 This release is a rewrite of MODFLOW-USG following an object oriented programming paradigm in Fortran, and provides a platform that includes the capabilities from several previous MODFLOW-2005 versions, including MODFLOW-NWT, MODFLOW-USG, and MODFLOW-LGR.There are still features lacking in the current release that are supported in MODFLOW-2005, such as subsidence, and stream flow routing (SFR) only supports rectangular wetted perimeters.The current version is 6.1.0, released December 12, 2019.