Field spectra were collected from the Kootenai River in northern Idaho during a tracer dye experiment on September 25, 2017, to assess the potential for inferring concentrations of Rhodamine WT dye from remotely sensed data. Reflectance measurements were made from a boat near the confluence with Ball Creek with the Kootenai River using an Analytical Spectral Devices FieldSpec3 spectroradiometer operated in reflectance mode. The Rhodamine WT dye concentration at each spectral measurement location was interpolated from field measurements of concentration obtained with a Turner Designs C3 submersible fluorometer (sonde) by pairing time stamps from the spectra with those from the sonde. This data release provides both the reflectance spectra and the interpolated dye concentrations, as well as locations and time stamps, and can be used to develop relationships between concentration and reflectance for mapping dye dispersion in river channels from field spectra or passive optical remotely sensed data.