DECIPHERING THE LONGITUDINAL CHANGES IN CHANNEL BOTTOM BEDFORM MORPHOLOGIES WITHIN TIDALLY-MODULATED FLUVIAL SYSTEMS: THE COLUMBIA RIVER, OREGON AND THE CHEHALIS RIVER, WASHINGTON

Although our understanding of bedform development and subsequent morphologies has vastly improved within the fully fluvial environment, similar advances have not occurred in regards to the hydrodynamics and bedform dynamics within tidally-influenced fluvial environments. The current paucity of knowledge regarding tidally-influenced environments stems from (a) modern tidally-influenced fluvial systems present a very challenging environment to obtain representative data measurements due to spatially and temporally varying flow conditions, and (b) the inherent complexity of landward to offshore changes in physical processes and sediment sources as well as the delicate interplay between dominant and subordinate flow sources (i.e. river, tides, and waves). However, recently collected single-beam (SBES) and multi-beam echo sounding (MBES) data sets from the Columbia River, Oregon and the Chehalis River, Washington allow us to gain a unique insight into the longitudinal change in bedform morphologies of two modern tidally-influenced fluvial systems of different scales and sediment supplies from their estuaries to their fully-fluvial environments. Moreover, these two unique data sets can be compared directly to examine how longitudinal bedform morphologies change between a relatively coarse-grained river-dominated system that is weakly modulated by tidal processes (Columbia River), and a tide-dominated fluvial system that possesses a large fraction of fine-grained sediment load relative to its coarse bedload (Chehalis River, Washington).