We investigate the applicability of the Two-dimensional, Runoff, Erosion and Export (TREX) model to simulate extreme floods on large watersheds in semi-arid regions in the western United States. Spatially-distributed extreme storm and channel components are implemented so that the TREX model can be applied to this problem. TREX is demonstrated via calibration, validation and simulation of extreme storms and floods on the 12,000 km2 Arkansas River watershed above Pueblo, Colorado. The model accurately simulates peak, volume and time to peak for the record June 1921 extreme flood

calibration and a May 1894 flood validation. A Probable Maximum Precipitation design storm is used to apply the calibrated model. The distributed model TREX captures the effects of spatial and temporal variability of extreme storms for dam safety purposes on large watersheds, and is an alternative to unit-hydrograph rainfall-runoff models.