Modeling Historic Salinity Intrusion in the Lower Columbia River Estuary

The salinity intrusion length, or the maximum extent that saline ocean water intrudes into an estuary is an important estuarine ecosystem parameter. In this presentation we detail the development of a 3-D baroclinic model of the 19^th century Lower Columbia River Estuary (LCRE) to assess how much salinity intrusion has changed. Comparisons of historic and modern data and model results indicates that the M₂ tide has increased by ~10% due primarily to channel deepening but also reduced freshwater flow (17% decrease in mean discharge and 40-45% in peak discharge). Bathymetric changes and seasonal shifts in flow input have increased salinity intrusion, such that the LCRE has shifted from a seasonally freshwater estuary to one with nearly permanent salinity intrusion. Available water temperature data from 1854-present and density data from 1925-present confirm that long term trends in tidal amplitudes are also associated with increases in salinity intrusion. Mechanisms for the changes include: increased estuarine circulation, increased internal asymmetry, reduced bottom friction and mixing, and decreased river flow. Changes to estuary bathymetry also appear to have changed the transport of buoyancy to the shelf; compared to the modern plume, which exits the estuary as a jet, initial model results suggest that the much shallower, more irregular bathymetry of the 19th century mouth produced a more bifurcated, dispersed near-field behavior.