Seasonal transfer and net accumulation of fine sediment on a muddy tidal flat: Willapa Bay, Washington

Tidal flats act as natural laboratories in which fundamental sediment-transport processes can be directly related to resulting seabed deposits. These environments represent important repositories for terrestrial particles (including organic carbon) entering marine dispersal systems. Along the coast of the US Pacific Northwest, tides, waves, currents, and storms create year-round energetic environments that evolve on myriad time scales, from semi-diurnal to interannual. In southern Willapa Bay, WA, an extensive tidal flat is accreting at a distance away from local fluvial sources. During winter, freshwater input and the peak suspended-sediment concentration (SSC) are one-to-two orders of magnitude greater than in summer, and wind- and wave-generated shear stresses prevent sediment from accumulating on the tidal flat. Temporary deposits form as a drape across secondary channels off the Bear River Channel. Sedimentary structures from these deposits reveal 15–30 cm of physically stratified sediment underlain by a discrete shell-hash layer 2–8 cm thick. The presence of relatively uniform excess 210Pb activities in the sediment above the shell hash, and only supported activities below indicate rapid deposition of the surficial sediment. During summer, the distribution of bed shear stresses is not significantly weaker than in winter due to the effect of local basin geometry on fetch, yet the SSC is much less, likely due to reduced fluvial sediment supply and enhanced benthic biological factors. Progressively through the summer, tidal currents and wind waves remove the temporary channel deposits and expose the buried shell-hash layer, and concurrent seabed changes allow the tidal flats to trap this remobilized sediment. Accumulation rates determined by 210Pb analysis for cores collected on the tidal flats show mean accretion at 1.4 mm/yr, which can be accounted for by local river sources. The mass of sediment stored in the temporary channel drapes during winter is approximately the same as the annual tidal-flat accumulation in the vicinity of these channels. This agreement suggests a mechanism by which sediment temporarily stored in winter channel deposits is subsequently reworked during summer, and transferred onto the tidal flats. The accumulation rate on the flats approximately matches local sea-level rise, which implies this system accretes over longer time scales in equilibrium with the space provided. The preservation of layered deposits (65–90 cm thick) buried 155–230 cm beneath the modern surface suggests that the past accommodation space was greater than at present. Coseismic subsidence during Cascadia earthquakes represents a possible mechanism by which vertical space can be created.

► Local geometry and wind fetch considerations can reduce the seasonal impact of winter wave events. ► Distinct seasonal hydrologic and morphodynamic conditions occur without changes in forces on the seabed. ► Deposits form in channels during winter when sediment supply increases. ► Sediment stored in channels is removed during summer and deposited onto the mudflat. ► Sediment accumulates annually at a rate of 1.4 mm/yr, which amounts to ∼8×106 kg/yr in southern Willapa Bay and is likely supplied by local rivers.