Abstract

The recession of Niagara Falls up the thalweg of Niagara River will be marked by five major events. Event No. 1: The Horseshoe Falls will first lower the level of the Chippawa–Grass Island Pool below the bottom of the American Channel and unwater the American Falls. The level of Lake Erie may be lowered a few feet at this time. The falls will split into two Horseshoes at Navy Island, and the Lockport Gorge, the extension of the Upper Great Gorge in the Lockport Dolomite, will fork into two gorges; the right hand one in the Tonawanda Channel carrying 70 percent of the flow of the river and the left one in the Chippawa Channel carrying 30 percent. The rate of erosion of the Lockport Gorge will vary with the configuration of the Horseshoe, faster during notch shape and slower during arch shape, with shallow and deep plunge pools, respectively, as the Upper Great Gorge has eroded. Event No. 2: The Horseshoe in the Tonawanda Channel will split into two waterfalls at the upper end of Navy Island and for a short period there will be three Horseshoe waterfalls until the left one in the Chippawa Channel, due to its faster erosion in the softer rocks of the Salina Group, intercepts the flow on the right side of Navy Island. This event will leave the right Horseshoe as a dry falls. Event No. 3: The Chippawa Horseshoe will decline in height as the Lockport Dolomite dips upstream to the south until, at a height of ∼50 ft, it will no longer have the capability of recession and will remain as a quasi-stationary waterfall. Event No. 4: The Salina Group will erode to a broad, gentler sloped gorge in the bottom of which the steep-sided Lockport Gorge will be eroded as far upstream as the quasi-stationary waterfalls. The rapid erosion to form stepped rapids in the Salina Gorge will lead it upstream to intercept the water in the Tonawanada Channel and divert it into the Chippawa Channel. Event No. 5: This will result in the unwatering of the Tonawanda Channel and the creation of another dry falls. Near the head of the river beneath the Salina Group, the more resistant Bertie Dolomite and its underlying Onondaga Limestone will be encountered into which the Bertie-Onondaga Gorge will be cut with a much slower rate of recession by a new Horseshoe, the Final Falls. The Final Falls; will be much lower than the preceding Horseshoes and may be so slow in recession as to be a quasi-stationary falls, without immediate change in the level of Lake Erie.

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