ALLUVIAL ARCHITECTURE OF FLUVIAL TO TIDALLY-INFLUENCED POINT BAR DEPOSITS: CHEHALIS RIVER, WA, USA

In this study, the alluvial architecture of tidally-influenced, backwater fluvial, and fully-fluvial point bars are investigated on the Chehalis River, WA, USA. Two field campaigns were conducted in September 2011 and 2012 where we (a) collected 16 shallow sediment cores (< 6m) using a vibracore, (b) acquired ground penetrating radar (GPR) data along exposed point bars using 100 MHz frequency antennas, (c) examined satellite and aerial photographs that reveal the rates of meander migration through time, and (d) collected sub-bottom profile transects within the channel using a Parametric Echo Sounder. These data allow examination of the migration of these bends and their internal sedimentary architecture, and how these change across the fluvio-tidal transition zone.

Preliminary results show that the internal stratigraphy of point bar deposits located in the fluvio-tidal transition zone is composed of cross-bedded sands and gravels that grade into mud-draped current ripples possessing interbedded silt laminae. These deposits represent IHS deposition associated with coarse-grained fully-fluvially derived sedimentation interbedded with finer-grained sediment associated with tidal processes. Conversely, the internal architecture of point bar deposits located in the backwater fluvial and pure fluvial zones comprises relatively high-angle (> 6°) and relatively low-angle (< 6°) dipping coarse-grained packages that can be bounded at their tops by gently dipping thin silty horizons that are most prominent in the downstream portions of these point bars. The backwater fluvial and fully-fluvial point bar deposits are interpreted to display the "classic" stratigraphic model for fluvially-derived point bar deposition.