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

• Published in: Journal of Geophysical Research Vol. 116; no. C8
• Main Authors: Halverson, M., Pawlowicz, R.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01.08.2011
• Subjects: Brackish; Entrainment; Estuaries; estuary; Fluctuations; Flushing; fresh water flushing time; Freshwater; Geophysics; Instrumentation; Marine; Nonlinear systems; Oceanography; Physical oceanography; River discharge; River flow; river plume; River plumes; Rivers; Saline water; Salinity; Salt balance; Scientific apparatus & instruments; ship of opportunity; Canada, British Columbia; Canada, British Columbia, Fraser R; INE, North America, Georgia Strait
• ISSN: 0148-0227; 2169-9275; 2156-2202; 2169-9291
• DOI: 10.1029/2010JC006793

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 m3s−1 at low river flow to a maximum of about 27,000 m3s−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. Key Points Plume salinity is determined by entrainment in the estuary and near‐field plume The freshwater flushing time is constant despite large changes in river flow The plume's 2‐day freshwater flushing time has dynamical consequences