Answer: Niagara Falls

The general rule of natural wonder stewardship is to leave nature alone: we set aside nature preserves, fence off geysers, and stop people from altering the land. In the case of Niagara Falls, however, a little intervention is required to keep all that rushing water, the very thing that makes Niagara Falls so incredible, from destroying the falls.

The combined falls (Horseshoe, American, and Bridal Veil), located on the Niagara River (which drains Lake Erie into Lake Ontario), form the highest flow rate of any waterfall in the world. During peak flow, 225,000 cubic feet of water can cascade over the falls each second (although the yearly average is 85,000 cubic feet per second). Historically the high rate of water flow lead to an erosion of approximately three feet per year. Engineering projects intended to both harness the huge amount of water for hydroelectric projects and to preserve the falls have reduced the yearly erosion rate to only one foot per year.

Still, even with human intervention the current rate of erosion means that 50,000 years from now the erosion will have completely worn the rock back all 20 miles to Lake Erie and the falls will cease to exist (replaced instead with a canyon carved down through the surrounding rock).