These data were collected to examine water mixing in the lower Maumee River to determine the extent to which water from Lake Erie backflows into the river, and under what conditions this occurs. We used a combination of fixed hydrologic gauges; water chemistry including stable isotopes and conservative tracers; and direct physical measurements to estimate the percentages of lake and river water at both fixed sites and transects. Those results were compared with concurrent flows and estimates of seiche activity. Data suggest that during 2013 mixing did occur, but was confined to the lowest river reach within the city limits of Toledo, Ohio. This tabular data set could be used for any questions involving large river water chemistry or physical processes. It includes boron and magnesium measurements, isotope data, sonde transect data, and sonde vertical profile data.