The rate of regression of the Horseshoe Falls of Niagara River in the Upper Great Gorge, the upstream portion of the Niagara Canyon, appears to have been quite variable. Historically, comparison of the surveys from 1842 to 1966 shows recession rates that are faster when the Horseshoe has a well-defined horizontal notch in the crest and slower when the crest is in the form of a horizontal arch. Examination of waterfalls in central New York State shows that plunge pools are generally deeper where the horizontal configuration of the crest of the waterfall is an arch. The horizontal arch is thus shown to be the more stable configuration. Soundings by the Canadian Government in the Maid of the Mist Pool in the Upper Great Gorge reveal the river bottom to be a series of basins—plunge pools—which are progressively lower in elevation in an upstream direction. The basins are separated by highs which likewise decrease in elevation upstream. The basins are interpreted as places where the Falls stood for a long time with a horizontal arch configuration in which the stresses were relatively low. The highs are interpreted as places where horizontal notches developed in the crest with resulting relatively higher stresses and faster recession. Photoelastic comparison of the notched crest of 1819 with the arched crest of 1966 indicates that the stresses in the notched crest are about three times those in the arched crest. The soundings reveal a narrow trough in the river bottom opposite the Prospect Point side of the American Falls. This trough is interpreted as a relict notched crest, during the formation of which the canyon wall near Prospect Point was subjected to much greater stress than during the formation of the arched crest of the Horseshoe Falls near the Goat Island side of the American Falls. The much greater amount of fracturing of the rock in the American Falls near Prospect Point than near Goat Island substantiates this interpretation.

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