Hennepin Island tunnel was a 2,500-foot (760 m)-long underground passage in Saint Anthony, Minnesota (now Minneapolis) dug beneath the Mississippi River riverbed between 1868 and 1869 to create a downstream spillway for hydro plants, milling and lumber business located upstream of St. Anthony Falls. The tunnel ran downstream from Nicollet Island, beneath Hennepin Island, and exited below St. Anthony Falls.
During construction of the tunnel, on October 5, 1869, the river broke through the thin layer of limestone separating the river's bed from the tunnel. The rushing river scoured the tunnel, caving in parts of Hennepin Island and causing the earth supporting St. Anthony Falls to collapse upstream. There was serious concern that the riverbed would crumble and reduce St. Anthony Falls to a long set of rapids. However, within a few weeks, dams were built to divert the river and stop St. Anthony Falls from being washed away. The fix was temporary as the 1870 spring floods damaged some of the new dams and swept away more of Hennepin Island. By the fall of 1870, the riverbed and banks were stabilized and a wooden apron capped St. Anthony Falls to stop the upstream progression of the collapsing falls. As a direct result of the collapse of the Hennepin Island tunnel, St. Anthony Falls ultimately received a sloping concrete apron to create an artificial falls that are held out as the only major waterfall on the Mississippi River.
The site of the tunnel is about a mile upstream from the 2007 collapse of the I-35W Mississippi River bridge.
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