As the fresh water from rivers continues to flow onto the surface of the Salish Sea, an estuarine current is formed that moves water from surface to bottom- seaward at the surface and towards the land at depths. This estuarine current has two major impacts. First it contributes the majority of dissolved nitrogen to the Salish Sea. The nitrogen is the primary nutrient that stimulates biological growth and results in a rich and diverse ecosystem. The estuarine circulation brings deep water from the ocean into the sea replacing the total volume of water of the Salish Sea each year. This is fundamental to keeping good water quality.
Estuarine Circulation and the box model
The movement of water in the Salish Sea has other forces than that resulting from just the river flow: tides, winds and bottom topography all have a part. At this point however, we want to understand only the effect of the rivers. Let’s assume that we have a Salish Sea estuary with only one river, constant river flow through the year, smooth sides, a smooth bottom, and with a single outlet to the ocean. This is called a box model and allows us to understand how the one variable we are interested in works. Our box model has no wind or tides.
How the Rivers and Ocean mix
Step 1. With no river flowing. Let’s assume for a moment that the river has stopped flowing. We have a layer of fresh water that floats on the salt water below and in the absence of energy to mix the two layers there is little mixing of the two layers. If we do not put any energy into this system the fresh water will very gradually mix with the salt below by diffusion. Note the light blue of the water that represents gradual mixing of the fresh and salt
Step 2. The River Moves. The continual flow of water from the river pushes the fresh surface water seaward. The fresh water on top still tends to stay as a discrete layer as it moves towards the ocean. However the closer the surface layer gets towards the ocean, we see that it gets saltier, than but not as salty as the deeper water below. How does that happen?
Step 3. Salt Water Entrainment. As the surface water is constantly pushed towards the ocean, there is friction between the moving surface layer and the saltier water below. The result of this friction is to “pull” some of the salt water into the fresher surface layer. We call this “salt water entrainment” In our box model estuary; the surface layer is still a little fresher than the deep water below. When it reaches the ocean it is just a little bit less salty.
Step 4. The deep water moves. As the deeper water in our estuary is constantly “entrained” by the moving surface layer, the result is that the water is sent out to the ocean and is replaced by water from the deeper ocean. This water is saltier and cooler than surface layers and tends to be rich in nitrogen nutrients.
Step 5. Equilibrium is reached. In our model once equilibrium is reached the surface water moves towards the ocean and deeper water is pulled in from the ocean at depth. If our model has the amount of fresh water flowing in that flows into the Salish Sea, the total volume would be exchanged in well under a year, and as you might expect the incoming deeper ocean water provides nitrogen nutrients that stimulates rich biological systems.
*circulation diagrams provided by Diedra Penner