Transportation infrastructure, river confinement, and impacts on floodplain and channel habitat, Yakima and Chehalis rivers, Washington, U.S.A.

Although floodplain roads and railroads are recognized as confining features with potentially large environmental impacts, few studies have explored the linkages between these structures and the natural disturbance regime that creates and maintains channel and riparian habitat. This study compares paired floodplain reaches with or without transportation infrastructure confining the riparian zone along the Yakima and Chehalis rivers in Washington State. Channel and floodplain habitat were degraded in the artificially confined reaches. Confined channels were narrower, simpler in planform, and relatively devoid of depositional surfaces such as bars and islands. Floodplains adjacent to confined channels exhibited degraded riparian forest and less refugium habitat such as side channels, ponds, and alcoves important for endangered salmonids and other biota. The results support hypotheses about how human modification of the floodplain landscape disrupts the flow regime and connectivity along riparian corridors. Neither simple buffer zones nor metrics such as valley width index adequately capture the disturbance-based landscape processes that drive riparian and channel habitat integrity. Future studies and indices of valley confinement, a critical driver of fluvial geomorphic processes, need to pay closer attention to artificial confinement of the channel, the riparian zone, and the active floodplain surfaces in order to portray the true constraints on fluvial processes.