<p>Climate change and increasing water demand are critical factors influencing water availability and the hydrologic cycle. This study examines their combined impacts on the Kaskaskia River watershed in Illinois, USA, using a comprehensive hydrologic modeling approach. The model integrates key watershed components, including existing reservoirs, multiple water withdrawal sources, and point source discharges, with necessary modifications to the governing algorithms to accurately represent system processes. A daily target release method was implemented for routing water through the watershed’s two main reservoirs, significantly improving the accuracy of storage and outflow simulations. A total of 68 hydrologic scenarios were simulated, combining 17 climate projections with four water demand patterns. Results reveal that climate change exerts a substantial influence on river flows and reservoir storages, while the effects of projected water demand are comparatively minor. Under RCP2.6, RCP4.5, and RCP8.5 scenarios, minimum storage volumes in both reservoirs are projected to decline over the next 25 to 50 years, while maximum storage volumes are expected to increase, indicating greater variability in reservoir levels. Despite these changes, future water yields from both reservoirs are projected to exceed current yields across all scenarios. These findings underscore the importance of adopting sustainable water resource management strategies to address the challenges posed by climate variability and evolving water demands, ensuring long-term water security in the region. Furthermore, as water availability is also constrained by water quality, future enhancements to the model should incorporate water quality simulations to assess the impacts of sediment and nutrient loads on overall water availability.</p>