A dye tracer study was completed on the lower Sandusky River between July 11-13, 2017, during a confirmed Grass carp (Ctenopharyngodon idella) spawning event. The data contained in this data release include time series of dye concentration at fixed stations downstream of the injection site. Six submersible fluorometers (Turner Designs C3 [Trademark] fluorometers) were deployed at predetermined stations between 1.19 and 16.05 river miles downstream of the injection point on July 10, 2017, approximately 24 hours prior to the injection. The fluorometers were mounted atop cinder blocks and deployed in approximately 3 to 4 feet of water with good circulation. The instruments were secured to shore using safety lines. The fluorometers were battery-powered and set to record dye concentration, temperature, and turbidity at 5-second intervals to internal dataloggers. At 8:54 am Eastern Standard Time on July 11, 2017, 13.5 liters of Rhodamine WT dye in a 20-percent solution was injected into the Sandusky River at the Tiffin Road bridge crossing (41.327110, -83.130339). The dye was injected as a slug from the downstream side of the bridge at the approximate center of the channel. At the time of the injection, the discharge 2.7 miles upstream of the injection site at the U.S. Geological Survey (USGS) streamflow gaging station on the Sandusky River near Fremont, Ohio, (USGS 04198000) was approximately 5,788 cubic feet per second and increasing. The deployed fluorometers recorded the dye plume as it passed each station; however, the fluorometer at station FL4 failed to log data to the datalogger and the fluorometer at station FL7 shifted in its mount at some point during the deployment causing limited flow of water past the sensors. Measured dye concentrations were corrected to remove the effects of turbidity on the background concentration (or fluorescence) using data collected by each instrument prior to the dye injection. The dye data were averaged over 1-minute intervals with the timestamp at the end of the averaging interval. Any data with suspicious values are flagged with an "S" in the flag column for each station. Rational for flagging suspicious data included known deployment issues (for example, FL7 sensors obstructed by mount), variability in the background concentration that was not due to turbidity, and professional judgement based on river mixing theory and 19 years of experience analyzing dye tracer data. Flagged data should only be used if the validity of the data can be proven (using modeling or theory, for example). All coordinates given in this data release are in (latitude, longitude) pairs in decimal degrees with North and East defined as positive and referenced to the World Geodetic System 1984 (WGS84) datum unless otherwise stated. All times are in Eastern Standard Time (EST) unless otherwise stated. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government.