Floodplain formation and cottonwood colonization patterns on the Willamette River, Oregon, USA

Using a series of aerial photographs taken between 1956 and 1996, we trace coevolution of floodplain and riparian forest on the Willamette River. With in-channel bar-forms appear to be the predominant incipient floodplain land form and habitat for primary succession. Interlinked development of bar(s) and erosion of near banks, filling of channels, and establishment and growt h of cottonwoods and willows results in coalescence with older floodplain. Size and internal structure of riparian forest patches reflect evolution of underlying barforms or chan nel beds. Floodplain matures as the active cha nnel migrates away by repetition of the bar formation and near-bank erosion process, or is progressively abandoned by infilling and/or constriction wi th a bar. Other parts of the floodplain are recycled as eroding banks provi de the coarse sediment and large woody debris for building new bars. A mult ichannel planform is maintained as building bars split flow, channels lengt hen as bars and islands join into larger assemblages. Avulsion appears to c ut new channels only short distances. Given the central role of bars and is lands in building new floodplain habitat, we identify their area as a geomo rphic indicator of river-flood plain integrity. We measure an 80% decline i n bar and island area between 1910 and 1988 within a 22-km section. Dams, r ip-rap, logging, and gravel mining may all be contributing to diminished ba r formation rates. Removing obstacles to natural riparian forest creation m echanisms is necessary to regenerate the river-floodplain system and realiz e its productive potential.