Three Waterfalls
Technically speaking, Niagara Falls belongs to Canada and the US, but there are some parts that solely belong to the US. The American Falls and the Bridal Veil Falls are exclusively American parts of Niagara Falls. On the other hand, there is no part that only belongs to Canada. The biggest part is called Horseshoe Falls which represents the natural border between the countries.
Some Concerns
People who go see Niagara Falls often enough will be able to notice the changes the waterfall experiences over time. With each new gallon of water that goes down the cliffs, the Falls lose a bit of its charm. At least, this is true according to people who live nearby and visit the Falls regularly.