The Tunnel and Reservoir Plan (TARP) is a system of 175 kilometers of deep underground tunnels and large reservoirs designed to capture and store combined sewer overflows of greater Chicago. Construction of the system began in the 1970's and currently, the tunnels are nearing completion and excavation of the reservoirs is underway. The hydraulic behavior of the system is inadequately understood, particularly the transition from gravity to surcharged flows. Because of the potential to form hydraulic transients under these conditions, an improved description of the hydraulic behavior of the system is a prerequisite to determining operational rules that better utilize the capacity of the system. In order to gain a better understanding of the hydraulic behavior in the tunnels, models are currently under development to model steady flow, unsteady flow, and hydraulic transients in the as-built system. A geographic information system (GIS) has been developed to provide a framework that facilitates managing the data, visualizing the output, and integrating the different modeling tools needed to describe this system. Of particular interest in this work is use of the GIS to store model results describing the steady-flow hydraulics of the system for the range of possible flow conditions. These stored descriptions of the hydraulics are used by more sophisticated models of the hydrodynamics of the system, thereby eliminating repetitive calculations that otherwise would be necessary for every time step in the unsteady-flow models. In addition, the GIS makes the modeling process more efficient by allowing multiple models to obtain necessary information, such as geometry and hydraulic coefficients, about the entire network from a single database. This allows changes made to the database to be quickly reflected in the models. Finally, the GIS provides an effective means of storing and displaying the model output clearly and concisely so that an assessment of the system performance can quickly be made.