Dynamics of Willapa Bay, Washington: A Highly Unsteady, Partially Mixed Estuary

Abstract Results from 3 yr of hydrographic time series are shown for Willapa Bay, Washington, a macrotidal, partially mixed estuary whose river and ocean end members are both highly variable. Fluctuating ocean conditions— alternations between wind-driven upwelling and downwelling, and intrusions of the buoyant Columbia River plume—are shown to force order-of-magnitude changes in salinity gradients on the event (2–10 day) scale. An effective horizontal diffusivity parameterizing all up-estuary salt flux is calculated as a function of riverflow: results show that Willapa's volume-integrated salt balance is almost always far from equilibrium. At very high riverflows (the top 15% of observations) the estuary loses salt, on average, while at all other riverflow levels it gains salt. Under summer, low-riverflow conditions, in fact, the effective diffusivity K is large enough to drive a net increase in salinity that is 3–6 times the seaward, river-driven salt flux. This diffusion process is amplified, not damped, by increased tidal forcing, contrary to the expectation for baroclinic exchange. Furthermore, K varies along the length of the estuary as ∼5% of the rms tidal velocity times channel width, a scaling consistent with density-independent stirring by tidal residuals. To summarize Willapa's event- and seasonal-scale variability, a simple diagnostic parameter space for unsteady estuarine salt balances is presented, a generalization from the Hansen and Rattray steady-state scheme.