Sedimentary Processes on Tidal Flats: Recent Studies of Mesotidal Settings in the US Pacific Northwest (Invited)

Tidal flats are coastal environments that are very sensitive to environmental change, especially sea level. The sedimentary processes operating on flats control the deposits that accumulate there, and preserve a high-resolution record of coastal history - with impacts from both land and ocean. Over the past century, many studies have examined tidal flats around the world. Recent investigations of mesotidal flats in Washington state have provided much new insight to the hydrodynamics of water and sediment transport, biological influences on sediment entrapment, resulting morphology of tidal-flat and channel surfaces, and the dynamics of sediment accumulation. This research has been done in northern Puget Sound near the mouth of the Skagit River, and at the south end of Willapa Bay removed from significant fluvial discharge. The physical processes (winds, waves, tides, river discharge) operating on the Skagit tidal flat are sufficient to rework the flat on most tidal cycles, and cause regular resuspension of the upper 1-3 cm of sediment. This causes effective removal of fine sediment discharged by the river, and tidal currents transport this material to distant subtidal areas for accumulation. The resulting sand flat is non-cohesive, and tidal channels are able to migrate easily. During periods of significant sediment discharge (winter floods, summer snowmelt) some fine-grained material (mud) can be buried on the flanks of the migrating channels. These processes are distinctly different than in southern Willapa Bay, where fine-grained sediment (mud) dominates accumulation and is supplied from distant river sources during winter floods. The mud moves between tidal flats and channels, but primarily resides in tidal channels during winter. Sea grasses and benthic algae become more prevalent during summer, and the mud is trapped on the flats as these biological components cause shear stresses to decrease and shear strengths to increase. The net result is a mudflat dominated by cohesive sediments, in which channel locations are relatively stationary due to the resistance to erosion imposed by consolidated muds. Together, the two study areas, with similar tidal ranges, provide interesting contrast regarding the net impacts on sedimentation - especially for the export and import of fine sediment to tidal flats.