Variations of the Fraser River Plume and their relationship to forcing by tide,wind and discharge

Abstract

Temporal and spatial variations of the Fraser River Plume, in the central Strait of Georgia (British Columbia, Canada), are monitored by continuous salinity sampling of the engine cooling water on two B.C. ferries. Travelling along two different routes between Vancouver Island and the mainland the ferries provide eight crossings per day both north and south of the river outflow. From each crossing, characteristic measures of the plume are extracted, such as the average salinity and the maximum salinity gradient. These parameters we then formulated as time series and used to compute cross‐correlations and cross‐spectra with the probable driving forces of wind and river discharge. The effect of the tides is examined using harmonic analysis.

Periods of high river discharge lead to decreases in the average salinity for each section, and peaks in the magnitude of the maximum salinity gradient. The correlation of the plume characteristics (average salinity, maximum salinity gradient) on the southern section with the along‐strait component of the wind is consistent with advection by the wind. No obvious correlation is found between the plume characteristics on the northern section and the wind, except during isolated events. Linear combinations of the wind and the discharge variations reproduce the general trend of the average salinities but cannot explain the level of variability. A shift to a non‐linear combination of wind and discharge improves this comparison. The phases of parameter fluctuations at tidal frequencies on the southern section agree with the expected effects of tidal currents and the modulation of the river discharge. The agreement is not as apparent for the northern section.