Estuarine Exchange Flow Variability in a Seasonal, Segmented Estuary

Journal Article Resource
June 2020

The diverse users of Oregon’s Coos estuary share a need to better understand water circulation and sediment transport in the estuary under present and future conditions. A collaborative research project led by the University of Oregon and South Slough National Estuarine Research Reserve filled data gaps and modeled how estuarine circulation and sediment respond to perturbations due to both natural and human-induced causes—such as dredging or inundation caused by sea level rise.

About this article

This journal article uses a hydrodynamic model of the Coos estuary to examine seasonal variability of salinity dynamics and estuarine exchange flow. In this tidally-driven estuary, modeling shows that salinity inflow is driven by tides, but outflow, stratification, and mixing are seasonally dependent and vary with river discharge.


Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.


Conroy, T., D.A. Sutherland, and D.K. Ralston, 2020: Estuarine exchange flow variability in a seasonal, segmented estuary. J. Phys. Oceanog., doi:10.1175/JPO-D-19-0108.1.

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