Sediment transport in ice occurs along shore as well as offshore into the deep lake. This episodic process is one of the principal mechanisms by which sand is lost from the nearshore system. USGS investigators monitor geological processes to determine the mechanisms for and extent of sediment movement. Bluff retreat between Wilmette and Waukegan, Illinois, varies from 10 to 75 centimeters per year, and averages 20 to 25 centimeters per year. Erosion rates north of Waukegan, however, approach 300 centimeters per year.
Sediment eroding from bluffs provides most of the sand to the nearshore zone.
As more structures are erected to protect the bluffs, less sand is available to the system. Consequently, the nearshore sand reservoir has shown a dramatic decrease in volume over the past 20 years. As this sand supply dwindles, the finer-textured lakebed sediments are exposed to wave attack, inevitably accelerating coastal retreat. Sidescan sonar surveys along Chicago's lakeshore showed where and why the revetments are failing.