Columbia River Plume Salinity: the effect of maintenance dredging in the mouth of the Columbia River

The Columbia River Plume provides a transitional habitat for juvenile salmonids moving from the river into the ocean, so there is public concern about possible changes in plume salinity associated with dredging at the mouth of the Columbia River, and other human activities on that salinity (e.g., flow divergence). The Army Corps of Engineers (ACE) conducts regular maintenance dredging in the mouth of the Columbia River, to aid the safe passage of ships through the "graveyard of the Pacific". The present study was undertaken to determine the impact of the channel dredging upon the initial formation of the Columbia plume and therefore on plume salinity. Constrictions in the mouth of the Columbia River act as hydraulic controls, determing the location and nature of plume liftoff, where the fresh water lifts off the bottom to form the plume. Prior studies indicated that the lateral constriction dominated the dynamics, and that plume formation was relatively unaffected by the sill a few kilometers seaward of the constriction. The ACE requested a more detailed study of the effect of dredging at this sill. We present a model study in three parts. First, a two-layer Eulerian model is used to describe the currents in the Columbia entrance channel, in the absence of any friction between the layers. The shape of the salt/ fresh interface is calculated at the location of plume liftoff, and different forcing and depth scenarios are compared. Then, a bulk Richardson number criterion is used to estimate the amount of mixing between the layers. The salinity and thickness of the nascent plume at peak ebb are extracted at the location of a depth constriction, and used to initialize a Lagrangian model. Finally, the Lagrangian model traces water parcels out over the continental shelf. Because the width control landward of the area of dredging provides the dominant influence on plume formation, dredging was found to have only modest impacts on plume conditions.