Salt flux and mixing in the Columbia River Estuary

Abstract

The Columbia River Estuary at low discharge falls in classes 1b and 2b of the Hansen & Rattray (1966) classification system with the former occurring upstream where the salinity gradients are weakest. During high discharge it falls in the relatively unexplored region bounding classes 1b, 2b and 4. It is typified by both strong tidal and mean currents modified by bathymetry and channel curvature. The dominant lateral dynamic balance is between the pressure gradient, centrifugal and Coriolis forces. The estuary has a strong vertical salinity gradient and also a marked transverse gradient required for the lateral dynamic balance. More than half the upstream salt flux, balancing the downstream mean flow advective salt flux is directly due to correlations between tidal components of velocity and salinity and between each of these and the tidal variation of cross-sectional area. The remaining upstream salt flux arises from the vertical gravitational circulation. The mean stratification and circulation for both high and low discharges yield theoretical estimates of the diffusive fraction of the upstream salt flux in reasonable agreement with the observed values. They also lead to reasonable estimates of P and Fm for low discharge condition but for high discharge only the estimate of P is reasonable. Neither the high discharge estimate of Fm nor the vertical profiles of velocity, U, and salinity, S, fit the theoretical models.