Keep an eye open when you cross the Strait … that might be the Fraser River Plume you're looking at.
by Paul LeBlond
Islands are defined by the waters that surround them. In our case, it's the Strait of Georgia and the channels that connect it to the ocean, from Puget Sound in the south, to Queen Charlotte Strait in the north, a coastal realm often called the Salish Sea.
The Salish Sea as a whole is an estuary: a semi-enclosed coastal area receiving significant freshwater runoff. This oceanographic definition is somewhat broader than the more familiar geographic concept of an estuary as the area around the mouth of a river.
In all estuaries, the interaction of winds, tides and runoff causes vigorous mixing and upwelling which create an environment rich in marine life. Everything that happens to the waters around our islands can be understood in terms of the estuarine context.
Under the action of the moon and the sun, the waters of the Pacific Ocean slosh back and forth twice a day: this is what we call the tide. As the sea level goes up at the mouth of Juan de Fuca Strait, water floods in towards the inner channels of the Salish Sea, gradually filling them. Later on, the sea level falls in the Pacific, and the coastal sea empties as the tide ebbs. Back and forth, twice a day.
Islands stand in the way of the rising tide, channelling flow to the passages between them. As the tide rises on the southern side of Active Pass, for example, water is pushed through to the Strait of Georgia. Because only a relatively small volume can flow through the pass, the difference in water levels between the ends of the pass continues to increase as the tide rises, and the currents become strong.
The rushing of the water causes waves to steepen and break and strong turbulence to be generated, mixing and homogenizing the waters in the pass. Eventually, the Strait of Georgia fills up, the water levels equilibrate and the current slackens. Then the tide falls and the current reverses. It's clearly no accident that Active Pass is such an interesting and biologically productive area: those tidal currents keep the waters well mixed and distribute nutrients and food through the water column.
Meanwhile, rivers continuously bring to the sea all that rain and snow that falls on our coast. Freshwater is lighter than saltwater and floats on top of it. As it spreads out on the sea, the fresh, muddy water layer, pushed by winds and tides, forms a pattern similar to that of smoke rising from a chimney, swept hither and fro by the winds. Engineers, ever poetic, call that pattern a "plume" (French for feather). French engineers, even more romantic, call it a "panache," meaning a bunch of undulating feathers, like those attached to the helmet of a hero.
The Fraser River spews out more water into the Salish Sea than all the other streams put together.
In summer, during freshet (the peak flow period), the plume fills the central Strait of Georgia and, on ebbing tides, spills out through Active and Porlier Pass. The plume covers the Strait with a thin (a few metres) layer of brackish muddy water, freshest and murkiest near the river, progressively saltier and clearer seawards. Winds and tidal currents push the plume up and down the Strait.
The strong ebbing flow from the Fraser propels the plume (and ships afloat on it, such as Captain Galiano's in 1792) towards Galiano Island, which rises as a barrier across its path. Our mainland-side shore is bathed by plume waters, turbid, brackish, and warm.
The low-density plume, floating as it does above the colder salt water, traps the heat of the sun. The raging tidal currents in Active Pass mix it with a lot of colder salt water, leaving the waters there less turbid and a lot more frigid.
As the tide rises, it backs up the flow out of the Fraser, causing upstream flow as far as the Deas Tunnel and pushing water up in tides all the way to Mission. The freshwater outflow at Steveston pulsates from nothing at flood tide to a strong jet at ebb. The plume is thus formed of a series of lenses of water, contributed by successive ebb tides. The latest lens, very fresh and muddy, spreads over an older lens, already partially mixed with salt water and thus denser and less muddy. The transition between new and old lenses is sometimes very abrupt and can be seen from above as a sharp change in colour over a distance of just a couple of metres. This rapid change in water properties is called a front. Fronts often accumulate flotsam; they are particularly noticeable under calm conditions.
The plume also supports internal waves. Everyone knows what waves do. Water, displaced from its equilibrium, falls back towards it, overshoots and bounces back again, pushes the water next to it, and so on … a wave is born, which can travel far away from its source, a radiant messenger of the disturbance that created it.
Surface waves travel on the interface between water and air. Internal waves travel on internal density changes, especially along sharp ones as between plume and salt water. The density contrast across from fresh to salt water at the bottom of the plume is about a hundred times smaller than that from air to water at the sea surface. Internal waves are thus a lot more sluggish than surface waves. Rising tides disturb the edge of the plume at strong tidal passages (especially Boundary Passage and Active Pass) and create internal waves which radiate away in broad spreading patterns.
In typical internal waves, the plume-salty interface dips down to about twice the plume depth every hundred metres or so in a sharp trough. The long snaking troughs advance at a speed of about one metre per second: a walking pace. In contrast, an ordinary surface wave one hundred metre long between crests (or troughs) would travel, in the deeper parts of the Strait, at a speed of 18 metres per second, or 65 kilometres an hour.
At the dips in the interface, the plume is thicker than elsewhere. From above, the water looks muddier because there is a deeper layer of near-surface muddy water. Coloured lines seen from above correspond to troughs (thick plume) of the internal wave; darker bands in between to crests (thin plume). These are visible from the air or the upper deck of the ferry.
The sea surface is hardly displaced at all by the internal motion, but shorter wind-waves riding above it are distorted by the water displacements associated with the internal wave. Over the dips, converging flows steepen waves, making the surface rougher and less mirror-like. Bands of shiny and darker water reveal internal wave trains.
Keep an eye open when you cross the Strait this summer. You are likely to travel through the Fraser River plume. Look out for fronts, and for the long bands of ruffled or slick water that betray the presence of internal waves.
* Reprinted from Archipelago Spring 2000, Galiano Conservation Association.
