Circulation, density distribution and neap-spring transitions in the Columbia River Estuary

Abstract

This paper has two purposes. The first is to use tidal-monthly variations in the density and velocity fields and the salt and water transports as key to understanding the circulation of the Columbia River Estuary and other river estuaries. The Columbia River Estuary is a good natural laboratory in this regard, because the flushing time of the system (a few days) is short relative to the tidal month during all seasons. This allows the occurrence of distinct transitions from a strongly to a weakly stratified water column (and back) during the tidal month. Furthermore, because atmospheric processes are secondary to riverflow and tidal influence in determining the circulation, most of the energy in circulatory phenomena is confined to distinct tidal, tidal-monthly and seasonal frequency bands. Observations of salt transport and neap-spring transitions reported herein should provide important constraints on future theoretical studies of estuarine circulation. The second purpose is to describe the circulation and density field of the Columbia River Estuary as background for understanding the geologic and biological investigations discussed in other papers in this volume. Previous investigations have focused on seasonal variations in riverflow as governing the turbidity maximum and biological productivity. Studies reported in this volume show that tidal monthly variations in circulatory processes are of comparable importance. With regard to the velocity field, the influence of stratification causes the tidal flow to show the largest vertical variations in phase and amplitude in the lower estuary. The vertical distribution of the mean current is controlled by the ebb-flood asymmetry in the time-dependent flow, vertical mixing processes, the baroclinic pressure gradient, and interaction of the flow with topography. Net upstream bottom flow is weak or absent during periods of weak stratification; it is substantially only when the system is highly stratified. Net upstream bottom flow tends to occur downstream of, and net downstream bottom flow upstream of, topographic highs. This pattern of convergences in the mean flow tends to preserve these topographic highs and lows and is not consistent with the traditional view that the baroclinic mean flow and mean salinity distribution structure one another; the former results instead primarily from ebb-flood asymmetry in the time-dependent flow. Comprehensive salt transport calculations were carried out, because knowledge of salt transport variations during the tidal month is vital to understanding the causes of the observed transitions in the density and velocity fields that occur between extremes of tidal range. These calculations show that neap-spring and seasonal variability of the gross features of the salinity intrusion in the lower estuary is limited by compensating adjustments of the tidal advective and mean-flow salt transports under most riverflow conditions. In contrast, the absence of such balancing mechanisms in combination with substantial neap-spring changes in vertical mixing allow large variations in the density field and salinity intrusion length in the upper estuary. Neap-spring variability is much higher during the low-flow season than during the high-flow season. Under the lowest riverflow conditions permitted by the present dam system, large tidal monthly changes in stratification occur throughout the system. The salt transport calculations also show that salt is carried into the estuary near mid-depth by tidal mechanisms acting primarily in the North Channel. Salt is transported out of the estuary closer to the surface of the South Channel by the strong mean flow. Mean velocities near the bottom are weak, and near-bottom, upstream salt transport by the mean flow is small. It is likely that inward tidal transport of salt occurs at mid-depth in many estuaries, as this is where salinity variations during the tidal cycle are usually greatest. Finally, the dynamical observations presented here, together with the known input of sediment from the river to the South Channel suggest that the concentration of fine sediment by the tidal asymmetry in a turbidity maximum should be stronger in the South than the North Channel. Limited observations confirm this.