An estimated 60 tons of dissolved minerals are swept over Niagara Falls every minute. The colour comes from the dissolved salts and “rock flour,” very finely ground rock, picked up primarily from the limestone bed but probably also from the shales and sandstones under the limestone cap at the falls.

The striking green color of the water at Niagara Falls is a natural result of the local geology and the sheer power of the falls. As the massive volume of water flows over the precipice, it erodes the soft shale and limestone layers beneath the harder Lockport Dolostone. This erosion creates "rock flour," which is extremely fine-grained particles of dissolved minerals and salts. When sunlight hits the water, these suspended minerals reflect the green part of the light spectrum back to our eyes. Scientists estimate that approximately 60 tons of dissolved minerals are swept over the falls every single minute. The intensity of the green can shift based on the time of day, cloud cover, and the amount of sediment being stirred up by the current. It is essentially a giant, natural light show powered by the pulverization of ancient rock that has been occurring since the falls were formed over 12,000 years ago at the end of the last Ice Age.