The Eurasian Lythrum salicaria is invading wetlands throughout North America. Our study sought to (1) document the extent of L. salicaria invasion along the lower West Branch of the Susquehanna River in central Pennsylvania and (2) assess the correlates of L. salicaria abundance including associated species and environmental/edaphic characteristics. We documented that L. salicaria is well established on the West Branch of the Susquehanna River, but only in that portion within the Ridge and Valley Province. Multiple analyses indicated that L. salicaria is associated with open-canopied, hydric sites with nutrient-rich, clay and/or silt soils. The vulnerability of a West Branch site to invasion by L. salicaria is best assessed by the presence of plant associates including Eleocharis obtusa, Phalaris arundinacea, and others.