In the summer/fall of 1969, the Army Corps of Engineers “dried up” the American side (not the Horseshoe Falls) of Niagara Falls in order to conduct a geological survey of the rock bed underneath. Water was diverted by way of a 600-foot cofferdam and once the water was stopped, surveyors found the remains of a man, a woman and a deer. Once the cliff face and base were dry, engineers began scaling (as in what you’d do to a fish) loose rock, cleaning the rocks of algae, drilling for rock samples and installing a system of hydration pipes under the rock face to keep it from crumbling.*
I imagine the sound must have been very strange. Niagara Falls makes a LOT of noise. Diverting a third of the water must have made it seem oddly quiet.
Some good pictures and more information here.