Before implementing the various USACE‑SWD policies, it is necessary to build and calibrate a model of the physical river basin network. This involves selecting objects and linking them together to accurately represent the physical features of the river basin. In addition, user methods are selected to model various physical processes (i.e. spill, evaporation, seepage, routing, etc.) depending on the level of modeling detail required.

Initially, the model should be run in Simulation mode. In Simulation, RiverWare solves an exactly determined system given a set of user inputs. No rules or policy constraints are involved in the solution. Each object performs a mass balance calculation and solves any user selected methods. The purpose of the simulation model is to ensure that the physical features of the basin are correctly modeled. In this stage of model development, the modeler checks input data for accuracy, checks for proper mass balance and routing, and calibrates the model. Only after the physical model has been calibrated, should the modeler begin implementing policy through the use of rules.