Powerful Potomac River waters sometimes cover this area at Great Falls. High water occurs at least once a year when all the rocks in the falls are covered, and floods occur about every 10 years after extremely heavy rainfall or a series of storms upstream in the Potomac River watershed. When the deluge reaches Mather Gorge, where the riverbed narrows from nearly 1,000 feet into a rocky, mile-long funnel 60 to 100 feet wide, the waters have no place to rise but upward. As high waters recede, they deposit organic matter, minerals, and seeds that help make this one of the nation's most biologically diverse areas. Major floods included the highest recorded flood on March 18-19, 1936, after rapid melting of snow and ice and torrential rains; April 26-27, 1937, from heavy rainfall; October 15-17, 1942, after 10-19 inches of rain in parts of the watershed; June 24, 1972, during Hurricane Agnes; November 4-7, 1985, during Tropical Storm Juan; January 19-22, 1996, after a blizzard followed by warming temperatures and rain; and September 6-8, 1996, during Tropical Storm Fran.