Fluvial Landforms and Landscape Transformations on a Large River Floodplain: Willamette River, Oregon, USA

Abstract:

Recent detailed mapping of the Willamette River floodplain in northwestern Oregon reveals insights into the floodplain landforms, their formative processes, and historical landscape transformations. Hierarchical mapping classification based mainly upon lidar topography, supplemented by aerial photographs, historical channel and soil maps, and targeted coring of floodplain soils, was carried out for 200 km of the mainstem Willamette River floodplain above Willamette Falls where floodplain landforms mainly reflect fluvial and anthropogenic influences.

Stark differences in the character and distribution of floodplain landforms and their underlying stratigraphy give rise to three distinct process regimes along the fluvial portion of the Willamette River. Floodplain surfaces along 60 km of the Upper Willamette River floodplain generally rise 1-2 m above the low-flow water surface and are bisected by complex assemblage of overflow channels and large-amplitude abandoned bends formed by avulsions along this historically multi-thread anastomosing reach. Downstream, the 90 km-long Middle Willamette River between Corvallis and Newburg Pool becomes increasingly entrenched within its floodplain, with floodplains gradually rising up to 8 m above the low flow water surface. These floodplain surfaces are dominated by ridge and swale topography with occasional floodbasins reflecting gradual meander migration and floodplain aggradation. The 50 km-long Newberg Pool is entrenched and confined by Pleistocene Missoula flood deposits and bedrock valley walls. This low-gradient reach extends to the lip of the15-m high Willamette Falls.

Historical declines in flood magnitude, bed-material supply, large wood, and bank erodibility result in a more stable modern-day floodplain with narrower active-channel corridor flanked by relict landforms formed by historical flow and sediment regime. Landscape transformations vary across the three process regimes but are greatest along Upper Willamette River which has transformed from an anastomosing channel to a more stable wandering planform. The inventory of habitats and landforms produced by this mapping provide a framework for developing restoration strategies that accord with present-day floodplain processes.