In the late 1800s, the U.S. Army Corps of Engineers removed the rocks forming the upper rapids to improve navigation. Later, the channel of the river narrowed in that area. To find out if any traces of the upper dam remained, researchers studied the area using side-scan sonar. That device was pulled by a boat. As the boat traveled, the sonar bounced sound off the river bottom and recorded a picture of the return signal.
It takes special training to interpret a side-scan sonar image! This one shows the pile of rubble that remains where a crib dam was near the upper rapids. Courtesy of the U.S. Army Corps of Engineers, Waterways Experiment Station, CERC-85-9.
Historical documents showed that three small dams had been at the upper rapids. The sonar survey confirmed that a part of one of the crib dams is still intact. However, there was no evidence of the other two dams, suggesting that they are no longer in place.
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