Forecasts of flows entering and leaving the Chain of Lakes on the Fox River in northeastern Illinois are critical information to water-resource managers operating the Stratton Dam at McHenry, Illinois. These managers determine the optimal operation of the Stratton Dam at McHenry, Ill., to manage Chain of Lakes pool levels and to help mitigate flooding in the Chain of Lakes system. In 2020, the U.S. Geological Survey (USGS) and the Illinois Department of Natural Resources–Office of Water Resources (IDNR–OWR) began a cooperative study to develop a system to enable engineers and planners to simulate and communicate water-surface elevations and flows and proactively prepare for runoff events forecasted for the Chain of Lakes. The hydraulic model may be helpful to the IDNR–OWR for optimizing the operation of the Stratton Dam and includes the implementation of three newly installed torque-tube crest gates that became operational in 2020. The hydraulic model for the Chain of Lakes was developed using the Hydrologic Engineering Center–River Analysis System (HEC-RAS) program (version 6.5; U.S. Army Corps of Engineers, 2022). The hydraulic model was used to simulate water-surface elevations and flows through the 18.5-miles Chain of Lakes system to 1.7 miles downstream from the Stratton Dam. Five USGS streamgages within the study area (Nippersink Lake at Fox Lake, USGS station 05548000; Fox River at Johnsburg, USGS station 05548500; Fox River at Miller Bridge near McHenry, USGS station 05549400; Fox River near McHenry, USGS station 05549500; Fox River (Tailwater) near McHenry, USGS station 05549501; U.S. Geological Survey, 2023) were used as reference points for model calibration and the water-surface elevations from each of these five USGS streamgages were used as an initial condition for the model simulation. This hydraulic model was calibrated to three runoff events (April–May 2022 [April 23–May 3], September 2022 [September 9–26], and March 2023 [February 28–March 15]) that incorporated the design specifications and observed gate operations of the Stratton Dam; furthermore, this hydraulic model simulated a validation event (January–February 2024 [January 28–February 22, specifically]) and a substantial flooding event during July 2017. The July 2017 event predated the torque-tube crest gate installation but nevertheless tested the performance of the model for such a substantial event. The model simulation results were a good fit to observed records at USGS streamgages with simulated peak water-surface elevations within −0.36 to 0.15 feet and peak flow −236 to 50 cubic feet per second. This data release contains a zip file that includes HEC–RAS model run files used in the calibration of the Chain of Lakes hydraulic model along with model performance and calibration metrics. References Cited: U.S. Army Corps of Engineers, 2022, HEC–RAS—River analysis system: U.S. Army Corps of Engineers software release, accessed October 15, 2023, at https://www.hec.usace.army.mil/software/hec-ras/download.aspx. U.S. Geological Survey, 2023, USGS surface-water data for the Nation, in USGS water data for the Nation: U.S. Geological Survey National Water Information System database, accessed May 13, 2024, at https://doi.org/10.5066/F7P55KJN. [Surface-water data directly accessible at https://waterdata.usgs.gov/nwis/sw.]