As the Yellowstone River runs north, leaving Hayden Valley, it passes over two falls. The first is the Upper Falls, which is 109 feet high. A quarter of a mile later it plunges over the spectacular lower falls, pictured above. At the Lower Falls the river falls 308 feet from the hard ryholite canyon walls, which were formed by volcanic activity about 600,000 years ago, down into the Grand Canyon of Yellowstone. Up to approximately 63,000 gallons of water per second goes over the falls during the spring run off season making it the largest waterfall, in volume, in the Rocky Mountains of the United States.