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.
LIVING WITH THE GREAT LAKES
BROUGHT TO YOU BY:
GRAND VALLEY STATE UNIVERSITY
DEPARTMENT OF GEOLOGY
ALLENDALE, MICHIGAN 49401