In 1969, the US Army Corps of Engineers “de-watered” the American Falls portion of Niagara Falls to assess the effects of erosion on the Falls’ rock face, and created a temporary public park in the process:

For a portion of that period, while workers cleaned the former river-bottom of unwanted mosses and drilled test-cores in search of instabilities, a temporary walkway was installed a mere twenty feet from the edge of the dry falls, and tourists were able to explore this otherwise inaccessible and hostile landscape. A riverbed, in other words, became an ephemeral public park, though as by-product of a potentially colossal geo-re-engineering project. The authorities even installed temporary interpretative signage explaining the Falls’ geology to inquisitive visitors.

(Via Bobulate)