western portion of the county and along

major transportation corridors. Agricultural land use, which now accounts for

only about 1 percent of the county, peaked

around 1920. However, stream channels

and floodplains may still have large surpluses of sediment resulting from poor

agricultural soil conservation practices

of that period (Trimble, 1969).

The major hydrologic feature of Gwinnett

County is the Eastern Continental Divide.

Streams southeast of the divide flow to

the Atlantic Ocean, draining 74 percent

of the county. Streams northwest of the

divide flow to the Gulf of Mexico,

draining 26 percent of the county. The

Chattahoochee River is the principal

water supply for Metropolitan Atlanta

and forms the 25-mile northwestern

boundary of the county. Buford Dam

impounds Lake Sidney Lanier at the

extreme northern boundary of the county.

Chattahoochee

River

Lake Sidney Lanier

Chattahoochee–

Flint River Watershed

Ocmulgee–

Oconee–Altamaha 

River Watershed

Georgia

Atlantic

Ocean

Buford 

Dam

Yellow River

Watershed

Suwanee

Creek Watershed

Alcovy

River

Watershed

Brushy Fork

Creek

Watershed

Big

Haynes

Creek

Watershed

Crooked

Creek

Watershed

84°15'

84°7'30"

84°

83°52'30"

33°52'30"

34°

34°7'30"

GWINNETT COUNTY

Snellville

Lilburn

Buford

Sugar Hill

Suwanee

Lawrenceville

Duluth

Norcross

About

15 miles

to Atlanta

Gwinnett County land use in 1998

(Gwinnett County Department of Public Utilities, 2000).

Park

3 percent

Transportation

9 percent

Agriculture

1 percent

Water

1 percent

Highdensity

residential

4 percent

Commercial, 

 industrial,

 and utilities 

 10 percent

Numbers have been rounded

Vacant

(undeveloped)

25 percent

Low-tomedium

density

residential

28 percent

Estate

residential

19 percent

Stream quality is a barometer of urban

land-use pressures on a watershed.

Managing land use in a watershed is vital

to protecting drinking-water supplies,

recreational opportunities, and stream

ecosystem health. However, the effect

of land use and management practices

on streams is difficult to assess and often

unmeasured. Pollution due to land use,

or nonpoint-source pollution, is complex

in its origin, transport, impacts, and

response to management practices. How

does urbanization affect sediment or

nutrient loads in streams? How effective

is a detention pond or a stream-buffer

requirement? In six watersheds of Gwinnett County, Georgia, monitoring results

of an ongoing study show the effects of

land use on streamflow and on loads of

suspended solids, metals, and nutrients.

Gwinnett County background

Gwinnett County lies in northeastern

Metropolitan Atlanta, Georgia, and is

one of the most rapidly developing counties in the United States. The population

of Gwinnett County grew by more than

250 percent from 1980 to 2000, and the

2001 population is estimated at 621,500

(U.S. Census Bureau, 2002).

Land use in the county is broadly mixed,

with residential being the largest. Urban

development is most dense in the south0 4 2 6 MILES

0 6 2 4 KILOMETERS

Population

0

200,000

400,000

600,000

190019201940196019802000

EXPLANATION

Eastern Continental 

 Divide

Elevation, in feet

High: 1,288

Low: 720

Monitoring station

Monitored watershed

Does land use affect our streams?

A watershed example from Gwinnett County, Georgia, 1998– 2001

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Water-Resources Investigations Report 02-4281

December 2002
designed and installed during 1996 as

a cooperative investigation of the U.S.

Geological Survey and the Gwinnett

County Department of Public Utilities.

Stream gages in six watersheds continuously monitor water level, streamflow,

and precipitation, and periodically collect

water-quality samples. At each stream

gage, three storm-composite samples and

three baseflow samples are collected

every 6 months. In baseflow conditions,

streamflow is from ground water that

seeps through the channel bed and banks.

Discharge measurements also are routinely

made to define stage-discharge relations.

Aquatic invertebrate sampling and habitat

assessments were conducted in four watersheds. This report includes data collected

through September 2001. During 2002,

monitoring began at six additional

watersheds in Gwinnett County.

Baseflow samples are collected using

depth and width integrating techniques.

Quality-assurance/quality-control measures ensure that sampled data are not

contaminated by outside sources and are

representative of the entire stream cross

section. All stormflow and baseflow

water-quality samples are processed

and analyzed at USGS laboratories.

Properties analyzed

During sample collection, standard field

properties are measured including pH,

specific conductance, water temperature,

and dissolved oxygen. Water samples are

analyzed for the following constituents:

biological oxygen demand, chemical

oxygen demand, total suspended solids,

turbidity, total dissolved solids, total

phosphorus, dissolved phosphorus,

several nitrogen species, and hardness.

Trace metals analyzed include cadmium,

copper, lead, zinc, chromium,

and magnesium.

Stream monitoring

Monitoring streams is critical to watershed assessment and informed management. Longterm monitoring using

consistent

methods is

essential to

compare

conditions

between

watersheds

and to

identify

trends. The

watershedmonitoring

network for

Gwinnett

County was

Stream-corridor land use

Streams are particularly sensitive to land

use in and near their floodplains. When

protected from development, these stream

corridors may filter out and substantially

reduce pollutants entering streams. The

Gwinnett County Stormwater Design

Manual (1999) provides incentives to

preserve additional stream-buffer areas

beyond the 25-foot requirement of

Georgia law.

Stream-corridor land use is compared

with watershedwide land use. A corridor

width of 30 meters (98 feet) was chosen

based on the limited resolution of

available land-use data. Land is much

less developed in the stream corridor

than watershedwide. In the more developed watersheds, however, a greater

percentage of the stream corridor is

developed. The bar chart on the right

illustrates this for transportation land use,

which has a high impact on water quality.

0

4

8

12

16

TRANSPORTATION LAND

USE, IN PERCENT

WATERSHED

WatershedwideStream 

corridor

of 30 meters

Less

developed

More

developed

Brushy

Fork

Creek

Big

Haynes

Creek

Alcovy

River

Suwanee

Creek

Yellow

River

Crooked

Creek

02207120 GWLT-02 Yellow River at SR 124 near Lithonia, Ga. 162

02207385 GWLT-04 Big Haynes Creek at Lenora Road near Snellville, Ga. 17.3

02207400 GWLT-05 Brushy Fork Creek at Beaver Road near Loganville, Ga. 8.15

02208150 GWLT-07 Alcovy River at New Hope Road near Grayson, Ga. 30.8

02334885 GWLT-12 Suwanee Creek at Buford Highway near Suwanee, Ga. 47

02335350 GWLT-01 Crooked Creek at Spalding Drive near Norcross, Ga. 8.89

USGS

station

number

Gwinnett

County

site number

Monitoring-station name

Area, in

square

miles

Assessment of nonpoint-source pollution

requires water-quality sampling during

rainfall runoff. Stormwater samples are

collected with automatic samplers to

obtain a discharge-weighted composite

sample of the storm runoff.

0

40

80

120

160

200

JANUARY 2001

STREAMFLOW, IN CUBIC

FEET PER SECOND

19 20

Sample taken

Big Haynes Creek

Stream-monitoring station

at Crooked Creek.

Crooked Creek during baseflow. Crooked Creek during stormflow.

Monitoring stations for the six monitored watersheds
The seasonal nature of streamflow is evident in the monthly mean water yield. Water yield is the total volume

of streamflow during a month or year divided by watershed area. The average yield for the wettest month

(March) is more than three times greater than that of the driest month (October). Annual variability is

evident in the annual mean flow and yield. The 1998 average flow was at least twice the flow of the drought

years 1999–2001. For the period 1998–2001 in the Yellow River watershed, the average total water yield

was 23.7 inches, which is 52 percent of the average precipitation.

Annual mean flow and water yield 

0

100

200

300

400

500

1998 1999 2000 2001 1998–

2001

FLOW, IN CUBIC

FEET PER SECOND

FLOW, IN CUBIC

FEET PER SECOND

Monthly mean flow and water yield, 1998–2001

0

1

2

3

4

0

114

228

342

456

570

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

YIELD, IN INCHES

Stormflow

Baseflow

0

7

14

21

28

35

42

YIELD, IN INCHES

 Yellow River

Watershed

Jackson Creek

Creek

Beaver

Ruin

Creek

Yellow

Jacks

River

Yellow

River

S

weetwater

Creek

84°7'30"

84°

33°57'45"

33°50'42"

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

Transportation

12 percent

Park

3 percent

High-density 

residential

2 percent

Numbers have been rounded. 

Areas less than

1 percent not shown

Low-to-medium

density residential

42 percent

Commercial, 

 industrial,

 and utilities 

 14 percent

Vacant

(undeveloped)

17 percent

Estate

residential

10 percent

A closer look at land use and hydrology,

Yellow River watershed

The Yellow River watershed is the largest

(162 square miles) in the county and lies

in the Ocmulgee River watershed. It is

situated in the more densely developed,

southwestern part of the county. Land use

(in 1998) is shown for the portion (97

percent) of the watershed located within

Gwinnett County. The predominate land

use (42 percent) is low-to-medium density

residential. Commercial and industrial development are concentrated along the major

transportation corridors. Four wastewatertreatment facilities discharge into the watershed upstream from the monitoring station.

Continuously monitored streamflow can be

divided into components of stormflow and

baseflow using hydrograph separation techniques (Sloto and Crouse, 1996). Stormflow

is the rainfall runoff that enters the drainage

network from overland flow. As a percent

of total flow, stormflow is greatest during

wet months and years. Baseflow enters the

drainage network through the streambed

and banks and is more stable.

Yellow River watershed land use in 1998.

Land use from

Gwinnett County

Department of Pubic

Utilities, 2000

 Yellow River

Watershed

Gwinnett 

County

EXPLANATION

Monitoring station

Wastewater-treatment facility
Streamflow concentrations and watershed

loads and yields, Yellow River watershed

Streamflow samples are analyzed to determine constituent concentrations. These data

define whether the stream quality is adequate

for uses such as recreation, for drinkingwater supply, for fishing, and as a habitat

for aquatic organisms. However, one must

know more than the condition of a stream in

order to mitigate pollution and manage the

resource. Is the pollution from point sources

or nonpoint sources? When and how are constituents transported? An intensive monitoring program, such as the one for Gwinnett

County, can address these questions.

Constituent load, shown to the right in

units of tons per year, is the product of

concentration and discharge. Constituent

yield, shown in units of pounds per acre

per year, is the load divided by the watershed area. Loads are computed using a

relatively simple rating-curve model

(Crawford, 1996). The model is calibrated and run using a stormflow load-todischarge curve, and a baseflow load-todischarge curve, and applied to the daily

time-step hydrograph for the Yellow River.

The results, summarized in the graphs to

the right, illustrate several important points.

• Water quality typically is worse during

stormflow than during baseflow.

• Concentration-to-discharge curves

are dissimilar for stormflow and

baseflow for suspended solids and for

constituents having an affinity for solid

phase particulates (such as metals);

whereas the curves are similar for

total dissolved solids.

• The absence (lead) or presence (total

nitrogen) of constituents from point

sources is evident in the baseflow

concentration-to-discharge curves.

Concentrations of constituents from

point sources are diluted and decrease

with increasing baseflow.

• Nonpoint-source constituent concentrations increase with increasing

streamflow because larger storms typically wash off more constituent mass

from a larger area of the watershed.

• In constituent loads and yields, the effect

of dissimilar baseflow and stormflow

concentration characteristics is multiplied. Stormflow loads of nonpointsource constituents vary exponentially

with stream discharge because concentration increases with discharge, and the

load is a product of concentration

and discharge.

• Seasonal and annual loads vary with

precipitation and total streamflow. Baseflow loads are relatively constant from

year to year.

These findings are relevant to analyses of

total maximum daily load, watershed assessments, and watershed protection strategies.

1

10

100

1,000

10

100

1,000

10,000

0

40

80

120

140

0

1

2

3

4

DISCHARGE, IN CUBIC FEET PER SECOND

0

10

20

30

40

0

100

200

300

400

500

600

700

Concentration and discharge Annual yield and load

0

1,000

2,000

3,000

4,000

5,000 260,000

195,000

130,000

6,500

0

36,000

27,000

18,000

9,000

0

1,300

975

650

325

0

8

6

4

2

0 0

4

8

12

16

1998

100 1,000

1999 2000 2001 1998–

2001

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRAMS PER LITER

CONSTITUENT YIELD, IN POUNDS PER ACRE PER YEAR

CONSTITUENT LOAD, IN TONS PER YEAR

CONCENTRATION,

IN MICROGRAMS PER LITER

CONCENTRATION, 

IN COLONIES PER

100 MILLILITERS

Stormflow

Stormflow

Baseflow

Baseflow

A. Total suspended solids A. Total suspended solids

B. Total dissolved solids B. Total dissolved solids

C. Total nitrogen C. Total nitrogen

D. Total lead

E. Fecal-coliform bacteria

D. Total lead

Collecting a stream sample at baseflow.

A. Excessive total suspended solids are a major cause of habitat degradation in

north Georgia streams. Stormflow and baseflow have dissimilar total-suspended-solid

characteristics; 99 percent of the annual load is carried by stormflow. B. Total dissolved

solids are naturally higher in baseflow, which has filtered through the ground. The total

dissolved solids-to-discharge curve is similar for baseflow and stormflow. C. The dilution

of point-source nitrogen is evident in baseflow. Nonpoint-source nitrogen increases with

discharge. These nitrogen levels do not indicate an impaired status for the Yellow River.

D. Lead concentrations are typically below detection limits during baseflow, indicating

no lead point sources. Stormflow carries 97 percent of the annual lead load. E. Fecalcoliform bacteria serve as an indicator for possible pathogenic bacteria in streams.

Fecal-coliform data are highly variable, but stormflow concentrations typically are

much greater than baseflow concentrations.
Monitoring data from six Gwinnett

County watersheds show strong

relations between transportation

land use in the stream corridor and

watershed yield of total suspended

solids and lead.

Effects of land use on stream hydrology

Bare or impervious areas and a more

developed stormwater drainage system

produce greater volumes of high-energy

stormflow and reduce baseflow in a

stream. A measure of this effect is the

average stormflow of a watershed, taken

as a percentage of total watershed flow.

The bar chart below shows that with

Nitrogen yield can be estimated from watershed land use and point-source indices.

Effects of land use on stream loads

and yields

Increased stormwater runoff accelerates

erosion of the land surface, particularly

for areas under construction, and causes

scouring of stream channels resulting in

much higher stream-sediment loads. Total

suspended solids are used as a measure

of overall development impacts in the

Gwinnett County Watershed Protection

Plan (2000). The usefulness of that

management strategy is illustrated here

in the strong relation shown between

total suspended solids and transportation

land use in the stream corridor.

Lead levels in most streams have decreased since the early 1970s as a result

of compliance with environmental laws.

Lead yields in the six monitored watersheds are closely related to transportation

land use in the stream corridor. This

may be due to increased lead deposition

in paved areas by transportation and

related land uses; and because of the

greater efficiency of wash off from

these surfaces, accompanied by reduced

filtration of runoff before it reaches the

drainage network.

Nitrogen loads in the watershed have both

nonpoint and point sources. Nonpoint

sources include natural organic detritus

and applied nitrogen, such as from

fertilizer. Nitrogen yield was estimated

based on an index of whether or not the

increasing watershed development,

stormflow increases as a percentage of

total watershed discharge.

Transportation land use can be an

indicator of the total impervious area

that is connected to a drainage network.

Stormflow was correlated with transportation land use in the stream corridor and

watershed contains wastewater-treatment

facilities, and based on land-use percentages in the following categories: low-tomedium density residential, high-density

residential, commercial/utilities,

and transportation.

The annual yield of a constituent in

streamflow is affected by an array of

land uses, point sources, hydrologic

factors, chemical processes, and other

factors. Effective water-resource management is difficult because of these many

factors. However, stream monitoring can

provide information and tools that are

valuable for watershed assessments,

land-use planning, and informed waterresource management.

watershedwide. The correlation is slightly

better for stream-corridor land use. Additional stream-corridor analyses are needed

using higher resolution land-use data.

4 6 8 10 12 14

MEASURED TOTAL NITROGEN YIELD,

IN POUNDS PER ACRE PER YEAR

4

6

8

10

12

14

ESTIMATED TOTAL NITROGEN YIELD,

IN POUNDS PER ACRE PER YEAR

BR

SU

BH

CR

YL

AL

0 5 10 15

TRANSPORTATION LAND USE, IN PERCENT 

30

25

35

40

45

50

55

60

STORMFLOW (1998–2001) AS

PERCENTAGE OF TOTAL FLOW

Watershed

wide

Stream

corridor

BR

SU

BH

CR

YL

AL

BR

SU

BH

CR

YL

AL

0

10

20

30

YIELD, IN INCHES

Stormflow

Baseflow

Mean annual water yield, 1998–2001

Brushy

Fork

Creek

(BR)

Big

Haynes

Creek

(BH)

Alcovy

River

(AL)

Suwanee

Creek

(SU)

Yellow

River

(YL)

Crooked

Creek

(CR)

Less WATERSHED

developed

More

developed

0 4 6 8 10 2

0.02

0.04

0.06

0.08

BR

SU

BH

CR

YL

AL

TRANSPORTATION LAND USE

IN THE STREAM CORRIDOR,

IN PERCENT 

TOTAL LEAD YIELD, IN POUNDS

PER ACRE PER YEAR

500

1,000

1,500

2,000

2,500

TOTAL SUSPENDED SOLIDS YIELD,

IN POUNDS PER ACRE PER YEAR

BR

SU

BH

CR

YL

AL

0 4 6 8 10 2

Sediment from stormflow into the

Yellow River.

Stormflow, as a percent of total flow,

increases with increasing transportation

land use.
Effects of land use on stream ecosystems

Just as stream-water quality is affected

by watershed land use, so are the aquatic

organisms. Stream communities in Gwinnett County are composed of a relatively

diverse group of organisms rarely seen

by most people. These stream inhabitants

spend all or parts of their life cycles in

aquatic environments and include insects

as well as crayfishes, snails, mussels, and

aquatic worms. Since these organisms

generally live in the same section of stream

during their entire lives, the invertebrate

community that develops is correlated with

water-quality and habitat conditions of that

stream section.

To determine the Georgia Ecological

Condition Scores (Georgia Department

of Natural Resources, 1997), invertebrates

collected during site visits are identified

and counted by a trained taxonomist. Similar species are grouped according to how

tolerant they are to pollution or how they

feed in the aquatic environment. Using

these data, indices that have been shown

to be related to environmental degradation

are calculated.

Part of the Georgia Ecological Condition

Score is composed of a separate index

known as the Georgia Visual Assessment

Habitat Score (Georgia Department of

Natural Resources, 1997). This index

provides managers with an assessment

of the physical condition of the stream

habitats present at a site and allows comparison among streams included in the

study as well as to regional reference

sites. Habitat data allow more accurate

interpretation of the biological data

especially when viewed in conjunction

with water-quality data.

Invertebrate samples were collected

near the stream gages of four of the six

monitored watersheds. A reference site,

located on the Alcovy river in the upper

Alcovy watershed, was selected and

sampled to represent relatively unimpaired

ecological conditions in Gwinnett County.

Georgia Ecological Condition Scores are

expressed as a percentage of the score of

the reference site and rated from very

good to very poor, in comparison with

conditions across northern Georgia,

including Gwinnett County.

The most densely urbanized watershed,

Crooked Creek, has the lowest Ecological

Condition Score. However, for the three

other watersheds, the comparative ecological condition does not agree with the

comparative land-use development. This

may be largely due to the localized effects

of past channelization on aquatic habitat in

For more information contact:

State Representative

U.S. Geological Survey

3039 Amwiler Road, Suite 130

Atlanta, GA 30360-2824

770-903-9100

http://ga.water.usgs.gov

Written by

Mark N. Landers

Paul D. Ankcorn

Keith W. McFadden

M. Brian Gregory

Design and layout by

Caryl J. Wipperfurth

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Big Haynes Creek and Brushy Fork Creek.

The effects of channelization also are

apparent in the habitat scores.

Streams are strongly affected by watershed

land use. Land-use effects are evident in

the amount of streamflow during storms,

in the amount of constituents in a stream,

and in the stream ecosystem. Monitoring

and understanding of these effects provide

information for the wise management of

land use and water resources.

Georgia Ecological Condition Scores

0

20

40

60

80

100 Very

good

Good

Fair

Poor

Very

poor

PERCENT OF REFERENCE CONDITION

Georgia Visual Assessment Habitat Scores

0

30

60

90

120

150

TOTAL HABITAT SCORE

Alcovy 

River

Big

Haynes

Creek

Brushy

Fork

Creek

Crooked

Creek

Reference site score

hydrograph separation and analysis:

U.S. Geological Survey Water-Resources

Investigations Report 96-4040, 46 p.

Trimble, S.W., 1969, Culturally accelerated sedimentation on the middle Georgia Piedmont:

Athens, Georgia, The University of Georgia,

unpublished masters thesis, 110 p.

U.S. Census Bureau, State and County QuickFacts, accessed on September 1, 2002, at

http://quickfacts.census.gov.

Base maps from Gwinnett County Board of Commissioners, Department of Support Services.

Gwinnett County Department of Public Utilities,

2000, Land-use classification for 1998.

Gwinnett County Stormwater Design Manual,

1999, revised October 2001: Gwinnett

County Georgia Department of Public

Utilities, Stormwater Management

Division, 277 p.

Gwinnett County Watershed Protection Plan,

2000: Gwinnett County Department of

Public Utilities, 54 p.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP:

A computer program for streamflow

References cited

Crawford, C.G., 1996, Estimating mean constituent loads in rivers by the rating-curve

and flow-duration rating-curve methods:

Bloomington, Indiana, Indiana University,

unpublished Ph.D. dissertation, 245 p.

Georgia Department of Natural Resources,

1997, Draft standard operating procedures—

freshwater macroinvertebrate biological

assessment: Atlanta, Ga., Environmental

Protection Division, Water Protection

Branch, 139 p.

U.S. Geological Survey Water-Resources Investigations Report 02-4281

December 2002

Collecting an invertebrate sample

from riffle habitat in a stream in

Metropolitan Atlanta.

Georgia’s Ecological Condition and

Visual Assessment Habitat Scores for

selected watersheds in Gwinnett County.