Currents are organized flows of water in a particular direction, like rivers in the sea. In the ocean, deep currents occur below 400 metres, and surface currents occur in the top 400 metres. Tidal currents, longshore currents and estuarine circulation are three types of local, surface currents that affect water movement and circulation around the Victoria area.
Global Ocean Currents
What causes local currents?
Local currents can be caused by tides, by a combination of wind and waves, or by freshwater streams or rivers entering the ocean. Tidal currents result when the flow of water with the changing tide is constricted by a channel or headland. Tidal currents generally change direction when the tide changes between the flood (incoming) tide and the ebb (outgoing) tide.
Longshore currents are produced when waves strike a shoreline on an oblique angle. The part of the wave that contacts the bottom first slows down relative to the rest of the wave. Refraction causes the wave to bend toward the shoreline and creates a current parallel to the shore. This current transports sediment and is important in creating landforms such as sand spits.
Waves and Coastal Sediment Processes
Rivers entering a body of salt water can also create currents that are known as estuarine circulation. Fresh water from a river is less dense than salty ocean water, therefore it moves seaward as a surface layer. More dense salt water moves landward to replace it. This process is similar to what occurs with deep ocean currents but on a smaller scale.
What types of currents affect the harbours in the Victoria area?
The waters around the CRD are subject to strong tidal currents, which account for 83-86% of the current variability in the Strait of Juan de Fuca. The flood tide flows into Juan de Fuca Strait, from southwest to northeast, and the ebb tide flows out in the opposite direction.
This tidal flow is superimposed on estuarine circulation. An enormous volume of fresh water enters Juan de Fuca Strait from the Fraser River near Vancouver. Although significant mixing of the layers occurs at the entrance to the strait, a general circulation pattern prevails: fresher water flows southwestward out of Juan de Fuca Strait near the surface and salt water flows into the strait at lower depths to replace it.
The combined effect of estuarine circulation and tidal flow creates currents as swift as one metre per second (or 3.6 km/h). This energy is sufficient to create sand dunes on the ocean floor in the middle of the Strait that are 25 metres high, 300 metres long and 1200 metres wide (see diagram below). Race Rocks, to the west of Victoria, is another area that experiences very strong tidal currents. These strong currents can be hazardous for small boats such as kayaks and sailboats.
Image: Computer-generated images of submarine dunes in Juan de Fuca Strait created by strong currents (courtesy of Natural Resources Canada)
In the harbours themselves, tidal currents flow basically into and out of the bays and inlets, but can also form eddies and other turbulent patterns as they swirl around land formations. Tidal currents predominate in the Gorge Waterway, particularly in the “narrows” at the Gorge bridge and between Aaron Point and Chapman Point. Some areas of Esquimalt Harbour also experience significant tidal currents, for example near Fisgard Island and Inskip Islands. In Victoria Harbour, tidal currents are strongest at the entrance and along the northern shore, for example near Harrison Island. Portage Inlet is unique due to its relative lack of tidal currents and in the lag effect caused by the narrow constriction of the Gorge Waterway. Esquimalt Lagoon experiences some tidal flushing at the entrance to the lagoon, but the lagoon itself is sheltered from strong currents.
Longshore currents have an important influence along the shoreline to the west of Victoria. For example, Coburg Peninsula, which encloses Esquimalt Lagoon, was formed by longshore transport of sediment from a gravel pit to the south. These currents continue to replenish Coburg Peninsula and maintain the sandy beach. (See also coastal sediment processes and Esquimalt Lagoon pages.)
What effects do currents have on marine life?
Currents are vital to the survival of all marine organisms. They transport nutrients, mix oxygen and other gases, aid in reproduction by dispersing spores, eggs and larvae, and act as transportation corridors for many animals. Many plants and animals are anchored to rocky shorelines, and depend on currents for all their needs. Kelp beds, for example, exist only where currents supply abundant nutrients and constantly mix the water.
Communities of filter feeding animals such as sponges, bryozoans, anemones, sea stars, mussels and oysters are often found in high current areas. Such communities can be observed in some of the locations in CRD harbours mentioned above, where tidal currents are strong.
The high nutrient levels associated with strong currents form the basis for rich marine life, from plankton up through fish, shellfish and marine mammals such as sea otters, harbour seals and whales.