Despite existing observational networks, large-scale flooding events in semiarid regions are poorly understood because of their infrequent occurrence and the lack of studies directed at quantifying their basic characteristics. Given the ephemeral nature of semiarid basins, large floods are critical events which can cause significant exchanges of water, sediment, and nutrients across the surface-groundwater interface. Our major objective in this study is to quantify the meteorological characteristics, watershed runoff response, and surface-groundwater interactions arising from a summer monsoon flood event in the R¿o Puerco and its downstream hydrologic effects in the R¿o Grande. Results indicate that only 3.6% of the storm rainfall resulted in tributary runoff which led to a propagating flood wave in the R¿o Grande. Along the main river reach we compute that 49% of the flood volume was lost to the shallow aquifer, with the remainder entering a downstream reservoir. Surface-groundwater interactions were further tracked in a network of shallow wells, suggesting significant changes in groundwater level, ranging from 0.05 to 0.6 m, along and across the river. The results of this study demonstrate the widespread effects of large-scale monsoon events in semiarid regions and serve as a basis for improved process understanding and modeling. On the basis of our observations we present a conceptual model of the generation, propagation, and attenuation of large floods in semiarid tributaries and main stem rivers during the North American monsoon. These linked processes have not been previously documented in a comprehensive manner in the American Southwest.