You’ve got to check out this great picture of the Niagara Falls area from the NASA Earth Observatory web site:
A blue-green veil of water tumbles 51 meters over the rocky precipice of the Niagara Falls in this Ikonos image, acquired on August 2, 2004. Every second, more than two million liters of water plummets over the half-circle of the Canadian/Horseshoe portion of the Niagara Falls, shown here, making it one of the world’s largest waterfalls. The force of the pounding water is sending a cloud of mist up from the bottom of the falls; this same force eats away at the rock behind the falls, pushing them back as much as two meters per year.
You can also see some less detailed photos on the NASA Gateway to Astronaut Photography web site. When you are on the site you keep clicking on the map to drill down to specific areas of the world.
The NASA Visible Earth project web site also has a lot of images. You can search for Niagara Falls and find a couple of images.
Lastly, here is a good shot of the entire Niagara River.
