ENERGETICS AND SEDIMENTARY PROCESSES IN THE COLUMBIA RIVER ESTUARY

The Columbia River Estuary is an energetic, sand-bedded system. Mixed tides with a spring tidal range of about 3.6m and a large freshwater discharge produce bottom shear stresses capable of transporting nearly all the sizes of sediment present in the estuary. As a result, the morphology of the estuary is closely related to the fluxes of tidal and fluvial energy. To investigate the relationship between energetics and sedimentary processes, an energy budget based on a one-dimensional harmonic tidal model was developed for the estuary-tidal river system and has been compared with the results of geological studies of the bedforms, large-scale morphology, sediment distribution, and suspended sediment field. The pattern of energy fluxes predicted by the model suggests the division of the system into three regimes: a tidally dominated lower estuary, a mid-estuary energy flux divergence (EFD) minimum region, and a fluvially controlled, tidal-fluvial reach. Model results also show that non-linear interactions between the tidal and fluvial flows are responsible for the suppression of the tides in the tidal-fluvial reach during periods of high flow. Finally, changes in tidal amplitude at the mouth result in a less than proportionate response in tidal elevations at points inside the estuary, because the cubic dependence of dissipation on tidal amplitude damps system responses to such changes. Many features of the observed sediment transport patterns and sedimentary environments can be related to the energy budget. Most of the medium to coarse sands entering the system from the river are permanently retained within the EFD minimum. Much of this deposition takes place upstream of the limits of salinity intrusion and is not, therefore, related to baroclinic circulatory effects. Most of the fine sands and the silts and clays entering the system are not permanently retained. Some of the silts and clays are, however, temporarily retained in a turbidity maximum, whose mean position is near the lower end of the EFD minimum. This position is dictated by the inability of salinity intrusion to extend up the fluvial potential energy gradient. Although some aspects of sedimentary processes cannot readily be related to the energy budget, our model provides a valuable conceptual approach to the dominant long-term sedimentary processes of sand transport and deposition in the Columbia River Estuary that structure the observed sedimentary environments. Biological processes are in turn strongly influenced by these geological patterns.