This web product summarizes water-quality data collected during water years 2000-2019 for a network of sites in the Big Thompson River Basin of Colorado (fig. 1 and table 1). Current data (2018-2019) for each site are compared to previously collected data and the State of Colorado instream standards. Lowess curves (Helsel and Hirsch, 2002) are provided on plots for selected constituents where a sufficient period of record and limited censored data are available. Although verified and reviewed, the summaries included in this product should not be considered a formal U.S. Geological Survey publication.