Gain-Loss Study Of Lower San Pedro Creek And The San Antonio . - USGS
In cooperation with the U.S. Air Force, Air Logistics Center,Kelly Air Force Base, Environmental Management OfficeGain-Loss Study of Lower San Pedro Creekand the San Antonio River, San Antonio,Texas, May–October 1999Open-File Report 02–023U.S. Department of the InteriorU.S. Geological Survey
U.S. Department of the InteriorU.S. Geological SurveyGain-Loss Study of Lower San Pedro Creekand the San Antonio River, San Antonio,Texas, May–October 1999By D.J. OckermanU.S. GEOLOGICAL SURVEYOpen-File Report 02–023In cooperation with the U.S. Air Force, Air Logistics Center,Kelly Air Force Base, Environmental Management OfficeAustin, Texas2002
U.S. DEPARTMENT OF THE INTERIORGale A. Norton, SecretaryU.S. GEOLOGICAL SURVEYCharles G. Groat, DirectorAny use of trade, product, or firm names is fordescriptive purposes only and does not implyendorsement by the U.S. Government.For additional information write toDistrict ChiefU.S. Geological Survey8027 Exchange Dr.Austin, TX 78754–4733E-mail: dc tx@usgs.govCopies of this report can be purchased fromU.S. Geological SurveyBranch of Information ServicesBox 25286Denver, CO 80225–0286E-mail: infoservices@usgs.govii
CONTENTSAbstract .Introduction .Purpose and Scope .Description of Study Area .Hydrologic Conditions Before and During Study .Approach .Streamflow Gain-Loss Measurements .Estimation of Ground-Water Inflow and Outflow .Gain-Loss Determinations and Estimation of Ground-Water Inflow .Tributary Inflow .Diversions .Measurement Summaries and Ground-Water Inflow Estimates .Summary .References .1113344777891415FIGURES1. Map showing location of study area and selected data-collection sites .2. Photograph showing USGS streamflow-gaging station San Antonio River at Theo Avenue (08178505) .3. Graph showing cumulative rainfall, daily mean streamflow, and days of streamflow-measurementsurveys, San Pedro Creek and San Antonio River, May–October 1999 .4. Photograph showing streamflow measurement at San Antonio River dam spillway near Padre Park,July 1999 .5. Schematic diagram of subreaches showing streamflow-measurement sites, streamflow-gaging stations,inflows, diversions, and return flows .6–10. Photographs showing:6. Confluence of San Pedro Creek and the San Antonio River, July 1999 .7. Seepage inflow along Concepcion Creek channel, July 1999 .8. Riverside Creek at San Antonio River, July 1999 .9. Seepage along west bank of San Antonio River, above Mission Road, July 1999 .10. San Antonio River diversion to Padre Park Acequia, July 1999 .2345689101011TABLES1. Monthly rainfall and departure from normal, January–October 1999 .2. Location of streamflow-measurement sites and streamflow-gaging stations, San Pedro Creek and SanAntonio River .3. Subreaches for gain-loss estimates, San Pedro Creek and San Antonio River .4. Summary of gain-loss determinations for subreaches along San Pedro Creek and the San AntonioRiver during streamflow-measurement surveys, May–October 1999 .5. Summary of tributary discharges (attributed to shallow ground-water inflow) to subreaches along SanPedro Creek and the San Antonio River during streamflow-measurement surveys, May–October 1999 .6. Summary of estimated shallow ground-water inflow for subreaches along San Pedro Creek and the SanAntonio River during streamflow-measurement surveys, May–October 1999 .357121314ABBREVIATIONSacre-ft/yr, acre-feet per yearft, feetft3/s, cubic feet per secondgal/min, gallons per minutein., inchesMgal/d, million gallons per daymi, milesCONTENTSiii
Gain-Loss Study of Lower San Pedro Creekand the San Antonio River, San Antonio,Texas, May–October, 1999By D.J. OckermanAbstractFive streamflow gain-loss measurement surveys were made along lower San Pedro Creek andthe San Antonio River from Mitchell Street toSouth Loop 410 east of Kelly Air Force Base in SanAntonio, Texas, during May–October 1999. All ofthe measurements were made during dry periods,when stormwater runoff was not occurring andeffects of possible bank storage were minimized.San Pedro Creek and the San Antonio River weredivided into six subreaches, and streamflow measurements were made simultaneously at the boundaries of these subreaches so that streamflow gainsor losses and estimates of inflow from or outflow toshallow ground water could be quantified for eachsubreach. There are two possible sources ofground-water inflow to lower San Pedro Creek andthe San Antonio River east of Kelly Air Force Base.One source is direct inflow of shallow groundwater into the streams. The other source is groundwater that enters tributaries that flow into the SanAntonio River. The estimated mean direct inflow ofground water to the combined San Pedro Creek andSan Antonio River study reach was 3.0 cubic feetper second or 1.9 million gallons per day. The meantributary inflow of ground water was estimated tobe 1.9 cubic feet per second or 1.2 million gallonsper day. The total estimated inflow of shallowground water was 4.9 cubic feet per second or 3.2million gallons per day. The amount of inflow fromsprings and seeps (estimated by observation) ismuch less than the amount of direct ground-waterinflow estimated from the gain-loss measurements.Therefore, the presence of springs and seeps mightnot be a reliable indicator of the source of shallowground water entering the river. Most of the shal-low ground water that enters the San Antonio Riverfrom tributary inflow enters from the west side,through Concepcion Creek, inflows near RiversideGolf Course, and Six-Mile Creek.INTRODUCTIONOrganic compounds, including tetrachloroethene(PCE) and trichloroethene (TCE), have been detected inthe shallow aquifer near Kelly Air Force Base (AFB) insouthwestern San Antonio, Texas. These compoundsmight be moving with shallow ground water eastwardtoward the San Antonio River (fig. 1). The interactionbetween the shallow aquifer and the San Antonio Rivereast of Kelly AFB, and the conditions (ground-waterlevels and river discharge) under which the river gainsor loses streamflow to the aquifer are not well understood. In May 1999, the U.S. Geological Survey(USGS), in cooperation with the U.S. Air Force, beganan investigation of lower San Pedro Creek, a tributary tothe San Antonio River northeast of Kelly AFB, and theSan Antonio River east of Kelly AFB to determine ifshallow ground water is entering the river and if so, thelocations and quantities.Purpose and ScopeThe purpose of this report is to present the resultsof a study to determine if the San Pedro Creek and SanAntonio River reaches identified above are gaining orlosing streamflow, where the gains or losses are occurring, and if streamflow gains or losses can be attributedto shallow ground-water interaction with the river.Results of streamflow gain-loss measurementsalong San Pedro Creek from Furnish Avenue to theSan Antonio River and along the San Antonio Riverbetween the USGS streamflow-gaging station atMitchell Street and the USGS streamflow-gagingstation at South Loop 410 during May–October 1999are presented.Abstract1
98o30'298o32'30"98o27'30"Study ooseveltionRiversMitcheRd.ionMissraSan Antonio08178050Creek 2e.Aveo08178505ThConcepcTEXASUS 90LOCATION MAPNew BraunfelsPedroI–10S. ZarzamoFurnAv ishe.ProbandtnSaUS nt site and number(table 2)Streamflow-gaging station and number(table 2)Rain gage and number3ANI–3529o22'30"Kelly Air Force Base0.5 mileTONERRIVsloreS. FSouthcross Blvd.0IOPadreParkDam4S.W. Military Dr.HarlandalEspadadam5.RdRoosevelt eek.RddapaEsPleasanton Rd.Gain-Loss Study of Lower San Pedro Creek and the San Antonio River, San Antonio, Texas, May–October, 19991S.PresaRd.08178565South Loop 410Figure 1. Location of study area and selected data-collection sites.0.51 Mile
Description of Study AreaThe study area comprises the reach of San PedroCreek from Furnish Avenue to the confluence with theSan Antonio River and the San Antonio River fromMitchell Street to South Loop 410 (fig. 1). The USGSoperates streamflow-gaging stations on the San AntonioRiver at Mitchell Street (08178050) (about 1,600 ftabove San Pedro Creek), at Theo Avenue (08178505)(about 600 ft below San Pedro Creek) (fig. 2), and atSouth Loop 410 (08178565). The stream lengthbetween Furnish Avenue and South Loop 410 is 7.81mi. The change in stream elevation between the twosites is about 110 ft.Base flow in the San Antonio River originatesfrom San Antonio Springs (an Edwards aquifer spring),which is above downtown San Antonio (not shown infig. 1). During the 1998 water year (October 1, 1997, toSeptember 30, 1998), the median daily mean flow was15 ft3/s at the Mitchell Street station. Only 10 percent ofthe daily mean flows exceeded 72 ft3/s at the MitchellStreet station (10 percent exceeded 127 ft3/s at theSouth Loop 410 station). Ninety percent of the dailymean flows exceeded 7.8 ft3/s at Mitchell Street (9.0ft3/s at South Loop 410). During storm events, runoffcan cause much greater discharges. The peak dischargesin 1998 were 3,150 ft3/s at Mitchell Street and 7,120ft3/s at South Loop 410.Two dams are on the San Antonio River in thestudy area. The upstream dam is near Padre Park(fig. 1). Espada dam is about 1.1 mi below Padre Parkand about 0.3 mi below S.W. Military Drive (fig. 1). Nomechanisms control releases from the dams, and flowoccurs over the tops of the dams.Hydrologic Conditions Before and DuringStudyRainfall was below normal before and during thestudy. Rainfall during January–October 1999 was about10 in. below normal (table 1). However, the June rainfallof 5.54 in. was above normal and might have helpedsustain base flow from tributaries and seeps through thedry months of July–October. Figure 3 is a rainfall anddischarge hydrograph for the period May–October1999 showing daily mean streamflow (South Loop 410station) and days of the streamflow-measurementsurveys.Figure 2. USGS streamflow-gaging station SanAntonio River at Theo Avenue (08178505).Table 1. Monthly rainfall1 and departure from normal2, January–October 1999[In 7-3.23-1.57-10.061 Rainfallduring study computed as average from two USGS rain gages: Harlandale Creek outfall at South Flores Street (08188520) (fig. 1)and Zarzamora Creek outfall at Alderette Park (08178430) (not shown in fig. 1).2Departure from normal computed as the difference between long-term average rainfall obtained from National Oceanic and AtmosphericAdministration weather station at San Antonio airport (about 10 miles north of Mitchell Street gaging station) and rainfall during study.INTRODUCTION3
121,00010RAINFALL, IN INCHES8Cumulative rainfall600Daily mean streamflow6Streamflow-measurement survey4004DISCHARGE, IN CUBIC FEET PER 99Figure 3. Cumulative rainfall, daily mean streamflow, and days of streamflow-measurement surveys, San PedroCreek and San Antonio River, May–October 1999.As seen in figure 3, rainfall events and resultingincreases in streamflow were more frequent during lateMay–mid-July than during August–October. Figure 3also shows when the measurement surveys were made.The streamflow-measurement surveys were made during base-flow conditions. During the five measurementsurveys, the San Antonio River discharge (average ofmeasurements at Theo Avenue and South Loop 410)ranged from 15.2 to 30.8 ft3/s. The average dischargewas 21.0 ft3/s.APPROACHStreamflow Gain-Loss MeasurementsTo determine whether the river is gaining orlosing streamflow and where gains or losses might beoccurring, streamflow-measurement surveys were madeon May 26, June 8, July 27, September 27, and October27, 1999. During each survey, simultaneous (withinabout 3 hours) streamflow measurements were made at4selected sites along the study reach. The measurementswere made by the current-meter velocity-area method(Rantz and others, 1982). Two measurements oftenwere made at each site during each measurement survey. When two measurements were made, the meanstreamflow was used for gain-loss computations. Theestimated typical error of streamflow measurementsduring this study, considering measurement and flowconditions, was 5 percent of the measured streamflow.Figure 4 shows a USGS technician making a streamflowmeasurement at the dam spillway near Padre Park.During each survey, measurements were made atthe Mitchell Street, Theo Avenue, and South Loop 410stations (table 2). Streamflow measurements also weremade at five intermediate sites, two on San Pedro Creekand three on the San Antonio River. In addition, tributary flows and diversions were measured or estimatedduring each survey. The location of measurement sitesand gaging stations determined subreaches for whichGain-Loss Study of Lower San Pedro Creek and the San Antonio River, San Antonio, Texas, May–October, 1999
Figure 4. Streamflow measurement at San Antonio River dam spillway near Padre Park, July 1999.Table 2. Location of streamflow-measurement sites and streamflow-gaging stations, San Pedro Creek and SanAntonio RiverStreamflowmeasurement siteor gaging station no.(fig. 1)DescriptionLocation1San Pedro Creek at Furnish Avenue1.60 miles above confluence with San Antonio River2San Pedro Creek at San Antonio RiverAt confluence with San Antonio River08178050USGS gaging station San AntonioRiver at Mitchell Street1,600 feet above San Pedro Creek08178505USGS gaging station San AntonioRiver at Theo Avenue600 feet below San Pedro Creek3San Antonio River at RooseveltAvenue1.54 miles below Theo Avenue gaging station4San Antonio River at Padre Park dam1.26 miles below Roosevelt Avenue, 1.12 miles aboveEspada dam5San Antonio River at Ashley Road0.85 mile below Espada dam, 1,100 feet above AshleyRoad bridge08178565USGS gaging station San AntonioRiver at South Loop 4101.14 miles below Ashley Road bridgeAPPROACH5
081780501SanPeSubreach Adro2CreeSubreach Bk08178505Subreach CConcepcion CreekRiverside Creek3SanSubreach DAntonioRiPadre Park damver4Subreach EEXPLANATION5Espada damStreamflow-measurement site and number(table 2)508178565Streamflow-gaging station and number(table 2)Dry-weather inflowsSix-Mile CreekSubreach FDiversions and return flows08178565Not to scaleFigure 5. Schematic diagram of subreaches showing streamflow-measurement sites, streamflow-gaging stations,inflows, diversions, and return flows.gains or losses could be computed as the difference instreamflow measured at the upstream and downstreamboundaries of the subreaches (fig. 5; table 3).During the May 26 survey, measurements werenot made at the Furnish Avenue and Roosevelt Avenueintermediate sites. Therefore, subreach A was not6included in that survey and subreaches C and D werecombined as a single subreach, C–D. Also, during theMay 26 and June 8 surveys, measurements were notmade at San Pedro Creek at the San Antonio River, sogain-loss estimates for subreaches A and B were notcomputed for those periods.Gain-Loss Study of Lower San Pedro Creek and the San Antonio River, San Antonio, Texas, May–October, 1999
Table 3. Subreaches for gain-loss estimates, San Pedro Creek and San Antonio RiverSubreach(fig. 5)Length(miles)Upstream streamflow-measurement siteor streamflow-gaging station (no.)Downstream streamflow-measurementsite or streamflow-gaging station (no.)San Pedro CreekA1.60Furnish Avenue (1)San Pedro Creek at San Antonio River (2)Mitchell Street station (08178050)Theo Avenue station (08178505)San Antonio RiverB.42C1.54Theo Avenue station (08178505)Roosevelt Avenue (3)D1.26Roosevelt Avenue (3)Padre Park dam (4)E1.97Padre Park dam (4)Ashley Road (5)F1.35Ashley Road (5)South Loop 410 station (08178565)Estimation of Ground-Water Inflow andOutflowGround-water inflow to the streams or outflow toground water are not directly measurable because theinflow and outflow processes usually cannot beobserved. Even visible inflow from springs and seepscannot always be measured accurately. Therefore,inflow from or outflow to ground water in subreaches ofthe streams was measured indirectly by measuring thedifference in streamflow at the upstream and downstream ends of the subreaches. Ground-water inflowand outflow are not the only sources of gain or lossalong the streams. Other sources are tributary inflow,diversions, return flows, and evaporation. By accounting for these factors, ground-water inflow or outflowcan be estimated. Ground-water inflow, G, is estimatedasG QD – QU – I D – R E,(1)whereQD measured streamflow at the downstreamboundary of the subreach,QU measured streamflow at the upstream boundaryof the subreach,I measured or estimated inflows from creeks ortributaries,D diversions in the subreach,R return flows in the subreach, andE estimated evaporation losses.Evaporation within a subreach was estimatedusing potential evaporation data from the NationalOceanic and Atmospheric Administration (1999) for astation at Sea World in San Antonio (not shown infig. 1). The daily evaporation rate was applied to thesurface area of each subreach to compute an averagedaily evaporation discharge. Because evaporative lossesare assumed to occur primarily during the daytimehours only, the daily pan evaporation was multiplied bythe ratio of 24 to hours of daylight to determine theevaporative rate during daytime hours (when measurement surveys were made). Changes in reservoir storagebehind the dams in the study reach were consideredinsignificant because the reservoirs remained full andcontinuous flow over the dams occurred during thestudy period.GAIN-LOSS DETERMINATIONS ANDESTIMATION OF GROUND-WATERINFLOWTributary InflowSeveral tributaries flow into the San AntonioRiver between Mitchell Street and South Loop 410. Theconfluence of San Pedro Creek with the San AntonioRiver (fig. 6) is about 1,600 ft below the Mitchell Streetstation. Base flow in San Pedro Creek originates primarily from San Pedro Springs (an Edwards aquiferspring) above the study area, rather than from a shallowground-water spring.Concepcion Creek enters the San Antonio Riveron the west bank about 1,200 ft below the Theo Avenuestation. Concepcion Creek had continuous base flowduring the study period. Most of the flow appeared to beshallow ground-water seepage entering the ConcepcionCreek concrete channel through numerous drainageGAIN-LOSS DETERMINATIONS AND ESTIMATION OF GROUND-WATER INFLOW7
Figure 6. Confluence of San Pedro Creek and the San Antonio River, July 1999.inlets extending a distance of at least 1 mi above thecreek’s confluence with the river (fig. 7).Three other inflows sustained by base flow duringthe study period are located near the Riverside GolfCourse between Mission Road and Roosevelt Avenue.Riverside Creek enters the San Antonio River from theeast (fig. 8). The source of flow in the creek is attributedto ground-water inflow. Two culverts discharge to theriver along the west bank between Mission Road andRoosevelt Avenue. The source of the discharge probably is shallow ground water. A golf course pond is adjacent to the upstream culvert and might contribute waterto the flow. The combined measured streamflow of allthree inflows was 1.0 ft3/s or less during the measurement surveys.Six-Mile Creek enters the San Antonio Riverbetween Ashley Road and South Loop 410. A smallbase flow (about 0.4 ft3/s) was observed during all of themeasurement surveys. Numerous springs and seepshave been observed along the banks of both sides of theSan Antonio River from Mitchell Street to Roosevelt8Avenue (B. Goodson, CH2M Hill, oral commun.,1999). Because of vegetation or rocks along the riverbanks, the occurrence of seeps is not always apparent.Springs or seeps were not observed from RooseveltAvenue to South Loop 410. The occurrence of springsand seeps also was more obvious during May and June.However, during the July measurement survey, andespecially the September and October surveys, thesprings and seeps had diminished. Most of the springsand seeps did not contribute substantial flow to the river.The seepage in figure 9 was estimated at 1.0 gal/min(about 0.002 ft3/s) or less.DiversionsTwo diversions are important for the gain-lossdeterminations. One diversion occurs above the damnear Padre Park. This diversion from the San AntonioRiver (fig. 10) flows through the Padre Park Acequia(historic irrigation ditch) and returns to the river aboveS.W. Military Drive. No water rights are associated withthis diversion (Texas Natural Resource ConservationGain-Loss Study of Lower San Pedro Creek and the San Antonio River, San Antonio, Texas, May–October, 1999
Figure 7. Seepage inflow along Concepcion Creek channel, July 1999.Commission, 1999). Another diversion occurs atEspada dam. Most of the water returns to the river justbelow the dam. Some of the water is diverted to Espadaditch (about 3.0 ft3/s). Two water rights are associatedwith the Espada ditch diversion (Texas NaturalResource Conservation Commission, 1999) for a totalpermitted diversion of 1,366 acre-ft/yr (1.9 ft3/s). Someof the Espada ditch diversion (about 1.0 ft3/s) isreturned to the San Antonio River below Ashley Road.Some of the water returns to the San Antonio Riverbelow the South Loop 410 station, beyond the studyreach.Measurement Summaries and Ground-WaterInflow EstimatesA summary of the gain-loss determinations foreach measurement survey is listed in table 4. For eachmeasurement survey, mean streamflow, measured difference between streamflow at upstream and downstream boundaries, tributary inflow, net diversions(diversions minus return flows), estimated evaporation,and estimated ground-water inflow are listed by subreach. The quantities of shallow ground water attributedto tributary inflow are listed in table 5. San Pedro Creekinflow to subreach B is excluded because the source ofthe San Pedro Creek flow is an Edwards aquifer spring.Estimated ground-water inflow (or streamflow loss toground water) by subreach for each measurement survey is listed in table 6. These estimated ground-waterinflows (or outflows) do not include ground-waterinflow into tributaries that eventually flow into the SanAntonio River.Gain-loss measurements for subreach A were notadequate during May 26 and June 8 to estimate groundwater inflows. On July 27, a small (0.4 ft3/s or 0.3Mgal/d) ground-water inflow to subreach A was indicated by the measurements. For September 27 andOctober 27, streamflow losses to shallow ground waterwere 0.3 and 1.1 ft3/s (0.2 and 0.7 Mgal/d). The mean ofthe three measurement surveys indicates a streamflowloss of 0.3 ft3/s to shallow ground water. No tributariescontribute to the subreach.GAIN-LOSS DETERMINATIONS AND ESTIMATION OF GROUND-WATER INFLOW9
Figure 8. Riverside Creek at San Antonio River, July 1999.Figure 9. Seepage along west bank of San Antonio River, above Mission Road, July 1999.10Gain-Loss Study of Lower San Pedro Creek and the San Antonio River, San Antonio, Texas, May–October, 1999
Figure 10. San Antonio River diversion to Padre Park Acequia, July 1999.Ground-water inflow estimates were made forsubreach B for only the July 27, September 27, andOctober 27 measurement surveys. On July 27, the estimated inflow was 2.1 ft3/s (1.4 Mgal/d). On September27 (after 2 months of below-normal rainfall), groundwater inflow was estimated to be 0.8 ft3/s (0.5 Mgal/d).On October 27, the subreach lost 0.3 ft3/s (0.2 Mgal/d).For the three measurement surveys, the mean estimatedground-water inflow was 0.9 ft3/s (0.6 Mgal/d). No tributaries contribute to the subreach.Subreach C–D had direct ground-water inflowduring all five measurement surveys. On May 26, theestimated direct ground-water inflow was 5.6 ft3/s (3.6Mgal/d). During subsequent surveys, subreach C–Dwas divided into separate subreaches C and D by theaddition of a measurement site at Roosevelt Avenue. OnJune 8, September 27, and October 27, subreach Creceived most of the direct ground-water inflow withinsubreach C–D. On July 27, most of the inflow occurredin subreach D. The mean estimated ground-waterinflow in subreach C–D for all of the measurements was3.1 ft3/s (2.0 Mgal/d).Subreach C receives tributary inflow fromConcepcion Creek, two inflows on the west bank (nearRoosevelt Avenue), and Riverside Creek. All of theseinflows probably originate from shallow ground water.The average tributary inflow in subreach C was 1.5 ft3/s(1.0 Mgal/d). Subreach C has no diversions. Subreach Dhas no tributary inflow but does include the diversionfor the Padre Park Acequia, which is returned to theriver in subreach E.Subreach E had an inflow of ground water onMay 26. However, during the other measurement surveys, streamflow was lost to ground water. The averagestreamflow loss (including the May 26 streamflow gain)was 1.1 ft3/s (0.7 Mgal/d). No tributaries, diversions, orreturn flows are in this subreach.Subreach F showed ground-water inflow duringall five measurement surveys. The average estimatedinflow was 1.0 ft3/s (0.6 Mgal/d). This subreach alsoreceives inflow from Six-Mile Creek. This estimatedinflow probably is from shallow ground water. Duringsome of the measurement surveys, an intermittent returnflow from Espada ditch entered the river below AshleyRoad.GAIN-LOSS DETERMINATIONS AND ESTIMATION OF GROUND-WATER INFLOW11
Table 4. Summary of gain-loss determinations for subreaches along San Pedro Creek and the San Antonio Riverduring streamflow-measurement surveys, May–October 1999[In cubic feet per second; positive indicates gain, inflow, or return flow entering the subreach; negative indicates loss, outflow,or diversion leaving the subreach. --, not measured or determined]May 26, 1999Subreach(see table 3fordescription)Meanstreamflowin subreachMeasureddifference instreamflow withinsubreachInflow from creeksor tributarieswithin .0-1.21.4002.02.000.41Net diversionin subreach0.1-30-3-.31.2Estimatedevaporationloss withinsubreachEstimatedground-waterinflow evaporationloss withinsubreachEstimatedground-waterinflow evaporationloss withinsubreachEstimatedground-waterinflow aches C and D combined for May 26, 1999, measurement survey.June 8, 1999Subreach(see table 3fordescription)Meanstreamflowin subreachMeasureddifference instreamflow withinsubreachInflow from creeksor tributarieswithin .91.4002.000.4Subreach(see table 3fordescription)Meanstreamflowin subreachMeasureddifference instreamflow withinsubreachInflow from creeksor tributarieswithin -3.92.1Net diversionin subreach0.10-3.0-.31.2July 27, 199912010.81.700.4Net diversionin subreach0.10-3.0-.31.2Gain-Loss Study of Lower San Pedro Creek
San Antonio River and the San Antonio River from Mitchell Street to South Loop 410 (fig. 1). The USGS operates streamflow-gaging stations on the San Antonio River at Mitchell Street (08178050) (about 1,600 ft above San Pedro Creek), at Theo Avenue (08178505) (about 600 ft below San Pedro Creek) (fig. 2), and at South Loop 410 (08178565).
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