Delivery, reworking and export of fine-grained sediment across the sandy Skagit River tidal flats

Abstract Tidal flats with fluvial input can have ample fine-grained sediment supply, and yet the Skagit River tidal flats remain mostly sandy. Therefore, hydrodynamic mechanisms must efficiently export mud off the flats. This study evaluates short-term deposition and long-term burial of mud on an intertidal flat and links seabed deposits to sediment-transport processes. To investigate the fine-grained sediment-transport mechanisms within the sandy Skagit River tidal flats, seabed and water-column observations were collected seasonally from 2008 to 2009 under varying river discharge conditions. Little mud was observed deposited on the seabed. Spatially, mud was most common near channels and on the seaward flat edge. Its presence was temporary, and was typically observed following high river discharge. Strongest current velocities and high suspended-sediment concentrations were measured during spring tidal cycles with peaks in the late ebb. Maximum velocities were observed within channels, permitting for rapid transport of fine-grained particles to the seaward edge of the flat and export beyond. These physical processes also act to mobilize the mostly sandy seabed, which is reworked at various frequencies: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1–2 cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1–2 m, making the limited mud deposits available for export. Overall, channel networks on sandy tidal flats with fluvial input not only deliver fine-grained sediment to the flat, but also aid in the export of suspended sediment at ebb tides and in the release of muddy deposits through reworking of the seabed.