YELLOWSTONE NATIONAL PARK — In the most recent Caldera Chronicles by the Yellowstone Volcano Observatory (YVO), U.S. Geological Survey Research Hydrologist Shaul Hurwitz uncovered how waterfalls are formed and why there are so many in the Yellowstone region.
The YVO described a waterfall as “a very steep commonly vertical fall of some magnitude in a river course.”
Hurwitz wrote that the most common model to explain waterfall formation suggests that they form where rocks of different hardness meet laterally or vertically.
“If a stream flows over harder rocks that are more resistant to erosion than the rocks immediately downstream, a ledge or bench will form across the streambed because the softer and less resistant rocks are worn away faster,” Hurwitz wrote. “As the ledge becomes higher, the softer downstream rocks will erode faster. This undercutting of the less-resistant rock causes the overhanging rock to shear off, and typically a plunge pool at the base of a waterfall is created where the water impacts.”
According to Hurwitz, the rivers flowing in Yellowstone National Park (YNP) are fed by large volumes of water from snow and rain falling over the Yellowstone Plateau. Some waterfalls in YNP formed where rocks with differences in hardness meet in these deep gorges, while others formed at the edges of thick rhyolite (a silica-rich volcanic rock) lava flows.
Hurwitz confirmed that there are approximately 350 waterfalls of a height of 15 feet or more in the park. Many of these can be viewed by hiking a short distance, for example, Gibbon Falls near Madison Junction, Tower Falls near Tower Junction, Mystic Falls near Biscuit Basin and Fairy Falls near Grand Prismatic Spring. A large number of waterfalls are also in Yellowstone’s southwest corner, which is unofficially named the “Cascade Corner.”
Hurwitz wrote that the park’s two most prominent waterfalls are in the Grand Canyon of the Yellowstone — the Upper (33 meters, or 109 feet) and the Lower Falls (94 meters, or 308 feet) — which exist because rock layers change laterally from soft to hard.
“The Lower Falls is the tallest waterfall in the park and is significantly taller that the total height of Niagara Falls (51 meters, or 167 feet for the Canadian and American Falls),” Hurwitze wrote. “The Upper and Lower Falls can be viewed from several locations along the rim of the Grand Canyon of the Yellowstone.”
According to the YVO, at the end of the last glacial period, about 14,000 years ago, ice dams at the mouth of Yellowstone Lake failed, and the large volumes of water that were released caused massive floods downstream. These floods led to erosion of the present-day canyon, which is a classic, narrow, V-shaped valley, indicative of erosion by rivers rather than by glaciation. The thick rhyolite flows at the base of the canyon were hydrothermally altered and weakened by hot groundwater. After the canyon formed, the weakened rhyolite was less resistant to flow of the Yellowstone River downstream from the falls.
