In 1969 the American Falls at Niagara ran dry on purpose

In June 1969 a temporary dam of rock and earth went across the Niagara River above the American Falls. The water stopped. For the next five months the falls was a bare cliff, and the river went over the Horseshoe Falls next door instead.

The American Falls carries only about a tenth of the river. Over the past 150 years, pieces of its rock ledge have kept breaking off, and the biggest recent fall came in 1954, when Prospect Point, at the north end, collapsed. The rock piled up at the foot of the falls. Today the water drops 70 to 110 feet onto that pile. The full drop to the river is 188 feet.

The U.S. Army Corps of Engineers built the dam to study the rockfall and to work out how to keep the falls from turning into a series of rapids.

With the riverbed dry, the engineers went over it, found the faults in the rock and bolted them together. Left alone, those weak spots would have made the falls wear back faster.

Reports disagree on whether that summer brought more visitors or fewer. A dry Niagara was news, and people came to look. Others may have stayed away, believing that both falls had been turned off.

There was also a plan to clear away the rock pile. It was dropped because of the cost. In November 1969 the dam was blown up with dynamite, and by December the water was going over the American Falls again.

In the mid-1970s it was decided to make no changes to the rock wall at all, on the grounds that the trend was now to let nature take its course. The fallen rock is still heaped at the base of the falls.