- To begin to understand freshwater seasonal floodplain fish communities in the context of human alteration of the physical system, species introductions and wetland restoration efforts, I studied fish assemblages in fifteen seasonal floodplain wetlands within four geographic regions (coastal, upper Columbia River estuary, Puget Sound and eastern Oregon/Washington) in the Pacific Northwest from November through June, 2001 to 2006. Temporal patterns of species richness and diversity in fish assemblages were examined within a hierarchical framework of spatial and temporal factors in the context of the dynamic and variable nature of floodplains. At broad regional scales, fish assemblages diverged along a regional climactic gradient from wet, cool coastal sites to drier sites further inland with greater seasonal temperature fluctuations. Even though wetlands provide a dynamic environment for fish, species richness and diversity at most sites were relatively stable over the winter and spring. Community stability may have been due, in part, to the predictable duration of wetland inundation during winter and spring, but also to the presence of species that fulfill life history requirements (spawning, rearing) in wetlands, and the presence of juvenile salmon throughout the winter and spring. Once fish entered wetlands, wholesale movement of species into and out of wetlands did not occur until summer desiccation. Comparison of fish species richness between each wetland and adjacent rivers revealed that only about 60% of species present in the rivers were found in wetlands, indicating floodplain wetlands were not compatible with the biological traits of almost half of the fish species present in adjacent rivers. Functional groups of species utilized wetlands at different times of year, during different developmental stages, for different durations, and movements into and out of wetlands were triggered mostly by changing water levels. Juvenile salmon movements were triggered by water level, temperature, barometric pressure and lunar phase. Juvenile salmon were present at every wetland site. No evidence was found to support that floodplain wetlands are a ‘predator trap’ for juvenile salmon. Further understanding of the ecological value of floodplain systems may be useful to guide efforts to restore these systems to dynamic, self-sustaining levels of ecosystem function.