Stream quality is a barometer of urbanland-use pressures on a watershed.Managing land use in a watershed is vitalto protecting drinking-water supplies,recreational opportunities, and streamecosystem health. However, the effectof land use and management practiceson streams is difficult to assess and oftenunmeasured. Pollution due to land use,or nonpoint-source pollution, is complexin its origin, transport, impacts, andresponse to management practices. Howdoes urbanization affect sediment ornutrient loads in streams? How effectiveis a detention pond or a stream-bufferrequirement? In six watersheds of Gwin-nett County, Georgia, monitoring resultsof an ongoing study show the effects ofland use on streamflow and on loads ofsuspended solids, metals, and nutrients.

Gwinnett County background

Gwinnett County lies in northeasternMetropolitan Atlanta, Georgia, and isone of the most rapidly developing coun-ties in the United States. The populationof Gwinnett County grew by more than250 percent from 1980 to 2000, and the2001 population is estimated at 621,500(U.S. Census Bureau, 2002).

Land use in the county is broadly mixed,with residential being the largest. Urbandevelopment is most dense in the south-

Agriculture1 percent

Transportation9 percent

.Commercial,

industrial,and utilities

10 percent

High-

densityI residential

4 percent

western portion of the county and alongmajor transportation corridors. Agricul-turalland use, which now accounts foronlyabout 1 percent of the county, peakedaround 1920. However, stream channelsand floodplains may still have larg~ SUf-pluses of sediment resulting from pooragricultural soil conservation practicesof that period (Trimble, 1969).Numbers have been rounded

Gwinnett County land use in 1998

(Gwinnett County Deportment of Public Utilities. 2000).

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-rnile northwt;:stern

boundary of the county. Buford Dam

impounds Lake Sidney Lanier at theThe major hydrologic feature of Gwinnett extreme northern boundary of the county.

County is the Eastern Continental Divide.

u.s. Geological Survey Water-Resources Investigations Report 02-4281December 2002

U.S. Department of the Interior

U.S. Geological Survey

"Stream-corridor land use

Streams are particularly sensitive to landuse in and near their floodplains. Whenprotected from development, these streamcorridors may filter out and substantiallyreduce pollutants entering streams. TheGwinnett County Stormwater DesignManual (1999) provides incentives topreserve additional stream-buffer areasbeyond the 25-foot requirement ofGeorgia law.

Baseflow samples are collected usingdepth and width integrating techniques.Quality-assurance/quality-control mea-sures ensure that sampled data are notcontaminated by outside sources and arerepresentative of the entire stream crosssection. All stormflow and baseflowwater-quality samples are processedand analyzed at USGS laboratories.

Stream monitoring

Monitoring streams is critical to water-shed assessment and informed manage-

ment. Long-term monitor-

ing usingconsistentmethods isessential to

compareconditionsbetweenwatershedsand to

identifytrends. Thewatershed-

monitoringStream-monitoring station network forat Crooked Creek. Gwinnett

County was

designed and installed during 1996 asa cooperative investigation of the U.S.Geological Survey and the GwinnettCounty Department of Public Utilities.Stream gages in six watersheds continu-ously monitor water level, streamflow,and precipitation, and periodically collectwater-quality samples, At each streamgage, three storm-composite samples andthree baseftow samples are collectedevery 6 months. In baseftow conditions,streamflow is from ground water thatseeps through the channel bed and banks.Discharge measurements also are routinelymade to define stage-discharge relations.Aquatic invertebrate sampling and habitatassessments were conducted in four water-sheds. This report includes data collectedthrough September 200 I. During 2002,monitoring began at six additionalwatersheds in Gwinnett County.

Monitoring stations for the six monitored watershedsJANUARY 2001

USGS

station

number

Gwinnett

Countysite number

Area, in

squaremiles

Assessment of nonpoint-source pollution

requires water-quality sampling duringrainfall runoff Stormwater samples arecollected with automatic samplers toobtain a discharge-weighted compositesample of the storm runoff

Monitoring-station name

02207120

02207385

02207400

02208150

02334885

02335350

GWLT-O2

GWLT-O4

GWLT-OS

GWLT-O7

GWLT-12

GWLT-O1

162

17.3

8.15

30.8

47

8.89

Yellow River at SR 124 near Lithonia, Ga.

Big Haynes Creek at Lenora Road near Snellville, Ga.

Brushy Fork Creek at Beaver Road near Loganville, Ga

Alcovy River at New Hope Road near Grayson, Ga.

Suwanee Creek at Buford Highway near Suwanee, Ga.

Crooked Creek at Spalding Drive near Norcross, Ga.Properties analyzed

During sample collection, standard fieldproperties are measured including pH,specific conductance, water temperature,and dissolved oxygen. Water samples areanalyzed for the following constituents:biological oxygen demand, chemicaloxygen demand, total suspended solids,turbidity, total dissolved solids, totalphosphorus, dissolved phosphorus,several nitrogen species, and hardness.Trace metals analyzed include cadmium,

copper, lead, zinc, chromium,and magnesium.Crooked Creek during base flow. Crooked Creek during stonnflow.

84A closer look at land use and hydrology,Yellow River watershed

The Yellow River watershed is the largest(162 square miles) in the county and liesin the Ocmulgee River watershed. It issituated in the more densely developed,southwestern part of the county. Land use(in 1998) is shown for the portion (97percent) of the watershed located withinGwinnett County. The predominate landuse (42 percent) is low-to-medium densityresidential. Commercial and industrial de-velopment are concentrated along the majortransportation corridors. Four wastewater-treatment facilities discharge into the water-shed upstream from the monitoring station.

Numbers have been rounded.Areas less than

1 percent not shown

Yellow River watershed land use in 1998.

Continuously monitored streamflow can bedivided into components of stormflow andbaseflow using hydrograph separation tech-niques (Sloto and Crouse, 1996). Stormflowis the rainfall runoff that enters the drainagenetwork from overland flow. As a percentof total flow, stormflow is greatest duringwet months and years. Baseflow enters thedrainage network through the streambedand banks and is more stable.

O 1 2 3 4 5 MILESIIi IIi III II

0 1 2 3 4 5 KILOMETERS

act Nov Dec Jan Feb Mar Apr May June July Aug Sept 1998 1999 2000 2001 1998-2001

The seasonal nature of stream flow is evident in the monthly mean water yield. Water yield is the total volumeof stream flow 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 isevident in the annual mean flow and yield. The 1998 average flow was at least twice the flow of the droughtyears 1999-2001. For the period 1998-2001 in the Yellow River watershed, the average total water yieldwas 23.7 inches, which is 52 percent of the average precipitation.

(/)wI(,)~

~

c5--1w>=

Concentration and discharge

A. Total suspended solids

Annual yield and loadA. Total suspended solids

Streamflow concentrations and watershedloads and yields. Yellow River watershed

Streamflow samples are analyzed to deter-mine constituent concentrations. These datadefine whether the stream quality is adequatefor uses such as recreation, for drinking-water supply, for fishing, and as a habitatfor aquatic organisms. However, one mustknow more than the condition of a stream inorder to mitigate pollution and manage theresource. Is the pollution from point sourcesor nonpoint sources? When and how are con-stituents transported? An intensive monitor-ing program, such as the one for GwinnettCounty, can address these questions.

260,000

1195,000

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~

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16,500

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5,000

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~ 600w 500a:U 400«a: 300~ 200(/) 100oz 05 c. Total nitrogena. 25z--20om 15>=1- 10z~ 51-i= 0(/)~ 16 D. Totalleed

U8

-a:z wO 1-

~6:J<a:a:a:uwI--ll.~~cnuz~z-<O a:U CD

12

8Collecting a stream sample at base flow.

4

Constituent load, shown to the right inunits of tons per year, is the product ofconcentration and discharge. Constituentyield, shown in units of pounds per acreper year, is the load divided by the water-shed area. Loads are computed using arelatively simple rating-curve model(Crawford, 1996). The model is cali-brated and run using a stormflow load-to-discharge curve, and a baseflow load-to-discharge curve, and applied to the dailytime-step hydrograph for the Yellow River.The results, summarized in the graphs tothe right, illustrate several important points.

019981999200020011998-

2001

z ffi 00 10,000Oa.a:-w

'<001-a: ~ ~ 1,0001- -JZ -Jw ~ 100

oo~ ~ 10

.....k:-!7 ...

100 1,000DISCHARGE, IN CUBIC FEET PER SECOND

A. Excessive total suspended solids are a major cause of habitat degradation innorth Georgia streams. Stormflow and baseflow have dissimilar total-suspended-solidcharacteristics; 99 percent of the annual load is carried by stormjlow. B. Total dissolvedsolids are naturally higher in baseflow, which has filtered through the ground. The totaldissolved solids-to-discharge curve is similar for baseflow and storm flow. c. The dilutionofpoint-source nitrogen is evident in base flow. Nonpoint-source nitrogen increases withdischarge. These nitrogen levels do not indicate an impaired status for the Yellow Rivet:D. Lead concentrations are typically below detection limits during baseflow, indicatingno lead point sources. Stormflow carries 97 percent of the annual lead load. E. Fecal-coliform bacteria serve as an indicator for possible pathogenic bacteria in streams.Fecal-coliform data are highly variable, but stormflow concentrations typically aremuch greater than base flow concentrations.

streamflow because larger storms typi-cally wash off more constituent massfrom a larger area of the watershed.

.In constituent loads and yields, the effectof dissimilar baseflow and stormflowconcentration characteristics is multi-plied. Stormflow loads of nonpoint-source constituents vary exponentiallywith stream discharge because concen-tration increases with discharge, and the

load is a product of concentrationand discharge.

.Seasonal and annual loads vary withprecipitation and total streamflow. Base-flow loads are relatively constant fromyear to year.

These findings are relevant to analyses oftotal maximum daily load, watershed assess-ments, and watershed protection strategies.

.Water quality typically is worse duringstormflow than during baseflow.

.Concentration-to-discharge curvesare dissimilar for stormflow andbaseflow for suspended solids and forconstituents having an affinity for solidphase particulates (such as metals);whereas the curves are similar fortotal dissolved solids.

.The absence (lead) or presence (totalnitrogen) of constituents from pointsources is evident in the baseflowconcentration-to-discharge curves.Concentrations of constituents frompoint sources are diluted and decreasewith increasing baseflow.

.Nonpoint-source constituent concen-trations increase with increasing

watershedwide. The correlation is slightlybetter for stream-corridor land use. Addi-tional stream-corridor analyses are neededusing higher resolution land-use data.

Effects of land use on stream hydrology

Bare or impervious areas and a moredeveloped stormwater drainage systemproduce greater volumes of high-energystormflow and reduce baseflow in astream. A measure of this effect is theaverage stormflow of a watershed, takenas a percentage of total watershed flow.The bar chart below shows that with

increasing watershed development,stormflow increases as a percentage oftotal watershed discharge.

Transportation land use can be anindicator of the total impervious areathat is connected to a drainage network.Stormflow was correlated with transpor-tation land use in the stream corridor and

60

(/)<

OoC\J

I(X)"'"'

~-JlL~[1:

r::(/)

45

40Mean annual water yield. 1998-2001

30(/) ~w Stormflow

B 20 I Baseflow~ r

Iz-,- I

9 01 250 5 10 15

TRANSPORTATION LAND USE, IN PERCENT

Stormflow, as a percent of total flow,

increases with increasing transportation

land use.

Sediment from stormflow into theYellow Rive1:

14

I!!I

Effects of land use on stream loadsand yields

Increased storrnwater runoff accelerateserosion of the land surface, particularlyfor areas under construction, and causesscouring of stream channels resulting inmuch higher stream-sediment loads. Totalsuspended solids are used as a measureof overall development impacts in theGwinnett County Watershed ProtectionPlan (2000). The usefulness of thatmanagement strategy is illustrated herein the strong relation shown betweentotal suspended solids and transportationland use in the stream corridor.

YL

CR

BH

AuAt

BF4

0 2 4 6 8 10 4 6 8 10 12 14MEASURED TOTAL NITROGEN YIELD,

IN POUNDS PER ACRE PER YEAR

Nitrogen yield can be estimated from water

shed land use and point-source indices.000z:JO(1.

~

9w>=0<~-J~

;Q

watershed contains wastewater-treatmentfacilities, and based on land-use percent -

ages in the following categories: low-to-medium density residential, high-densityresidential, commercial/utilities,and transportation.

The annual yield of a constituent instreamflow is affected by an array ofland uses, point sources, hydrologicfactors, chemical processes, and otherfactors. Effective water-resource management is difficult because of these manyfactors. However, stream monitoring canprovide information and tools that arevaluable for watershed assessments,land-use planning, and informed water-resource management.

0 2 4 6 8 10

TRANSPORTATION LAND USE

IN THE STREAM CORRIDOR,

IN PERCENT

Lead levels in most streams have de-creased since the early 1970s as a resultof compliance with environmental laws.Lead yields in the six monitored water-sheds are closely related to transportationland use in the stream corridor. Thismay be due to increased lead depositionin paved areas by transportation andrelated land uses; and because of thegreater efficiency of wash off fromthese surfaces, accompanied by reducedfiltration of runoff before it reaches thedrainage network.

Monitoring datafrom six GwinnettCounty watersheds show strongrelations between transportationland use in the stream corridor andwatershed yield of total suspendedsolids and lead.

Nitrogen loads in the watershed have bothnonpoint and point sources. Nonpointsources include natural organic detritusand applied nitrogen, such as fromfertilizer. Nitrogen yield was estimatedbased on an index of whether or not the

:s:

9I!.-J~

~I!.Ow(!)~zw

~w0.

55

50

35

30

a:<w>- 12a:wIl.

I:\! 10

~a:~ 800QZ:J 6

~

~

z~ 100

~O 80(.)w(.)z 60wa:

~ 40 ~

~ 20 Fzw(.)a:w0.

Georgia Ecological Condition Scores

Verygood

Good

Fair

Poor

Verypoor

wa:Ou(/)

~1-ffi<:I:~<

~

Effects of land use on stream ecosystems

Just as stream-water quality is affectedby watershed land use, so are the aquaticorganisms. Stream communities in Gwin-nett County are composed of a relativelydiverse group of organisms rarely seenby most people. These stream inhabitantsspend all or parts of their life cycles inaquatic environments and include insectsas well as crayfishes, snails, mussels, andaquatic worms. Since these organismsgenerally live in the same section of streamduring their entire lives, the invertebratecommunity that develops is correlated withwater-quality and habitat conditions of thatstream section.

To determine the Georgia EcologicalCondition Scores (Georgia Departmentof Natural Resources, 1997), invertebratescollected during site visits are identifiedand counted by a trained taxonomist. Sirni-lar species are grouped according to howtolerant they are to pollution or how theyfeed in the aquatic environment. U sing

Alcovy Big Brushy CrookedRiver Haynes Fork Creek

Creek Creek

Georgia s Ecological Condition and

Visual Assessment Habitat Scores for

selected watersheds in Gwinnett County

these data, indices that have been shownto be related to environmental degradationare calculated.

Part of the Georgia Ecological ConditionScore is composed of a separate indexknown as the Georgia Visual AssessmentHabitat Score (Georgia Department ofNatural Resources, 1997). This indexprovides managers with an assessmentof the physical condition of the streamhabitats present at a site and allows com-parison among streams included in thestudy as well as to regional referencesites. Habitat data allow more accurateinterpretation of the biological dataespecially when viewed in conjunctionwith water-quality data.

Invertebrate samples were collectednear the stream gages of four of the sixmonitored watersheds. A reference site,located on the Alcovy river in the upperAlcovy watershed, was selected andsampled to represent relatively unimpairedecological conditions in Gwinnett County.Georgia Ecological Condition Scores areexpressed as a percentage of the score ofthe reference site and rated from verygood to very poor, in comparison withconditions across northern Georgia,including Gwinnett County.

The most densely urbanized watershed,Crooked Creek, has the lowest EcologicalCondition Score. However, for the threeother watersheds, the comparative eco-logical condition does not agree with thecomparative land-use development. Thismay be largely due to the localized effectsof past channelization on aquatic habitat in

Big Haynes Creek and Brushy Fork Creek.The effects of channelization also areapparent in the habitat scores.

Streams are strongly affected by watershedland use. Land-use effects are evident inthe amount of streamflow during storms,in the amount of con~tituents in a stream,and in the stream ecosystem. Monitoringand understanding of these effects provideinformation for the wise management ofland use and water resources.

Collecting an invertebrate samplefrom riffle habitat in a stream inMetropolitan Atlanta.

References cited

Crawford, C.G., 1996, Estimating mean con-stituent loads in rivers by the rating-curveand 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 biologicalassessment: Atlanta, Ga., EnvironmentalProtection Division, Water ProtectionBranch, 139 p.

Gwinnett County Department of Public Utilities,2000, Land-use classification for 1998.

Gwinnett County Storrnwater Design Manual,1999, revised October 2001: GwinnettCounty Georgia Department of PublicUtilities, Storrnwater ManagementDivision, 277 p.

Gwinnett County Watershed Protection Plan,2000: Gwinnett County Department ofPublic Utilities, 54 p.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP:A computer program for streamflow

hydrograph separation and analysis:U.S. Geological Survey Water-ResourcesInvestigations 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 Quick-Facts, accessed on September 1,2002, athttp: / / quickfacts. census. gov.

Base maps from Gwinnett County Board of com-missioners, Department of Support Services.

u.s. DEPARTMENT DFTHE INTERIDRGALE A. NORTON, Secretary

U.S. Geological SurveyCharles G. Groat, Director

Written byMark N. LandersPaul D.AnkcornKeith W. McFaddenM. Brian Gregory

Design and layout byCaryl J. Wipperfurth

For more information contact:

State RepresentativeU.S. Geological Survey3039 Amwiler Road, Suite 130Atlanta, GA 30360-2824770-903-9100http://ga.watel:usgs.gov

u.s. Geological Survey Water-Resources investigations Report 02-4281

December 2002