Well, firstly it’s neither of these. It’s the International Control Structure that extends 1,550 feet from the Canadian shore to the roughly the middle of the Niagara River, about a half a mile above the Canadian Horseshoe Falls. It just looks like someone started to build a bridge or dam and just stopped.
It was built both by Canadians and Americans in 1954 for two reasons.
The first was to hold back some of the water going over the Canadian Falls and force more water from the upper river to the American Rapids and over the American Falls and Bridal Veil Falls. The American Falls is about 10 feet higher than the Canadian side. If more water was diverted from the American Falls it would have turned them into a stream.
The second reason was to divert water from the upper river into intakes for Canadian and American power plants downriver of the falls at Lewiston N.Y. and Queenston, Ontario.
 I’ve shown the International Control Structure plus one of these intakes found on the Canadian side of the border.