Rainfall during the warm season (June 15-October 15) is the most important of the year in terms of flood generation and erosion in rivers of the southern Colorado Plateau. Fluvial erosion of the plateau decreased substantially in the 1930s to early 1940s, although the cause of this change has not been linked to variation of warm-season rainfall. This study shows that a decrease of warmseason rainfall frequency was coincident with and probably caused the decreased erosion by reducing the probability of large floods. Warm-season rainfall results from isolated thunderstorms associated with the Southwestern monsoon and from dissipating tropical cyclones and (or) cutoff low-pressure systems that produce widespread, general rainfall. Warm-season rainfall is typically normal to above normal during warm El Nin??o-Southern Oscillation (ENSO) conditions. A network of 24 long-term precipitation gages was used to develop an index of standardized rainfall anomalies for the southern Colorado Plateau for the period 1900-85. The index shows that the occurrence of anomalously dry years increased and the occurrence of anomalously wet years decreased after the early 1930s, although 1939-41, 1972, and 1980-84 were anomalously wet. The decrease in warm-season rainfall after the early 1930s is related to a decrease in rainfall from dissipating tropical cyclones, shifts in the incidence of meridional circulation in the upper atmosphere, and variability of ENSO conditions. ?? 1992 Kluwer Academic Publishers.