The influence of Baker Bay and Sand Island on circulations in the mouth of the Columbia River

Abstract

The Columbia River estuary is the scene of a collision between strong river discharge, considerable tidal magnitude and violent surface gravity waves that have claimed thousands of vessels and hundreds of lives. Lagrangian GPS measurements of position and surface velocity and Eulerian measurements of vertical velocity structure, temperature and salinity were used to study circulation patterns in the mouth of the Columbia River, paying particular attention to the influence of Baker Bay and the Sand Island pile dikes. Tidal range and river discharge are the key drivers in the dynamic lower Columbia River, and the presence of Baker Bay, a shallow sub-embayment, adds further complexity. Drifter velocities were greatest during maximum ebb flows and were greater in the South Channel, which is dominated by river discharge, than the North Channel, which is more heavily influenced by tidal oscillations. Drifters occasionally entered Baker Bay via Baker Inlet during flood flows, especially in conjunction with strong southwesterly winds. During ebb flows, some drifters were trapped in the Sand Island pile dike system and conducted clockwise circulations for 2–3 hours. Eulerian water temperature and salinity depend on many variables, including the semi-diurnal tides, neap-spring cycle, season, ocean upwelling and downwelling, river discharge and bathymetry. Baker Bay served as a source of contrasting water characteristics to the main channel, injecting pockets of warmer water adjacent to Sand Island following neap higher high tides during a period of high river discharge. Velocity profiles at Baker Bay, Baker Inlet, between the Sand Island pile dikes and in the main channel were analyzed to determine the degree of logarithmic behavior at each location. Unique velocity perturbations occurred in the vicinity of the pile dikes, including reversed (upriver) flow between the pile dikes during maximum ebb. Understanding unique flow patterns in river estuaries can be critical for littoral naval operations.