The U.S. Geological Survey (USGS), in cooperation with the Grand Valley Drainage District and the Colorado Water Conservation Board, conducted a loading analysis for selected constituents and tributaries to the Colorado River in the Grand Valley, western Colorado, using data from 1991 to 2018, to characterize concentrations, stream loading, and load reductions for Escherichia coli, total recoverable iron, and dissolved selenium for stream segments on the State of Colorado 303(d) list of impaired waters. Escherichia coli, total recoverable iron, and dissolved selenium concentrations and streamflow data were compiled from the Water Quality Portal (WQP). The data were submitted to the WQP by the USGS, Colorado Department of Public Health and Environment (CDPHE), and Colorado River Watch. The data tables include information on sites, data collection time periods, concentrations, computed loads, and regression model diagnostics. Dissolved selenium annual loads, percentage load reductions required to meet State regulatory standards, mean daily loads computed for irrigation and nonirrigation seasons, and regression model diagnostics and results are presented for tributary sites where sufficient data were available. This data release was revised in 2021 with the addition of folder "Datarelease_inputandoutput zip." Included in this folder are the (1) Load Estimator (LOADEST) regression model used to estimate daily mean dissolved selenium concentrations and loads for water year 2003 for a 303(d) listed segment represented by the site Lewis Wash near Grand Junction, Grand Valley, western Colorado and the (2) Weighted Regressions on Time, Discharge, and Season regression models used to characterize dissolved selenium conditions in the Grand Valley, western Colorado, from 1980 to 2018 at selected main-stem sites. First posted February 2020 (contact author for originals). Revised August 2020, version 2.0. Revised August 2021, version 3.0.