Stability analysis of waterfall cliff face at Niagara Falls: An implication to erosional mechanism of waterfall

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Although recession of waterfalls or knickpoints in bedrock rivers is a common geomorphological process, detailed mechanics of waterfall recession has only been examined in a few cases. Caprock recession model at Niagara Falls, in which gravitational collapse of caprock induced by undercutting notch plays a significant role, has been one of the well-known models describing the waterfall erosion, but the validity of the model has hardly been examined in a quantitative context. Here we assess the stability of the cliff of waterfall face of Niagara Falls in terms of the strength of bedrock and the length of undercutting notch. The result of a cantilever model analysis shows that the caprock remains stable until the undercut reaches tens to over a hundred meters. However, the actual length of undercutting notch of waterfall face is up to 10 m, and such a long notch to cause gravitational collapse of the caprock can hardly be formed. The recession of the waterfall could therefore be caused by gradual detachment of the rock of the waterfall face induced by fluvial erosion of surface water flow, rather than by elongation of undercutting notch and episodic gravitational collapses of the caprock.