Flash flood hydrographs were examined using water stable isotopes (deuterium and oxygen) and a plug-flow lumped catchment model to assess the origin and routing processes of flood water in a semiarid basin in southwestern United States. Precipitation and stream water were sampled during storm and flood conditions at high and low elevations. Isotope mixing relationships readily determined the predominance of three sources for the representative summer monsoon events: high elevation precipitation from two major subbasins and base flow. Each flood progressed through a series of source water contributions, as indicated by several segments of linear mixing between these end-members. We developed a plug-flow lumped catchment model to test possible governing processes for specific watershed and forcing conditions. Results suggest two main findings: First, these flood events were generated primarily from event water runoff in high elevations that mixed at the flood bore with pre-event base flow resident in the stream. The power, speed, and turbulence of the flood bore cause it to mix with, ride atop and push the resident in-stream water in the front of the rising limb. Second, the timing and volume of flood waves from subbasins are identified by their combined isotopic signature at the basin outlet; this approach may provide an effective mesoscale constraint for rainfall-runoff models.