Tides

How tides move the water in the Salish Sea

We know that when we stand at the edge of the Salish Sea the water level is always changing. Dropping as the tide ebbs and rising as it floods. The ebb and flooding occurs on approximately 6 hours cycles. This tidal cycle also causes the water to move horizontally. Using the box model, (described for the estuarine circulation description), we will show how this horizontal movement works. First our assumptions. To start we assume that our sea has smooth sides and a flat bottom.

Step 1. First lets see what happens if we put a dam at the mouth of the Strait of Juan de Fuca that stops water exchange between the ocean and the Salish Sea. The tides in the Salish Sea stop. The water no longer ebbs or floods. Tides are a property of the open ocean. The Salish Sea is too small to have its own tides. Instead it responds to the rising and falling of the Pacific Ocean.

Step 2. Now let’s have a dam at the entrance to the Salish Sea that drops slowly for 6 hours. We see that the waters of the Salish Sea flow towards the Ocean. After the six hour period the surface waters of the Salish Sea have moved some distance towards the Ocean.

Step 3. Now we replace the physical dam at the mouth of the Strait of Juan de Fuca with the Ocean itself. As the Ocean tide rises, water moves back into the Salish Sea. At the end of the 6 hours the water is almost back to the point it started from. But not all the way. There is a net seaward movement of water caused by the estuarine circulation. So at the end of an ebb and flood tide the water is somewhat closer to the Ocean than when it started.

Step 4. Now we will introduce an area of shallow irregular bottom to our otherwise smooth bottom. Like we see in Haro and Rosario Straits, or in Deception Pass. As the tidal current moves over the shallow bottom, turbulence causes mixing of the water all the way to the surface. These are the tidal currents that boaters in the Salish Sea know so well. The current flows towards the Ocean as the tide ebbs.

Step 5. As the tide floods the current flows inward. As we saw in number three above. At the end of the tidal cycle the net movement of the water is just a little bit seaward, again the result of estuarine circulation. As you can see in boxes 4 and 5, there is an increase in surface salinity resulting from this tidal mixing. The estuary tends to have the fresh water float on top of the salt water (see estuarine circulation). Tidal currents, with mixing from surface to bottom, mix the less salty water at the surface with the saltier water at depths. The net result is that, except for areas near to the mouth of a river, the surface waters of the Salish Sea are closer to the salinity of the Ocean.

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*tide diagrams provided by Diedra Penner

Page Updated 07.13.2012