Emerging theory suggests that biotic communities in floodplain wetlands should change longitudinally along a river’s length and be affected by lateral connectivity of floodplain habitats with main river channels. However, useful invertebrate indicators of floodplain condition remain poorly developed. Snails as a group possess several attributes that should make them useful as potential environmental indicators. In this study, we sampled snail assemblages in 12 floodplain wetlands along Wusuli River (headwater, mid, and downstream reaches) in Northeastern China’s Sanjiang Plain. In addition, at 10 sites, comparisons between river-connected and levee-isolated wetlands were performed. Results demonstrated that snail assemblages were concurrently being impacted by river reach, and whether the wetlands were isolated from the river, or not. Using multivariate analyses, the studied wetlands were divided into three major groups based on snail assemblages: downstream, mid, and headwater reaches. Seven snail species were significant indicators of these wetland groups. Because snail assemblages vary strongly both longitudinally along and laterally across floodplain habitats of Northeastern China, management of floodplains should recognize that landscape position plays an important role in local habitat ecologies, and that efforts to limit hydrologic connectivity across floodplains will have important consequences for resident biota.