Hydrodynamic and Water-Temperature Model of a 21-Mile Reach of the Upper Illinois River, Illinois, 2020 – 2022

A hydrodynamic and water-quality model (CE-QUAL-W2) was developed of a 21-mile reach of the upper Illinois River including a 3-mile reach of a major tributary, the Fox River.  The CE-QUAL-W2 model is 2-dimensional in the vertical and longitudinal directions and averaged over the lateral direction.  Continuous water quality and streamgage data provided time-series data for model boundary conditions.  Discrete velocity, cross-section area, and temperature profiles at several locations within the study reach provided model calibration data. The model was calibrated to 2021 and 2022 observed data and validated with 2020 data.  Model output consisted of 2-dimensional, laterally averaged hydrodynamic and water-temperature time series. The model may provide insight into the role of hydrodynamics and temperature in the development of harmful algal blooms (HABs) in the study reach. Utilizing the observed and simulated hydrodynamic conditions in the Illinois River study reach it was possible to compare and contrast streamflow, velocity, and temperature conditions in years with varying distributions of HABs. There were documented occurrences of extensive HABs in the study reach in June 2020 and June 2021 but there was a limited HAB in the summer of 2022. The objective of model development was to find similarities in site physical conditions in 2020 and 2021 that may contrast with the conditions in 2022.