This dataset is attached to the following article: Logozzo, L.A., Martin, J.W., McArthur, J., Raymond, P.A. Contributions of Fe(III) to UV-vis absorbance in river water: A case study on the Connecticut River and argument for the systematic tandem measurement of Fe(III) and CDOM. Biogeochemistry 160: 17–33 (2022). https://doi.org/10.1007/s10533-022-00937-5 This dataset includes concentrations of ferric iron, Fe(III), and dissolved organic carbon (DOC) along with UV-vis absorbance at 254 nm and 412 nm on the operationally dissolved size fraction (<0.22 μm), as well as the truly dissolved (<0.02 μm) and colloidal (0.02-0.22 μm) size fractions. Samples were collected every two weeks at five sites along the Connecticut River mainstem from 2018 to 2020 and at 7 sites in the Connecticut River watershed sampled once synoptically during the summer 2019.