A wastewater reuse model was applied to the Upper Colorado River basin to assess the percent of accumulated wastewater (ACCWW%) from municipal and industrial wastewater treatment plants (WWTPs) in each initialized (that is, has an assigned hydrosequence number for flow-routing) National Hydrography Dataset Version 2.1 (NHDPlus V2.1) stream segment. Model inputs included (1) NHDPlus V2.1 stream geometry and hydrologic information (Table 1); (2) WWTP information, provided by the U.S. Environmental Protection Agency (Table 2), including average daily effluent discharges and outfall locations. Outfall locations were linked to the corresponding NHDPlus V2.1 stream segment receiving the effluent prior to calculating ACCWW%. ACCWW% values were calculated for years 2007 to 2020 using mean-monthly and mean-annual NHDPlus V2.1 gage-adjusted Enhanced Runoff Method streamflow estimates.