Entrainment and flushing time in the Fraser River estuary and plume from a steady salt balance analysis

A passenger ferry equipped with oceanographic instrumentation made eight transects across the Fraser River plume in the Strait of Georgia, British Columbia, each day for 4 years. Estimates of average plume salinity and volume were made from the observations and used in a quasi-steady budget to estimate the net upward flux of ambient salt water into the plume. This entrainment flux increases from a minimum of 17,000 m(3)s(-1) at low river flow to a maximum of about 27,000 m(3)s(-1) at intermediate flow. The entrainment flux then becomes essentially independent of river flow, although we argue on physical grounds it must eventually decrease. Entrainment velocities of a few mm s(-1) were estimated from the entrainment flux when assuming that mixing primarily occurs along the salt wedge estuary or in the near-field plume. These values are consistent with previous estimates. This, in turn, suggests that entrainment is not important over most of the plume's full extent and that bulk plume properties are set in the near field. Finally, the observations are used to estimate a plume freshwater flushing time of 2.2 days, with essentially no dependence on river discharge, even though discharge varies seasonally by almost an order of magnitude. This value lies in between the expected time scales for near-field and far-field plumes and is long enough for rotation to play an important role. There is no evidence, however, of a coherent bulge as observed in other systems.