As was reported previously, there are studies that are saying that the increased mist at the Falls is from temperature, not from the wall of hotels. The New York Times has an excellent article about the 2 sides of this debate:
From his seventh-floor office at the State University of New York at Buffalo, Marcus Bursik can watch the mist rising from Niagara Falls, about 20 miles away. In size and shape, the mist resembles the volcano plumes that Dr. Bursik studies as his primary topic of research. Noticing that the dimensions of the Niagara plume changed from day to day, he decided to investigate them.
Across the border in Canada, the Niagara Parks Commission was doing the same thing for a different reason: over the past decade, the number of misty days had more than doubled. The mist may have looked nice from a distance, but it was spattering patios, ruining Kodak moments and chasing tourists under awnings and umbrellas.