The Basics of Grain Size within a Lake System


   Grain size within a lake system is generally related to the amount of energy that exists in a particular environment.  Below is a simple graphic showing a cross section of a typical lake with significant wave action.  Notice on the diagram that different areas of the lake have different relative energy levels.  Nearshore, wave action results in a relatively high energy environment.  In deeper water, the relative energy is less.  Deep water (greater than wave base, which is equal to one half the wavelength) is not affected by surface waves.  Hence, the deep water environment is normally a low energy environment, unless it is affected by strong currents.
 
 


How is grain size related to energy?
   Water with high energy is able to move many particles as bed load (sand and gravel) or in suspension (silt and clay).  As the amount of energy decreases, the size of particles the water can hold in suspension also decreases.  Thus, as we move along the lake bottom away from shore and into deeper water, grains moved as bed load or held in suspension decrease in size.  In the relatively high energy area nearshore, the first particles to be deposited will be the heavier particles such as pebbles and coarse sand.  In the medium energy area offshore, we tend to find finer sand or silt.  In deep water, water movement is usually negligible.  In order for clay sized grains to drop out of suspension, the water must be very still for an extended period of time.  Since the deep water region is usually very still, this is the area in which we tend to find very fine-grained particles such as clay.

Is grain size within a lake system really that easy?
   No.  The basic concept outlined above is correct; however, many factors can affect energy distribution within a lake.  Storms, deep-water currents, and upwelling are just a few examples of things that alter the energy distribution within a lake.  These changes in energy will affect grain size.  Therefore, grain size can actually be more complicated than simple lake geometry predicts.


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