Estimates of extreme floods and probabilities are needed for hydrologic engineering and dam safety risk analysis. Physically-based, distributed watershed models are used as an avenue to estimate extreme floods, and as a basis to extrapolate frequency curves. The main elements of this research include improving and using a two- dimensional, physically-based rainfall-runoff model (CASC2D) to estimate extreme floods and probabilities for dam safety on a large (12,000 km2) watershed, the Arkansas River above Pueblo, Colorado. New tools have been developed so the model can be applied at this scale. CASC2D can be successfully used to model extreme floods based on observations of extreme rainfall (from both rain gage networks and weather radar) for large watersheds. Future research will focus on the storm transposition concept and linkages with radar.