Between the year 1931 and 1954 the rocks at the American Niagara Falls were eroding so badly that the Falls were in danger of extinction. In 1969 the United States Army Corps of Engineers undertook the job of preparing the Falls for maintenance.  A cofferdam (a watertight enclosure pumped dry) was constructed and after working almost around the clock for three days the flow of water  (60,000 gallons per second) was cut back by 75%.