The River Plume

    An interesting phenomenon can be observed where a stream empties into a lake.  A distinct color difference between the river and lake water can commonly be observed.  The area that appears to be an extension of "river-colored" water into a lake is called a “plume.”  Figure 1 is a cartoon of an aerial view of a river plume.  Note the distinct color difference.  Although this is  a cartoon representation, the color difference in actuality may be just as distinct.

What causes the river plume?
    A river plume is visible for several reasons.  River water commonly appears more brown in color than the lake because the river is ordinarily carrying more suspended particles such as silt and clay.  However, the plume is not just different in color, but also a distinct line is commonly observed between the two water masses.  Typically, the river water is relatively warm and, therefore, less dense than the relatively cool, lake water.  Water of different densities will not mix easily.  Consequently, a distinct line forms where the waters of different densities meet.  This boundary continues under the water as well, so the "line" is actually a surface.

    Figure 2 illustrates a cross section of a plume.  Notice how the river water lies over the more dense, lake water.  This results in the formation of a wedge of river water overlying the lake water.  A boat is shown testing water at the plume.  If the researchers test the water at depth “a”, the water will have the characteristics of the river water.  However, if the researchers test the water further down the line at depth “b”, the water will have the characteristics of the lake water.   A similar wedge of water occurs where river water (relatively low-density, fresh water) flows into the ocean (relatively high-density, salty water).

    A photograph of the plume at the mouth of the Kalamazoo River at Saugatuck, Michigan, is shown in Figure 3.  The geometry of a river plume as well as the sharpness of its boundary may vary considerably depending on a number of factors (e.g., wind, wave, and current conditions as well as the amount of material carried in suspension).  But by studying the geometry of the plume, you can determine the direction of littoral transport at any one time (see "Longshore Current and Beach Drift").  For example, in Figure 3 you can see the plume extends to the left (south) indicating that littoral transport was from north to south when the photograph was taken.  You can also get a sense from the photograph of  the tremendous amount of sediment that is carried to the Great Lakes daily by rivers.  Whether or not the sediment is accompanied by toxic materials, it decreases the water quality of the lakes by increasing  turbidity.  As a result, control of the erosion of sediment into streams is critical to maintaining high water quality in the Great Lakes.

Figure 3  Photograph of the mouth of the Kalamazoo River at Saugatuck, Michigan.
Note the distinctive, brown, river water (plume) that extends into Lake Michigan.

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