Long Term Changes to the Lower Columbia River Estuary (LCRE) Hydrodynamics and Salinity Patterns

Changing the morphological and hydrological conditions of an estuary can affect the estuarine hydrodynamics. The hydrograph of the Lower Columbia River Estuary (LCRE) and its bathymetry have been altered significantly over the past 150 years, such that the spring-freshet has decreased by 40-50% while winter flow has increased by 50%. In addition, the inlet width has been narrowed from 9.7 to 3.2km by the construction of jetties, and the controlling depth of the navigation channel has been deepened from 6 to 13m by continuous dredging. Also, ~70% of the shallow water habitat has been lost due to diking and wetland reclamation. Finally, the main shipping channel was changed from the north to the south side of the estuary. These system alterations lead to the following question: how have changes since the mid-1800s altered the salinity intrusion, freshwater distribution, transport processes, and water levels in the Lower Columbia River estuary? In this study, I use a 3D hydrodynamic model to gain insights into changes in circulation and salinity intrusion from the mid-1800s to the present. Two models were constructed: one based on bathymetric measurements made between 1869 and 1900, and a second based on bathymetric surveys made from 2008 to 2010. The horizontal resolution is typically 50m inside the estuary and 2000m offshore, with 36 sigma layers of vertical resolution. The models have been calibrated and verified using field measurements distributed over the lower 40km of the estuary. The average skill assessment of the calibration is 0.98; the root mean square error is between 0.11m and 0.15m for water level, 2-4psu for salinity, and 0.25m/s for velocity. Analysis of flow bifurcation (water distribution between channels at a junction) showed that morphology, tidal forcing, and water surface slope have played major roles in freshwater distribution in the multi-channel estuary. However, the asymmetry in the water surface slope between the North and the South Channels is what controls the distribution of flow between the two channels. In the historical model, the two channels surface slopes are equal. Therefore, the hydraulic radius and the cross-sectional area control the flow distribution, but the percentage of freshwater in each channel is still a function of river discharge. For the modern model, the percentage of freshwater in the South Channel decreases as the river discharge increases. During spring tide, 88% of freshwater passes through the South Channel during low river discharge conditions (3,000m3/s), while 48% passes through during high river discharge (15,000m3/s). Conversely, in the historical model, the percentage of freshwater in the South Channel increases as the river discharge increases. This implies that the morphological changes in the estuary causes the South Channel to export more freshwater today, as compared to 150 years ago. Finally, model results suggest that M2 amplitude has increased by 10% in the city of Astoria due exclusively to channel deepening, and about 12.5% (~0.11m) when both channel deepening and river discharge alteration are included. The LCRE system alteration is modeled to change the average location and the seasonal cycle of salinity intrusion. The simulation results found that average winter-time salinity intrusion has decreased due to a 50% increase in river discharge. During the annual spring freshet, salinity intrusion has increased because of decreased river flow. Due to changes in river discharge, the seasonality of maximum salinity intrusion has shifted from wintertime to late summer/early fall. Channel deepening exerts a strong control on salinity intrusion, and has caused greater landward salinity intrusion landward due to increased stratification. Channel deepening also amplified spring-neap variations in intrusion and altered the sensitivity of intrusion to river discharge. Altogether, the change in freshwater distribution, salinity intrusion, and stratification produced a change in the LCRE system classification. A parameter space classification based on Geyer & MacCready, (2014), suggests that morphological changes and river discharge alteration have shifted the LCRE from partially mixed 41% of the time to a salt wedge estuary 34% of the time. Finally, the effects of future sea-level rise and a subduction zone earthquake on tidal amplitudes and salinity intrusion are investigated. A future sea-level rise of 0.21m will have an insignificant effect on salinity intrusion and tidal variation inside the estuary. On the other hand, sudden land subsidence resulting from a potential M9 earthquake and/or a future 1.5m SLR (extreme cases) could produce a substantial effect on tidal amplitudes and circulation. Modeling analyses suggest that either scenario could elevate the major tidal constituent (M2) by 0.05m. Moreover, the maximum value could be moved upstream by 4km. Combined together, the combined effect of 1.5m SLR and sudden land subsidence might raise M2 by 0.11m and advance the location of maximum M2 by 10km landward. Similarly, salinity intrusion could increase 4km as a result of either the SLR or land subsidence cases, and might increase by 10km with both scenarios combined. Moreover, both cases have altered the habitat water depth and inundation by increasing the mean depth