Tidal Modulation of the Fraser River Plume

Abstract

Properties of the Fraser River plume in the Strait of Georgia, BC, are significantly influenced by the tide. However, the dynamics and magnitude of this tidal influence on the plume area are not known. Here, we use 17 years of daily satellite observations of suspended particulate matter to understand the tidal variability of the plume area. A consistent inverse relationship between the Fraser River plume area and the tidal elevation with a phase lag of 1–2 hr is revealed from two independent analyses: one by correcting for temporal aliasing and extracting tidal signal from the whole image set and the other using only same-day image pairs. The plume area increases/decreases by about 20% after ebb/flood tides, and a lower river discharge typically leads to a more dramatic tidal variation in the plume area. A simple analytical model based on the volume conservation and salinity balance equations is developed to analyze the mechanism of the tidal variability in the plume size. The observed tidal patterns of the plume area are largely reproduced using tidally modulated plume salinity (observed from instrumented ferries) and river discharge (from numerical model outputs). Tidal flux in both river discharge and entrainment rate is found to be important in explaining plume area variability.