Utah Division of Water Resources, Idaho Department of Water Resources, Wyoming State Engineer's Office, PacifiCorp and CADSWES jointly developed a RiverWare model of the Bear River. The Bear River is an interstate stream that provides irrigation water for 150,000 agricultural acres, hydroelectric power, recreational opportunities and managed ecological habitat before flowing into Great Salt Lake. The purpose of the first phase of model development was to determine if more water could be stored in Bear Lake by changing flood control operations then assessing the impact to system performance. The model was run in a hypothetical historic framework by predicting how the system would have performed from 1980-2020 had operations been different than they actually were. Several challenges arose including numerical modelling of the offstream dual reservoir system, decision making for how to handle forecast informed reservoir operations in a predictive mode and how to address unknown water-use scenarios in the simulation framework. The model was automated to run over a range of parameter values to generate scenarios. The outcome was a well-suited, shared technical tool as well as mproved relationships and trust between states, a research institution and a private utility.