The 308-foot Lower Falls at Yellowstone National Park may have formed because the river flows over volcanic rock more resistant to erosion than the downstream rocks, which are hydrothermally altered. The 109-foot Upper Falls flows over similar rocks.
Past and current hydrothermal activity at Yellowstone National Park altered and weakened the rhyolite, making the rocks softer. The Yellowstone River eroded these weakened rocks to deepen and widen the canyon, a process that continues today. The current canyon begins at Lower Falls and ends downstream from Tower Fall. Here is a view of the Lower Falls.