ASSESSING CONTROLS ON PALEODRAINAGE MORPHOLOGY ON THE CONTINENTAL SHELF OFFSHORE CENTRAL OREGON

The continental shelf offshore central Oregon provides a glimpse of how eustatic sea level has affected coastal fluvial systems since the Last Glacial Maximum (LGM, ~20 ka). Using high-resolution CHIRP and sidescan sonar data, we mapped and characterized submerged paleolandscapes of the region to highlight the factors controlling the evolution of fluvial morphology over a sea-level cycle. During the LGM, with eustatic sea level ~120 m below present, the shoreline was located near the shelf edge. During this time, the Umpqua and Siuslaw Rivers drained across the shelf as they adjusted to, and extended towards, a lower base-level. We observe major variations in acoustic character and morphology of projected paleodrainages offshore from each of the modern rivers. Both rivers are influenced by the same regional climate and convergent margin tectonics, and provide an opportunity to compare other small-scale factors that could explain the observed variability in paleodrainage morphology. We examined several of these characteristics including drainage basin area, channel gradient, surficial geology, and continental shelf morphology. These results could be used for future paleolandscape modelling to assess submerged natural and archaeological resources that may be preserved along the Pacific Margin.