Some Oceanographic Features of Juan de Fuca Strait

Abstract

The distribution and structure of dissolved oxygen, salinity, temperature and density, and their seasonal and tidal variations are summarized, and related to the tidal and estuarine mechanisms.Juan de Fuca Strait is a complex, deep, positive estuary. It is divided into inner and outer parts by a sill extending southward across the channel from Victoria, B.C. The Inner Strait is separated from the Strait of Georgia by the San Juan Archipelago. The water structure in the Strait of Georgia is highly stratified due to the shallow brackish upper zone maintained by the Fraser River discharge. This brackish water tends persistently seaward due to the estuarine mechanism. In the passages through the San Juan Archipelago the shallow and deep waters are mixed to near homogeneity by the turbulent tidal flows. In the Inner Strait the stratification is small. Part of this mixed water is fed back into the lower zone of the Strait of Georgia, and part escapes seaward in the upper zone of the outer part of Juan de Fuca Strait, where it overruns the intruding ocean water, creating a new stratification. The ebb flow is stronger than the flood in this upper zone, and the halocline is deepest on the northern side of the strait.The flood flow, augmented by the deep inflow required by the estuarine mechanism, is strongest in the lower zone. Here the ocean waters advance over the sill during the flood flow, but do not retreat during the ebb flow, which is relatively weak. These ocean waters are incorporated with the mixed waters in the Inner Strait. This mechanism is a tidal pump.The concentration of fresh water in the upper zone of Juan de Fuca Strait varies from 2 to 6% during the year. The amount (depth of fresh water when separated from the ocean water in the system) varies from 1 to 7 m. In this and all other properties there is a gradient from the Strait of Georgia into the Inner Strait. In the Outer Strait there are cross-channel gradients, but none longitudinally.Throughout the system the density structure is salinity dominated. During the summer the thermocline coincides with the halocline. In winter the waters are isothermal, or the upper waters become slightly colder than the deep waters. Then the stability depends on the salinity structure alone.The salinity is a linear function of temperature within 0.1 °C, except at the surface in summer. The slope of the relation varies with time (season) and location. The relation shows that the waters throughout the system are mixtures of ocean water and brackish water from the Strait of Georgia, and tributary inlets.