annual precipitation of 54.7 inches, and high evapotranspiration rates during the growing season. Precipitation in WYs 

2004–09 was about 18 percent below average, and the county 

experienced exceptional drought conditions in WYs 2007 and 

2008. Low precipitation resulted in below average runoff. 

Watershed water yields, the percentage of precipitation that 

occurs as runoff, ranged from 34.3 to 59.4 percent for WYs 

2004–09; water yields typically are lower in low precipitation 

years. The watersheds with the highest percentage of impervious areas have the highest water yields. 

A comparison of base-flow and stormflow water quality 

indicate that turbidity, TKN, TN, TP, TOC, Pb, Zn, TSS, and 

SSC concentrations increased with increasing discharge at all 

watersheds, yet SC decreased during stormflow at all watersheds and TDS exhibited dilution at a few of the watersheds. 

Total suspended solids and SSC concentrations typically 

were two orders of magnitude higher in stormflow samples, 

turbidities were about 1.5 orders of magnitude higher, TP and 

Zn were about one order of magnitude higher, and TKN, TN, 

TOC, and Pb were about twofold higher. 

Seasonal patterns in TN, TP, TDS, TSS, and Zn were 

all fairly similar, with higher concentrations in the summer 

and lower in the winter. A linear long-term trend analysis 

for stormflow composite concentrations of TN, TP, TDS, 

TSS, and Zn identified significant trends in 36 of the 60 

constituent-watershed combinations. When a watershed did 

have a significant trend, the trends typically were decreasing 

for TN (4 decreasing, 1 increasing), TP (11 decreasing), 

TSS (7 decreasing, 1 increasing), and Zn (4 decreasing) and 

typically were increasing for TDS (7 increasing, 1 decreasing). 

Total nitrogen, TP, and Zn changes in concentrations over 

time were small. Total dissolved solids and TSS trends were 

larger. Despite the strong relation between TSS yields and 

the percentage of watershed impervious area, trends in TSS 

concentrations did not always correlate with the observed 

changes in impervious areas. No linear trends were detected 

in annual runoff for any of the 12 watersheds that could 

have produced trends in TSS concentrations. But suspendedsediment concentrations and transport are controlled 

particularly by the magnitude of storm runoff, rather than 

the amount of annual runoff. Trends in the variations of the 

magnitude of storm runoff were not assessed. The cause of 

trends in TSS has not been attributed, but is likely the result 

of a combination of climatic patterns (resulting in variability 

of magnitude, intensity, and duration of storms), changes in 

impervious areas, and the continued implementation of best 

management practices (BMPs) (such as stormwater detention 

ponds, diversion structures, and stream buffers) and other 

erosion controls used to minimize sediment transport. 

Stream water constituent loads were estimated for 10 

water-quality constituents. These estimates represent the 

cumulative effects of watershed characteristics, hydrologic 

processes and variability, biogeochemical processes, and 

human influences on watershed water quality. Yields, in load 

per unit area, were used to compare loads from watersheds 

with different sizes. For the Gwinnett County LTTM program, 

storm samples were collected as a composite so the load 

estimation approach developed and used for the Gwinnett 

County LTTM program for 1998–2003 (Landers and others, 

2007) was used with some minor modifications. This approach 

employs the commonly used regression-model method, but 

develops separate relations for base-flow and storm-based 

average concentrations. Concentrations were modeled as a 

function of discharge, time, season, and turbidity to improve 

model predictions and reduce errors in load estimates.

The amount of annual runoff is the primary factor in 

determining the amount of annual constituent loads. Below 

average precipitation during the WYs 2004–09 study period, 

and especially during WYs 2006–08, resulted in corresponding below average runoff and loads. Variations in constituent 

yields between watersheds appeared to be related to various 

watershed characteristics. Suspended sediment (TSS and 

SSC) along with constituents transported predominately in 

solid phase (TP, TOC, Pb, and Zn), and TDS typically had 

higher yields from watersheds that had high percentages of 

impervious areas (Yellow River, Crooked Creek, and North 

Fork Peachtree Creek) or high basin slope (Richland Creek). 

High TN yields were also associated with watersheds with 

high percentages of impervious areas. Higher percentages of 

impervious area result in increases of runoff over infiltration, 

flood magnitudes, and the “flashiness” of flooding, which can 

result in increased land-surface erosion and stream transport 

of sediments.

Watersheds with higher percentages of NO2+NO3 loads 

relative to TN loads were frequently associated with watersheds where treated wastewater was known to contribute much 

of the nitrogen load associated with base flow. Low TN, TSS, 

Pb, and Zn yields appear to be associated with watersheds 

that have a low percentage of high-density development, 

but this characteristic did not preclude having higher yields. 

Watersheds with either high or low Pb and Zn yields typically 

also had high or low TSS yields, respectively. 

Annual patterns in TSS yields between watersheds were 

quite variable. TSS yields were lower in drought years, WYs 

2007 and 2008, from the combined effects of lower runoff and 

fewer, lower magnitude storms, which likely resulted in less 

surface erosion and lower stream sediment transport during the 

drought. Suspended-sediment transport is particularly sensitive to storms, as evident from the two orders of magnitude 

increase in concentration observed between base-flow and 

storm-event samples. Total suspended solids and SSC yields 

differed, which is possibly the result of low-biased TSS 

concentrations for samples with sand fractions of greater than 

25 percent.

Interpreting the aggregate effects of watershed characteristics, hydrologic processes and variability, biogeochemical 

processes, and human influences on watershed water quality 

is complex. Although some factors, such as impervious area, 

may relate to a pattern observed in water-quality yields, other 

factors, both measured and unmeasured, can obscure this relation. Some factors may be correlated with each other, which 
References Cited 69

may make it difficult to determine what specifically might be 

the cause of a particular water-quality relation. For example, 

the effectiveness of the implementation of BMPs to counteract 

the effects of impervious areas is difficult to assess because 

BMPs are correlated with impervious area: more BMPs are 

implemented in watersheds that have higher percentages of 

impervious area. In this case, more BMPs have a positive 

effect on water quality, yet higher impervious area has a 

negative effect. Patterns and trends in climate also affect water 

quality. Despite the exceptional drought observed, no linear 

trends in runoff were detected for any of the 12 watersheds. 

This study did not determine whether there were any patterns 

or trends in the frequency, magnitude, or duration of storm 

events, which could also affect the results.

Long-term, comprehensive records of climatic, hydrologic, and water-quality conditions can be used by watershed 

managers to evaluate, protect, and enhance the rivers and 

streams. The monitoring stations used in this study represent 

a range of watershed conditions, including water-quality 

attainment status, point-source discharges, and other physical 

characteristics. The computation and analysis of constituent 

loads is a critical element and useful tool in the management 

and protection of the diverse watersheds in Gwinnett County, 

Georgia. Total suspended solids annual loads have been 

identified in the Gwinnett County Watershed Protection 

Plan for target performance criterion. The Gwinnett County 

Department of Water Resources uses these computations 

along with the assessment of trends in imperviousness for the 

management and protection of aquatic habitat. Real-time data 

and additional USGS tools, such as WaterAlert, are valuable 

assets available to the public and the managers of Gwinnett 

County for flood warning and emergency management and for 

the identification of potential pollution issues.

Acknowledgments

This report is possible because of the field data-collection 

efforts and analysis expertise of the U.S. Geological Survey, 

Gwinnett Project personnel: Paul Ankcorn, Kerry Caslow, 

Jonathan Evans, Brandon Reyes, and Karen Stull. Thanks 

are extended to Susan Grams and Edward Martin of the U.S. 

Geological Survey for assisting in the modeling and computing of constituent loads. Thanks are also extended to U.S. 

Geological Survey employees Jamie Painter for GIS support, 

Caryl Wipperfurth and Kimberly Swidarski for preparations of 

illustrations, and Kay Naugle for editing the report.

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Appendix

Statistical summaries of the water-quality sample concentrations for 13 field and laboratory constituents 

(table 2) are provided by watershed in tables 1–1 through 1–12. 

1–1. Yellow River at Georgia 124 near Lithonia, Georgia, station number 02207120

1–2. No Business Creek at Lee Road, below Snellville, Georgia, station number 02207185

1–3. Big Haynes Creek at Lenora Road, near Loganville, Georgia, station number 02207385

1–4. Brushy Fork Creek at Beaver Road, near Loganville, Georgia, station number 02207400

1–5. Alcovy River at New Hope Road, near Grayson, Georgia, station number 02208150

1–6. Wheeler Creek at Bill Cheek Road, near Auburn, Georgia, station number 02217274

1–7. Apalachee River at Fence Road, near Dacula, Georgia, station number 02218565

1–8. Richland Creek at Suwanee Dam Road, near Buford, Georgia, station number 02334480

1–9. Level Creek at Suwanee Dam Road, near Suwanee, Georgia, station number 02334578

1–10. Suwanee Creek at Suwanee, Georgia, station number 02334885

1–11. Crooked Creek near Norcross, Georgia, station number 02335350

1–12. North Fork Peachtree Creek at Graves Road, near Doraville, Georgia, station number 02336030
74 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Table 1–1. Statistical summary for selected constituents, Yellow River at Georgia 124 near Lithonia, Georgia, station number 02207120. 

Samples collected 1996–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 68 90 162 2,900 39 69 104 140 172 233

Turbidity (FNU) 68 89 102 1,002 0.5 4.0 7 31 138 290

Nitrate+nitrite as nitrogen, total 

(mg/L as N)

69 77 1.66 5.90 0.02 0.72 0.90 1.60 2.00 2.80

Ammonia plus organic nitrogen 

(mg/L as N)

65 69 0.71 3.60 0.11 0.23 0.30 0.50 0.92 1.50

Nitrogen, total (mg/L as N) 69 77 2.36 6.40 0.22 1.37 1.81 2.18 2.76 3.60

Phosphorus, total (mg/L as P) 69 76 0.079 0.640 0.006 0.013 0.020 0.032 0.090 0.200

Phosphorus, dissolved (mg/L as P) 69 90 0.019 0.140 0.003 0.009 0.010 0.020 0.029 0.040

Organic carbon, total (mg/L as P) 28 34 3.4 10.0 1.1 1.9 2.4 3.1 4.2 5.2

Lead, total (μg/L) 69 77 5.3 38 0.1 1.0 2.0 3.0 8.7 12

Zinc, total (μg/L) 69 77 30 200 4.8 7.8 10 18 38 70

Dissolved solids, total (mg/L) 69 77 95 230 44 54 73 94 111 140

Suspended solids, total (mg/L) 68 77 126 1,300 1 2 4 29 170 346

Suspended sediment (mg/L) 15 45 206 988 2 7 32 134 288 537

Table 1–2. Statistical summary for selected constituents, No Business Creek at Lee Road, below Snellville, Georgia, station number 

02207185. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 52 76 83 171 39 52 61 76 100 125

Turbidity (FNU) 50 74 87 765 0.8 3.9 7 35 129 234

Nitrate+nitrite as nitrogen, total 

(mg/L as N)

49 59 0.94 2.50 0.03 0.39 0.54 0.77 1.45 1.70

Ammonia plus organic nitrogen 

(mg/L as N)

49 59 0.7 5.1 0.1 0.2 0.3 0.5 0.8 1.4

Nitrogen, total (mg/L as N) 49 59 1.66 5.61 0.28 0.84 1.08 1.52 2.01 2.43

Phosphorus, total (mg/L as P) 49 58 0.066 0.400 0.005 0.009 0.020 0.033 0.078 0.170

Phosphorus, dissolved (mg/L as P) 49 58 0.022 0.120 0.002 0.006 0.008 0.020 0.020 0.050

Organic carbon, total (mg/L as P) 45 56 3.1 7.2 1.0 1.7 2.1 2.8 3.8 5.0

Lead, total (μg/L) 32 39 6.8 47 0.2 0.3 1.6 3.8 9.0 14

Zinc, total (μg/L) 32 39 30 310 2.0 5.3 6.1 12.0 27 79

Dissolved solids, total (mg/L) 49 59 61 110 26 43 51 60 72 80

Suspended solids, total (mg/L) 49 59 108 1,300 1 2 4 23 122 270

Suspended sediment (mg/L) 24 49 238 3,780 3 7 22 69 202 459
Appendix 75

Table 1–3. Statistical summary for selected constituents, Big Haynes Creek at Lenora Road, near Loganville, Georgia, station number 

02207385. Samples collected 1996–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 106 91 59 160 20 42 50 59 65 75

Turbidity (FNU) 106 91 100 1,105 0.8 5.0 7.2 19.6 104 265

Nitrate+nitrite as nitrogen, total 

(mg/L as N)

88 79 0.85 2.20 0.22 0.52 0.62 0.80 0.99 1.20

Ammonia plus organic nitrogen 

(mg/L as N)

88 79 0.7 2.6 0.1 0.2 0.3 0.4 0.9 1.6

Nitrogen, total (mg/L as N) 88 79 1.50 3.37 0.80 1.00 1.12 1.34 1.72 2.40

Phosphorus, total (mg/L as P) 88 78 0.110 0.590 0.006 0.016 0.033 0.074 0.150 0.280

Phosphorus, dissolved (mg/L as P) 88 83 0.039 0.340 0.000 0.007 0.014 0.020 0.050 0.093

Organic carbon, total (mg/L as P) 43 37 3.4 23 0.9 1.4 1.9 2.8 3.9 6.0

Lead, total (μg/L) 88 79 5.1 44 0.16 1.0 1.1 2.1 6.9 12

Zinc, total (μg/L) 87 79 25 580 2.0 4.4 6.3 12 31 55

Dissolved solids, total (mg/L) 88 79 57 1,400 20 34 40 49 56 62

Suspended solids, total (mg/L) 87 79 128 1,100 1 3 5 14 180 390

Suspended sediment (mg/L) 17 46 758 10,500 4 6 25 199 457 1,038

Table 1–4. Statistical summary for selected constituents, Brushy Fork Creek at Beaver Road, near Loganville, Georgia, station 

number 02207400. Samples collected 1996–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 89 91 43 95 10 34 39 44 47 50

Turbidity (FNU) 88 91 113 2,062 3 9 14 31 131 282

Nitrate+nitrite as nitrogen, total 

(mg/L as N)

89 75 0.30 1.40 0.02 0.17 0.22 0.28 0.35 0.46

Ammonia plus organic nitrogen 

(mg/L as N)

89 75 0.8 4.9 0.1 0.3 0.3 0.5 0.9 1.6

Nitrogen, total (mg/L as N) 89 75 1.07 5.16 0.26 0.51 0.61 0.79 1.27 2.09

Phosphorus, total (mg/L as P) 89 75 0.106 1.400 0.005 0.020 0.030 0.050 0.125 0.250

Phosphorus, dissolved (mg/L as P) 89 75 0.024 0.120 0.003 0.015 0.020 0.020 0.020 0.038

Organic carbon, total (mg/L as P) 27 37 4.0 11 1.6 2.4 2.7 3.6 4.8 6.6

Lead, total (μg/L) 89 75 5.0 89 0.15 1.0 1.1 2.85 4.9 11

Zinc, total (μg/L) 88 75 18 440 1.4 3.3 5.6 9.5 21 38

Dissolved solids, total (mg/L) 89 76 43 300 20 30 34 40 46 55

Suspended solids, total (mg/L) 88 75 158 3,300 1 5 8 19 197 390

Suspended sediment (mg/L) 16 46 364 3,817 5 11 28 156 358 948
76 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Table 1–5. Statistical summary for selected constituents, Alcovy River at New Hope Road, near Grayson, Georgia, station number 

02208150. Samples collected 1997–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 70 81 72 202 35 52 58 72 82 89

Turbidity (FNU) 68 82 174 3,036 2 6 10 39 232 450

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 68 74 0.44 0.88 0.18 0.28 0.37 0.43 0.51 0.56

Ammonia plus organic nitrogen 

(mg/L as N) 70 74 0.8 4.7 0.1 0.2 0.21 0.5 1.0 1.4

Nitrogen, total (mg/L as N) 70 74 1.18 5.15 0.38 0.61 0.70 0.95 1.46 1.89

Phosphorus, total (mg/L as P) 70 73 0.105 0.800 0.004 0.008 0.020 0.050 0.160 0.270

Phosphorus, dissolved (mg/L as P) 70 82 0.021 0.027 0.002 0.003 0.007 0.020 0.020 0.050

Organic carbon, total (mg/L as P) 27 31 3.1 14 0.5 1.1 1.4 2.4 4.5 5.7

Lead, total (μg/L) 67 73 13 150 0.1 1.0 2.0 5.9 15 32

Zinc, total (μg/L) 65 73 43 490 2.0 2.5 5.2 18 57 110

Dissolved solids, total (mg/L) 68 74 58 220 18 40 47 54 63 76

Suspended solids, total (mg/L) 67 73 224 1,600 1 3 5 39 355 705

Suspended sediment (mg/L) 19 40 467 2,247 5 7 18 306 724 1,259

Table 1– 6. Statistical summary for selected constituents, Wheeler Creek at Bill Cheek Road, near Auburn, Georgia, station number 

02217274. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 50 65 50 190 20 36 42 49 55 60

Turbidity (FNU) 51 71 196 2,043 2 4 9 48 182 451

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 50 58 0.59 0.93 0.10 0.33 0.39 0.60 0.78 0.86

Ammonia plus organic nitrogen 

(mg/L as N) 50 58 0.8 3.3 0.1 0.2 0.3 0.5 1.1 1.8

Nitrogen, total (mg/L as N) 50 58 1.39 3.92 0.51 0.88 1.00 1.19 1.53 2.28

Phosphorus, total (mg/L as P) 50 57 0.111 0.650 0.004 0.012 0.020 0.050 0.170 0.280

Phosphorus, dissolved (mg/L as P) 50 58 0.032 0.150 0.000 0.007 0.020 0.020 0.050 0.067

Organic carbon, total (mg/L as P) 47 49 3.3 10 0.6 1.1 1.3 2.9 4.6 6.5

Lead, total (μg/L) 31 41 11 170 0.1 0.3 3.0 6.0 9.1 18

Zinc, total (μg/L) 31 41 37 390 2 5.3 7.9 20 38 83

Dissolved solids, total (mg/L) 50 58 52 220 8 32 38 45 54 68

Suspended solids, total (mg/L) 50 58 256 5,400 1 2 4 46 218 611

Suspended sediment (mg/L) 23 48 430 4,060 1 4 25 127 592 1,094
Appendix 77

Table 1–7. Statistical summary for selected constituents, Apalachee River at Fence Road, near Dacula, Georgia, station number 

02218565. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 49 67 56 140 20 40 47 57 63 71

Turbidity (FNU) 48 68 171 1,180 3.7 5.1 8 73 293 497

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 47 53 0.44 0.78 0.13 0.28 0.34 0.43 0.51 0.62

Ammonia plus organic nitrogen 

(mg/L as N) 28 65 0.90 3.40 0.08 0.20 0.26 0.50 1.25 2.30

Nitrogen, total (mg/L as N) 47 53 1.39 4.20 0.42 0.56 0.68 0.92 1.94 3.00

Phosphorus, total (mg/L as P) 47 53 0.102 0.500 0.003 0.010 0.020 0.050 0.140 0.280

Phosphorus, dissolved (mg/L as P) 47 53 0.021 0.078 0.003 0.005 0.010 0.020 0.020 0.050

Organic carbon, total (mg/L as P) 45 52 3.5 10 0.3 1.4 1.8 2.9 4.7 6.8

Lead, total (μg/L) 30 36 9.2 38 0.2 0.3 3 8.9 12 24

Zinc, total (μg/L) 30 36 47 850 1.3 4.4 6.3 20 53 91

Dissolved solids, total (mg/L) 47 53 50 140 18 32 42 47 55 70

Suspended solids, total (mg/L) 47 53 274 2,000 1 3 4 52 380 815

Suspended sediment (mg/L) 23 46 544 3,992 1 6 22 135 557 1,869

Table 1–8. Statistical summary for selected constituents, Richland Creek at Suwanee Dam Road, near Buford, Georgia, station 

number 02334880. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 49 57 83 1,100 25 46 59 77 84 97

Turbidity (FNU) 51 64 434 3,827 1.8 6.9 14 89 527 1,133

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 49 56 0.54 1.20 0.27 0.32 0.42 0.54 0.63 0.76

Ammonia plus organic nitrogen 

(mg/L as N) 49 56 0.99 5.30 0.08 0.20 0.34 0.50 1.30 2.10

Nitrogen, total (mg/L as N) 49 56 1.54 5.77 0.50 0.80 0.91 1.16 1.87 2.59

Phosphorus, total (mg/L as P) 50 55 0.267 3.000 0.012 0.020 0.030 0.093 0.330 0.730

Phosphorus, dissolved (mg/L as P) 50 55 0.031 0.440 0.005 0.009 0.017 0.020 0.035 0.050

Organic carbon, total (mg/L as P) 48 53 3.0 12 0.4 1.1 1.4 2.4 4.1 5.9

Lead, total (μg/L) 31 40 18 89 0.1 0.5 3 9 24 48

Zinc, total (μg/L) 31 40 76 780 2 5 7 30 88 220

Dissolved solids, total (mg/L) 50 57 71 300 19 43 51 58 73 100

Suspended solids, total (mg/L) 50 56 592 6,900 1 3 10 150 640 1,530

Suspended sediment (mg/L) 25 50 1,234 9,301 3 20 37 380 1,636 3,326
78 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia

Table 1– 9. Statistical summary for selected constituents, Level Creek at Suwanee Dam Road, near Suwanee, Georgia, station 

number 02334578. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 48 75 99 3,600 26 37 50 64 77 82

Turbidity (FNU) 48 75 235 2,570 2.3 4.2 7.1 78 328 602

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 48 58 0.36 1.30 0.02 0.17 0.27 0.34 0.43 0.55

Ammonia plus organic nitrogen 

(mg/L as N) 48 53 0.86 3.80 0.15 0.20 0.28 0.51 1.10 2.20

Nitrogen, total (mg/L as N) 48 58 1.28 4.90 0.22 0.46 0.64 0.91 1.69 2.90

Phosphorus, total (mg/L as P) 48 57 0.140 1.000 0.004 0.014 0.020 0.050 0.210 0.430

Phosphorus, dissolved (mg/L as P) 48 57 0.027 0.110 0.004 0.008 0.016 0.020 0.039 0.050

Organic carbon, total (mg/L as P) 46 55 3.6 22 0.6 1.1 1.3 2.9 5.2 7.1

Lead, total (μg/L) 30 42 9 56 0.1 0.61 2.8 5.8 9 18

Zinc, total (μg/L) 30 42 48 470 2.9 5.9 8.2 20 51 140

Dissolved solids, total (mg/L) 48 58 57 120 25 39 48 55 63 72

Suspended solids, total (mg/L) 48 58 271 2,500 1 3 4 67 320 1,000

Suspended sediment (mg/L) 24 49 538 5,121 2 5 23 165 674 1,678

Table 1–10. Statistical summary for selected constituents, Suwanee Creek at Suwanee, Georgia, station number 02334885. Samples 

collected 1996–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 99 102 116 3,700 34 60 74 97 110 140

Turbidity (FNU) 94 100 134 1213 2.7 5.9 13 30 205 390

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 86 85 0.75 4.20 0.10 0.39 0.50 0.66 0.88 1.10

Ammonia plus organic nitrogen 

(mg/L as N) 71 71 0.67 2.30 0.14 0.20 0.29 0.50 1.00 1.40

Nitrogen, total (mg/L as N) 72 84 1.44 4.80 0.49 0.86 1.03 1.28 1.73 2.24

Phosphorus, total (mg/L as P) 85 84 0.114 0.790 0.004 0.015 0.020 0.040 0.140 0.310

Phosphorus, dissolved (mg/L as P) 86 85 0.023 0.310 0.002 0.008 0.014 0.020 0.020 0.043

Organic carbon, total (mg/L as P) 38 34 3.0 7.0 0.6 1.7 1.9 2.4 3.9 5.1

Lead, total (μg/L) 70 85 6 28 0.2 1 2 3 9 15

Zinc, total (μg/L) 69 85 29 720 2 4 6 19 35 59

Dissolved solids, total (mg/L) 68 85 67 140 25 44 55 66 78 90

Suspended solids, total (mg/L) 79 85 130 870 1 4 8 30 192 410

Suspended sediment (mg/L) 22 53 232 1,739 2 10 22 150 302 545
Appendix 79

Table 1–11. Statistical summary for selected constituents, Crooked Creek near Norcross, Georgia, station number 02335350. Samples 

collected 1996–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 81 98 116 4,500 28 43 57 89 103 110

Turbidity (FNU) 78 95 151 2,439 2.3 3.9 6.4 38 200 428

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 81 82 0.36 2.70 0.03 0.17 0.27 0.34 0.42 0.49

Ammonia plus organic nitrogen 

(mg/L as N) 67 69 0.82 3.80 0.13 0.20 0.26 0.52 1.25 1.80

Nitrogen, total (mg/L as N) 70 82 1.21 4.14 0.25 0.48 0.61 0.92 1.68 2.25

Phosphorus, total (mg/L as P) 82 81 0.125 0.820 0.006 0.016 0.020 0.037 0.195 0.380

Phosphorus, dissolved (mg/L as P) 83 81 0.023 0.280 0.002 0.008 0.016 0.020 0.020 0.040

Organic carbon, total (mg/L as P) 37 38 3.7 13 0.5 1.3 1.6 2.7 5.2 7.1

Lead, total (μg/L) 72 82 10 120 0.1 1 2 6 12 24

Zinc, total (μg/L) 71 82 66 710 4 6 10 39 97 160

Dissolved solids, total (mg/L) 71 82 60 170 14 36 44 60 70 78

Suspended solids, total (mg/L) 81 82 270 3,120 1 2 4 41 370 700

Suspended sediment (mg/L) 17 41 687 3,141 2 4 16 362 1,241 1,686

Table 1–12. Statistical summary for selected constituents, North Fork Peachtree Creek at Graves Road, near Doraville, Georgia, 

station number 02336030. Samples collected 2001–2009.

[μS/cm, microsiemens per centimeter; FNU, formazin nephelometric unit; mg/L, milligrams per liter; %, percentile percentage of samples that are less than 

listed concentrations for each station]

Constituent

Number and 

type of samples Concentration

Base 

flow

Stormflow Mean Maximum Minimum 10% 25% 50% 75% 90%

Specific conductance (μS/cm) 49 73 68 120 4 31 42 71 95 100

Turbidity (FNU) 50 70 97 994 0.3 5 8 48 124 225

Nitrate+nitrite as nitrogen, total 

(mg/L as N) 50 58 0.48 1.20 0.04 0.30 0.38 0.45 0.53 0.68

Ammonia plus organic nitrogen 

(mg/L as N) 50 58 0.8 4 0.1 0.2 0.3 0.5 1 1.6

Nitrogen, total (mg/L as N) 50 58 125.00 4.86 0.44 0.61 0.72 0.94 1.50 2.28

Phosphorus, total (mg/L as P) 50 57 0.097 0.680 0.004 0.010 0.020 0.050 0.140 0.200

Phosphorus, dissolved (mg/L as P) 50 72 0.023 0.120 0.000 0.000 0.009 0.020 0.027 0.050

Organic carbon, total (mg/L as P) 48 54 3.8 16 0.5 0.9 1.1 2.7 5 9.1

Lead, total (μg/L) 31 39 9 50 0.11 0.8 3 8 11 21

Zinc, total (μg/L) 31 39 89 380 5 14 22 61 120 220

Dissolved solids, total (mg/L) 50 58 58 170 10 32 44 62 71 81

Suspended solids, total (mg/L) 50 58 123 870 1 2 4 28 180 350

Suspended sediment (mg/L) 24 46 357 2,758 4 10 22 118 337 1,098

Manuscript approved July 23, 2014

Prepared by the USGS Science Publishing Network

Raleigh Publishing Service Center (PSC)

Edited by Kay P. Naugle

Illustrations by Kimberly A. Swidarski and 

Caryl J. Wipperfurth

Layout by Kay P. Naugle

Cover by Carol A. Quesenberry, Denver PSC

For more information concerning this report, contact:

Director, Georgia Water Science Center

1770 Corporate Drive, Suite 500

Norcross, GA 30093

(678) 924-6700

http://ga.water.usgs.gov/ 
Joiner and others—Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia—Scientific Investigations Report 2014–5141

Printed on recycled paper

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20145141