Rainbow over the American and Horseshoe Falls in Niagara Falls, Ontario, Canada.
Nikon D700/28-300VR, 1/160s, f/16, ISO 200, EV 0, 28mm focal length.

Spend a day at Niagara Falls in Ontario, Canada, on a warm summer day with little breeze and you may be rewarded with a beautiful rainbow in the mist thrown into the air above the waterfalls. Along the Queen Victoria Park, I stood with thousands of people marveling at the beauty and power of the waterfalls and river which formed the Niagara Gorge and wonders before them. It is both overwhelming and humbling to stand within feet of millions of cubic feet of water dropping almost 200 feet (60 meters) over the falls into the Niagara River below.

To come face to face with the power of Niagara Falls, many tourists were donning blue raincoats and boarding one of the Maid of the Mist boats.  These boats bring people within feet of the base of the waterfalls.  Horseshoe Falls are about 2,600 feet (790 meters) wide and drop 173 feet (53 meters) and is the most powerful waterfall in North America, as measured by vertical height and flow rate.

Maid of the Mist VII tour boat full of blue raincoat clad people face Horseshoe Falls on the Niagara River at Niagara Falls in Ontario, Canada. Nikon D700/28-300VR, 1/160s, f/16, ISO 200, EV -0.3, 28mm focal length.

If you are wondering why the water looks green, so did I.  I found the explanation on Wikipedia:

The verdant green colour of the water flowing over the Niagara Falls is a byproduct of the estimated 60 tonnes/minute of dissolved salts and “rock flour” (very finely ground rock) generated by the erosive force of the Niagara River itself. The current rate of erosion is approximately 1 foot (0.30 m) per year down from a historical average of 3 feet (0.91 m) per year. However, it is estimated that 50,000 years from now, even at this reduced rate of erosion, the remaining 20 miles (32 km) to Lake Erie will have been undermined and the falls will cease to exist.

That is an even more humbling thought.