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In this paper, we examined twelve riparian forests along urban-rural gradients in Austin, TX (USA), on the relationship among watershed urbanization and the invasion of alien woody species. We assessed the degree of biological invasion by measuring relative alien cover (RAC) of the riparian forests. We also measured environmental variables (15 in total) that characterize the study forests, including impervious surface percentage of corresponding watersheds, stream hydrology of adjacent streams, species diversity, canopy gap percentage, and soil nutrient contents of the riparian forests. Stream hydrology was quantified by the transfer function model. The results indicate that impervious surface percentage was related to stream hydrology: the more the impervious surface in a watershed, the faster streamflow recedes after the storm, and the longer dry period the riparian forest experienced (R(2) = 0.722). Impervious surface percentage was also related to RAC (R(2) = 0.498). Nonmetric multidimensional scaling (NMDS) grouped the 15 environmental variables into five dimensions. Multiple regression analysis of RAC on the five NMDS dimensions shows that RAC was related only to the dimension related to hydrological drought. Based on these results, we concluded that watershed urbanization facilitates the invasion of alien species in riparian forests by causing hydrologic drought, particularly in hot and semi-arid regions. Published by Elsevier B.V.