Roads, Railroads, and Floodplain Fragmentation Due to Transportation Infrastructure Along Rivers

River floodplains comprise about 7 percent of the land area of the United States and are areas of great biodiversity and ecological productivity, much of which is due to the many connections between terrestrial and aquatic systems in these settings. The flat topography of floodplains, however, means that they are also ideal sites for transportation infrastructure that can disconnect the river from the surrounding landscape. Few studies have examined the role of roads and railroads as components of river system structure and function at landscape scales. In this study, we use geographic information systems (GIS) and easily obtainable data to map the extent of floodplain disconnection caused by transportation infrastructure across two river basins in Washington State. Digital geologic or soils data, along with digital elevation data, provide the extent of total floodplain area, and transportation data define the extent of disconnection. Our results show that 44 percent to 58 percent of the total floodplain area in the three study basins is disconnected by these roads and railroads. Transportation infrastructure disconnected between 17 percent and 64 percent of the floodplain area in the individual study reaches. Federal Emergency Management Agency floodplain data often show where floodplains are truncated by infrastructure, particularly by large features or in urban areas, but do not capture the loss of total floodplain area. Relatively simple broad-scale documentation of infrastructure and floodplain disconnection has potential for guiding further study of floodplain fragmentation at multiple scales, providing an impetus for improving infrastructure design and repair and helping inform aquatic and floodplain management activities.