In 1969 they blocked off the water above the American Falls to do a geological study to see if they could (or should!) remove the boulders at the bottom, which were a result of many large rock slides (the last of which happened in 1954). After a 6-7 month period of time they determined that the rocks needed to stay there otherwise if they were removed it could cause the falls to actually collapse and turn into rapids. Not to mention the fact that it would be a huge undertaking, especially in 1969! They found a couple of bodies and a lot of coins down there when the water was not there. The Horseshoe Falls were still flowing during this time period