If you've ever been to Niagara Falls and traveled along the Canadian side on the upper river, you'd notice a long shallow dam-like structure stretching from the Canadian shoreline out to about the mid point of the river. I'd always assumed that this structure, called the International Control Structure, was used to divert water for Canadian hydropower. In fact, it is not. A quick Google search resulted in the answer at Niagara Falls Hydrology. Surprisingly, it is for scenic purposes:
In 1912, "existing diversions have already seriously interfered with and injured the scenic grandeur of Niagara Falls at the Horseshoe, which injury and interference will be emphasized by the effects of lower stages sure to recur on Lake Erie and the upper lakes due to natural causes" (W. L. Marshall, Chief of Engineers, U. S. Army, War Department for Maj. Charles Keller, Corps of Engineers, in 28 Ann Rep Comm, 1912). Mr. Marshall also suggested the remedy was to place a "submerged dam placed in the bed of the river immediately above Horseshoe Fall, with the object of diverting a portion of the great volume passing over the center ... so as to increase the streams feeding the depleted ends of that fall ..."
Incredibly, the American and Canadian power projects can nearly drain the Niagara River empty:
At present, fifty percent of the water in the Niagara River is removed for the purposes of generating power, etc. between April first and October 31 (tourist season); seventy-five percent is removed during the winter (non tourist) months (Bastedo in Tesmer, 1981).More on the unsightly International Control Structure:
Recession at the Horseshoe Falls was intensified from the effects of early water diversion activities, which tended to concentrate the volume of river water in the center of the Horseshoe Falls (Tiplin, 1988). To compensate for this concentration, a submerged weir was built in 1942 to raise the level of water in the Grass Island Pool. The conspicuous, unsubmerged International Control Structure, built in 1953-54 to spread the remaining water allowed over the cataracts more or less evenly over the Horseshoe and American Falls, was accompanied by "remedial work" at the exposed edges of the Horseshoe Falls -including the Terrapin Point section on Goat Island. Remedial work included excavating deeper channels in certain areas above the falls. "Viewing areas" were areas of reclaimed land, shortening the width of the falls at Terrapin Point, on Goat Island and the Table Rock viewing area in Ontario.