Riparian ecosystems in the western USA have been invaded by non-native woody species deliberately introduced for stream bank stabilization, agricultural windbreaks, and urban shade. Recent work suggests that the non-native tree Ulmus pumila (Siberian elm) is capable of significant spread in western riparian ecosystems, that range infilling is still incomplete, and that the invasion is dispersal-limited. Our objective was to understand the interacting roles of propagule pressure from upland U. pumila, human influences, and river geomorphology in promoting riparian U. pumila invasion along the South Platte River, Colorado, USA. We used linear regression and information-theoretic model selection to evaluate the relative importance of these factors to riparian U. pumila stem density. U. pumila stem density increased with increasing channel and floodplain restriction and increasing human influence from both urban and rural development. Model selection indicated that local upland U. pumila seed sources were relatively unimportant to riparian U. pumila stem density, suggesting that upland propagule pressure is currently contributing less than other human influences to U. pumila spread along the South Platte River. In particular, higher road density was the most important predictor for the proportional abundance of smaller U. pumila individuals (DBH<5-cm and 5-15-cm), suggesting that human influence in densely populated areas has been the primary driver of recent U. pumila population expansion. U. pumila stem density was only weakly associated with abundance of other common riparian tree species. Land managers and other entities concerned with non-native tree invasion into important riparian habitat may be able to reduce U. pumila spread most effectively by focusing U. pumila control efforts where human influences are greatest.