The Illinois River is one of the major tributaries of the Mississippi River in the central United States with a drainage area of 75,156 square kilometers. The Illinois River connects the Great Lakes with the Mississippi River through the Illinois Waterway that consists of eight lock and dams along the Illinois River and its tributaries. Because of its central location in the state and being the receiving river downstream of the Greater Chicago metropolitan area, the Illinois River has experienced significant changes and pollution over the last 100 years. At present, however, the Illinois River has become the focus of state and federal restoration efforts. One of the major restoration concepts is the reconnection of the Illinois River with its floodplain. Since much of the floodplain has been leveed off for agricultural uses, the reconnection issue is controversial and expensive. However, large floodplain areas that previously were agricultural lands have been purchased by governmental and nongovernmental agencies for restoration purposes. It is anticipated that there will be more such purchases in the future. This paper evaluates the interaction of the Illinois River with its floodplain using hydraulic models to better understand the influence of restoration efforts on river hydraulics. The models will be used to evaluate changes in water discharges, elevation, and velocities as different parts of the river and floodplain are reconnected. Different alternatives for reconnection are also to be evaluated. Copyright ASCE 2004.