Modelling salinity and circulation for the Columbia River Estuary

Simulations of the time and depth-dependent salinity and current fields of the Columbia River Estuary have been performed using a multi-channel, laterally averaged estuary model. The study simulated two periods. The first, in October 1980, with low riverflow of about 4,000m³s⁻¹, which showed marked changes in the salinity intrusion processes between neap and spring tides; and second, in spring 1981, with high riverflow varying between 7,000 and 15,000m³s⁻¹, which showed the rapid response of the salinity intrusion to changes in riverflow and that vertical mixing did not change character with increasing tidal energy because of the maintenance of stratification by freshwater flow. An extreme low flow simulation (riverflow of 2,000m³s⁻¹) showed a more partially mixed character of the estuary channels with tidal dispersion of salt across the Taylor Sands from the North Channel to the upper reaches of the Navigation Channel. Asymmetries in the non-linear tidal mean flows, in the flood and ebb circulations, and salinity intrusion characteristics between the two major channels were observed at all riverflows. The model confirms Jay and Smith's (1990) analysis of the circulation processes in that tidal advection of salt by the vertically sheared tidal currents is the dominant mechanism by which the salinity intrusion is maintained against large freshwater flows. An accurate finite-difference method, which minimized numerical dispersion, was used for the advection terms and was an important component in reasonably simulating the October neap-spring differences in the salinity intrusion. The simulations compare favorably with elevation, current and salinity time series observations taken during October 1980 and spring 1981.