This software archive is superseded by Hydrologic Toolbox v1.1.0, available at the following citation: Barlow, P.M., McHugh, A.R., Kiang, J.E., Zhai, T., Hummel, P., Duda, P., and Hinz, S., 2024, U.S. Geological Survey Hydrologic Toolbox version 1.1.0 software archive: U.S. Geological Survey software release, https://doi.org/10.5066/P13VDNAK. The U.S. Geological Survey Hydrologic Toolbox is a Windows-based desktop software program that provides a graphical and mapping interface for analysis of hydrologic time-series data with a set of widely used and standardized computational methods. The software combines the analytical and statistical functionality provided in the U.S. Geological Survey (USGS) Groundwater (Barlow and others, 2014) and Surface-Water (Kiang and others, 2018) Toolboxes and provides several enhancements to these programs. The main analysis methods are the computation of hydrologic-frequency statistics such as the 7-day minimum flow that occurs on average only once every 10 years (7Q10); the computation of design flows, including biologically based flows; the computation of flow-duration curves and duration hydrographs; eight computer-programming methods for hydrograph separation of a streamflow time series, including the BFI (Base-flow index), HYSEP, PART, and SWAT Bflow methods and Eckhardt’s two-parameter digital-filtering method; and the RORA recession-curve displacement method and associated RECESS program to estimate groundwater-recharge values from streamflow data. Several of the statistical methods provided in the Hydrologic Toolbox are used primarily for computation of critical low-flow statistics. The Hydrologic Toolbox also facilitates retrieval of streamflow and groundwater-level time-series data from the USGS National Water Information System and outputs text reports that describe their analyses. The Hydrologic Toolbox supersedes and replaces the Groundwater and Surface-Water Toolboxes. The Hydrologic Toolbox was developed by use of the DotSpatial geographic information system (GIS) programming library, which is part of the MapWindow project (MapWindow, 2021). DotSpatial is a nonproprietary, open-source program written for the .NET framework that includes a spatial data viewer and GIS capabilities. This software archive is designed to document different versions of the Hydrologic Toolbox. Details about version changes are provided in the “Release.txt” file with this software release. Instructions for installing the software are provided in files “Installation_instructions.pdf” and “Installation_instructions.txt.” The “Installation_instructions.pdf” file includes screen captures of some of the installation steps, whereas the “Installation_instructions.txt” file does not. Each version of the Hydrologic Toolbox is provided in a separate .zip file. Citations: Barlow, P.M., Cunningham, W.L., Zhai, T., and Gray, M., 2014, U.S. Geological Survey groundwater toolbox, a graphical and mapping interface for analysis of hydrologic data (version 1.0)—User guide for estimation of base flow, runoff, and groundwater recharge from streamflow data: U.S. Geological Survey Techniques and Methods 3–B10, 27 p., https://doi.org/10.3133/tm3B10. Kiang, J.E., Flynn, K.M., Zhai, T., Hummel, P., and Granato, G., 2018, SWToolbox: A surface-water toolbox for statistical analysis of streamflow time series: U.S. Geological Survey Techniques and Methods, book 4, chap. A–11, 33 p., https://doi.org/10.3133/tm4A11. MapWindow, 2021, MapWindow software, accessed January 9, 2021, at https://www.mapwindow.org/#home.