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Don’t Dump. Drains to the Bay:
The Fate of River Water in the Coastal Ocean

Alex Horner
Civil and Environmental Engineering
Stanford University
March 2000

Picture your favorite coastal city. Wander down to the harbor front and look around for the river mouth (most great coastal cities have them). Have you ever wondered where all of that river water comes from and where it is going? A quick look at a map will give you a good idea of where it has come from, and probably a good guess about what it may have picked up along the way. But where is it going? And where will it leave all of the contaminants and nutrients that it carried to the coast? In order to answer some of these questions, I conduct laboratory experiments to understand what determines the fate of river water after it has been expelled into the coastal ocean.

Rivers are collectors. They collect the sediments from erosion, the nutrients from decaying plants and animals, and everything that we pour into them. At the coast, where the rivers end, what happens to the river water will determine where all of these go. The Mississippi basin, for example, collects water from 40% of the lower 48 states and parts of Canada as well. Most importantly, it drains the heavily fertilized farmlands of the Midwest and empties an average of 4000 tons of dissolved nitrogen into the Gulf of Mexico each day. During the summer, when this nutrient load is at its peak, biological growth and decay are so intense near the river mouth that most of the oxygen in the water is used up. This is called hypoxia and the affected areas are often called dead zones as very little can live in them until the oxygen balance is restored. The freshwater leaving the river mouth is referred to as the river plume, and the shape of this plume determines the location of these dead zones. Near the mouth if the Mississippi, along the Louisiana coast, the location of the river plume is obviously very important to the local fishing industry. In other parts of the world, river plumes are important to the offshore oil drilling industry, the tourism industry, marine biologists, and anyone who is interested in maintaining the look and function of the coastlines.

I am interested in learning what determines the shape and size of river plumes so that we can predict how they will affect the surrounding coast. The fate of the river water is probably determined by a few physical parameters; the flow rate of the river, the geometry of the river mouth, the salinity difference between the river and the coastal ocean, and the rotation of the earth. We can make some educated guesses about the effects of these parameters. For instance, if the flow rate of the river increases, we expect the river water to travel further offshore. If the river mouth were especially wide and shallow, we expect the river water to be more dilute and to remain closer to the shore.

The effect of salinity is less obvious. River water tends to "float" on the surface of ocean water instead of mixing down to the bottom because the lack of salt makes it less dense. This means that completely fresh river water flowing into (or onto) the salty ocean water will stay on the surface. River water that becomes slightly salty in an estuary, on the other hand, will mix down into the ocean water more readily since its density is closer to that of the ocean water. In this case, the river water forms a deeper, more dilute current.

The effect of the rotation of the earth is referred to as the Coriolis force. While it is nearly imperceptible in everyday life, the Coriolis force has a significant effect on very large-scale motions such as ocean currents. In the northern hemisphere, the Coriolis force deflects currents clockwise so we expect the out-flowing river water to move to the right as it leaves the mouth. If the relative effect of rotation is strong enough, the water will be turned back into the coast and guided away from the river mouth. However, when we consider simultaneously the effects of flow rate, salinity, geometry, and rotation, the results are more difficult to predict. For most large rivers in the northern hemisphere, far from the equator (where the Coriolis force is weakest), the combination of all these effects is a freshwater surface current that turns to the right and travels along the coast.

The picture described above has been observed and studied at the mouth of the Columbia, Delaware, and Mississippi rivers and the Chesapeake Bay. In all natural settings, however, winds, upwelling currents or other local phenomena frequently obscure these features. In addition, the large scale of the river plume makes it very difficult to take measurements before the wind or other currents change it. In order to understand the physics that govern the movement of the river water, we can devise laboratory experiments or computer simulations to model the real flow.

In general, computer simulations and laboratory experiments have reproduced observed river plumes well and are currently being used to provide new insight into the dynamics of the natural river plumes. Recent computer simulations suggest, for example, that river water, with all of its nutrients and pollution, accumulates near the mouth of the river and only a fraction of it moves down the coast in the current described above.

I have built a laboratory experiment to determine what causes river water to accumulate near the mouth and to examine how the flow rate, geometry, salinity and Coriolis force combine to determine the fate of the river water. The experiment consists of a circular tank, 2 meters in diameter, filled with salt water and set upon a rotating table. In the tank, I built a straight wall with a small diffuser from which I can introduce freshwater to represent the mouth of a river. I use a system of lasers, digital cameras, fluorescing dyes and particles in the flow to measure the velocity and salinity. We currently think that a balance exists between the momentum of the river water and the Coriolis force with the density difference and river geometry only playing a secondary role. In other words, if we knew the ratio of momentum to the Coriolis force, we would be able to predict the fraction of river water that accumulates at the mouth.

In the future, the models that we develop of coastal river plumes may be used by city engineers to set water quality standards, by coastal engineers to determine how oil slicks or other contaminants will disperse, or by biologists to understand the food sources of coastal organisms.