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 http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://store.usgs.gov.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. DatumVertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88).Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).Altitude, as used in this report, refers to distance above the vertical datum. vii Supplemental InformationSpecific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C).Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Abbreviations AbstractThe hydrogeology and hydrologic characteristics of the Ozark Plateaus aquifer system were characterized as part of ongoing U

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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 http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod/.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Quantifying hydrologic alteration in the Mississippi Alluvial Plain (MAP) of the southcentral United States is particularly difficult because of the lack of current reference, or even relatively undisturbed, streams and associated streamflow data. Impacts, such as water withdrawals for agriculture, weirs, dams, channelization, and other forms of regulation, within the MAP increased substantially beginning around 1960 suggesting that streamflow has since been altered. Using historical streamflow and climate data and explanatory variables, the U.S. Geological Survey developed random forest regression models to estimate expected reference monthly streamflows (pre-1960) at 76 sites in the MAP and two adjacent Level III Ecoregions. To compensate for the lack of current reference stream sites in the study area, the pre-1960 streamflow data were used as a surrogate to estimate current streamflow conditions without anthropogenic influence (inferring current reference conditions). Overall, nearly every site within the study area had less zero-flow days than what historically has been observed and there were more low-pulse spells. However, the frequency of floods remained relatively consistent.

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