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We evaluated the importance of alien species in existing vegetation along wadeable streams of a large, topographically diverse river basin in eastern Oregon, USA; sampling 165 plots (30 x 30 m) across 29 randomly selected 1-km stream reaches. Plots represented eight streamside community types associated with varying elevation, precipitation, and landform. Mantel comparisons, non-metric multidimensional scaling (NMS), and Spearman correlation identified relationships of alien species (n = 60) distribution and relative alien cover (RAC) to native species (n = 355) composition, 28 vegetation descriptors, 31 environmental variables, and 30 disturbance conditions. Alien species occurred in 93% of sample plots, in all community types, and along all sampled stream reaches; with RAC ranging from 0.1% to 47% and 1 to 24 alien species occurring along individual stream reaches. Alien richness and RAC were positively related to native diversity where invasion was limited (RAC < 5%), but negatively where invasion was more severe (RAC > 5%). RAC differed among community types: greatest in arid associations (shrubland/grassland), followed by associations with limited tree canopy cover (meadows, dry forest), and lowest in moist, closed forest associations; suggesting differences in invasion status or vulnerabilities to alien invasion among community types. Alien species, as a group had wider ecological amplitude than natives, and species composition among community types was less distinct when both alien and natives were considered compared to native species only. RAC was negatively related to elevation, precipitation, and tree cover; while positive relationships of RAC occurred with grazing pressure, upstream watershed size, stream order, overall level of exogenous disturbance, limited vegetated buffer, agriculture in the upstream watershed, floodplain or south facing slope locations, and proximity to roads. Taken together, these results suggest possible ecological consequences to streamside vegetation related to alien species and identify indicators of conditions, where invasion may be greatest, offering potential for informing decisions for monitoring and managing alien species.