The falls could be replaced by a series of rapids. 50,000 years from now, at the present rate of erosion, the remaining 20 miles to Lake Erie will have been undermined. There won't be a falls anymore, but there will still be a river at work.

Niagara Falls is not expected to disappear for at least another 50,000 years. The falls are currently "retreating" or migrating upstream toward Lake Erie due to the erosion of the soft shale and sandstone beneath the hard limestone caprock. Historically, the falls moved at a rate of about 3 feet per year, but modern engineering and hydroelectric water diversion have slowed this to roughly 1 foot (30 cm) per year. Scientists estimate that in about 15,000 years, the falls will reach a different rock layer that may change their shape significantly. By the time the river erodes all the way back to Lake Erie in approximately 50 millennia, the falls will effectively cease to exist as the lake begins to drain into a series of rapids, fundamentally altering the Great Lakes landscape as we know it today.