Prepared in cooperation with the South Carolina Department of Health and Environmental Control Low-Flow Characteristics of Streams in South Carolina Open-File Report 2017–1110 U.S. Department of the Interior U.S. Geological Survey Cover. South Carolina Botanical Garden, Clemson, South Carolina. Photograph by Toby D. Feaster. Low-Flow Characteristics of Streams in South Carolina By Toby D. Feaster and Wladmir B. Guimaraes Prepared in cooperation with the South Carolina Department of Health and Environmental Control Open-File Report 2017–1110 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Acting Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. 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ISSN 2331-1258 (online) iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose and Scope ..............................................................................................................................2 Previous Studies ...................................................................................................................................2 Description of the Study Area ............................................................................................................4 Physical Setting.....................................................................................................................................4 Climate ....................................................................................................................................................5 Low-Flow Characteristics............................................................................................................................6 Diversions...............................................................................................................................................7 Frequency Statistics .............................................................................................................................8 Flow-Duration Analysis ........................................................................................................................9 Statistical Analysis of Trends in Annual Minimum 7-Day Average Flows and Precipitation Data ...................................................................................................................10 Kendall’s Tau Trend Assessments for Unregulated Streamgages .............................................10 Kendall’s Tau Trend Assessment for Regulated Streamgages ...................................................14 Kendall’s Tau Trend Assessment for Total Annual Precipitation by South Carolina Climate Divisions ...................................................................................................................15 Flow Variability .............................................................................................................................................18 Comparison With Previously Published Low-Flow Statistics ...............................................................20 Estimates of Low-flow Statistics at Ungaged Sites ...............................................................................21 Summary........................................................................................................................................................22 Selected References ...................................................................................................................................22 Glossary .........................................................................................................................................................26 Figures 1. Map showing the eight major river basins in South Carolina as defined by the South Carolina Department of Health and Environmental Control as well as continuous-record streamgages in South Carolina included for analysis in this investigation and as published in Feaster and Guimaraes and Guimaraes and Feaster ....................................................................................................................................3 2. Graphs showing South Carolina statewide annual precipitation for 1895 to 2015 and the 10 driest and wettest years for the same period ......................................................5 3. Map showing South Carolina annual precipitation, by climate division, for the period 1895 to 2015 and the climate normal period 1981 to 2010 .............................7 4. Graphs showing annual minimum 7-day average flows and the 7Q10 estimate, and the log-Pearson Type III frequency curve for the annual minimum 7-day average flow for U.S. Geological Survey streamgage 02177000 Chattooga River near Clayton, GA ...........................................................................................................................9 5. Graph showing an example of a regulated streamgage for which the log-Pearson Type III frequency distribution does not adequately fit the logarithms of the annual minimum 1-day flows ...................................................................................................................9 6. Map showing the trend direction (downward, upward, or no trend) for the continuous-record streamgages in South Carolina for which a Kendall’s tau trend assessment was made. ...................................................................................................11 iv 7. Map showing annual minimum 7-day average flow and results of Kendall’s tau statistical test for detection of trends for eight long-term streamgages in South Carolina .............................................................................................................................12 8. Graphs showing annual minimum 7-day average flow and results of Kendall’s tau statistical test for detection of trends for U.S. Geological Survey streamgage 02136000 Black River at Kingstree, SC, for the full period of record from climate year 1930 to 2006 and the truncated period of record from climate year 1944 to 2006 ...13 9. Graphs showing annual minimum 7-day average flow and results of Kendall’s tau statistical test for detection of trends for U.S. Geological Survey streamgage 02147500 Rocky Creek at Great Falls, SC, for the full period of record from climate year 1951 to 2011 and the truncated period of record from climate year 1951 to 2006 ...13 10. Graphs showing selected periods of record at U.S. Geological Survey streamgage 02132000 Lynches River at Effingham, SC, for which a Kendall’s tau statistical test for detection of trends was assessed: 1930 to 2006; 1936 to 1957; 1951 to 1965; and 1971 to 2006 ..........................................................................................................................14 11. Graphs showing annual precipitation for calendar years 1895 through 2015, and the results of Kendall’s tau statistical test for detection of trends for those years for the climate divisions in South Carolina .............................................................................15 12. Graph showing the total population for South Carolina from 1790 to 2010 .......................18 13. Map of the unit annual minimum 7-day average flow with a 10-year recurrence interval (7Q10 divided by drainage area at the streamgage) at unregulated U.S. Geological Survey continuous-record streamgages in South Carolina ....................19 14. Graph showing annual minimum 7-day average unit flow for the 2-, 5-, 10-, 20-, 30-, and 50-year recurrence intervals for eight long-term streamgages in South Carolina .........................................................................................................................20 15. Graph showing duration of daily mean flow for eight long-term streamgages in South Carolina .........................................................................................................................20 Tables 1. South Carolina Department of Health and Environmental Control schedule for basin data analysis and statistics availability ..........................................................................2 2. Continuous-record streamgages in or near South Carolina for which low-flow frequency statistics were computed and published in Feaster and Guimaraes and Guimaraes and Feaster ..............................................................................................................28