Hennepin Island tunnel was a 2,500-foot-long (760 m) 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. 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 late 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 site of the I-35W Mississippi River bridge that collapsed in 2007.
Hennepin Island was named after explorer, Catholic priest, and Franciscan missionary Louis Hennepin (May 12, 1626 – c. 1705). In 1865, entrepreneurs William W. Eastman and John L. Merriam had bought its sister island, Nicollet Island, with the idea of creating a tunnel from Nicollet Island under St. Anthony Falls to provide for-profit spillway services to the many milling and lumber business located upstream of St. Anthony Falls. The tunnel was to be part of a system of waterworks that supported the industries driving Minneapolis' growth. The project was designed to create more industry on Nicollet Island through the waterpower.
St. Anthony Falls are made up of a hard limestone cap over soft sandstone. 10,000 years ago the falls were located near present-day Fort Snelling, Minnesota, at the mouth of the glacial River Warren. In the succeeding millennia, the river had been washing away the sandstone and undermining the limestone lip of St. Anthony Falls, causing the falls to slowly retreat upriver near its present location.
In 1868, workers began to dig a 2,500-foot-long (760 m) tunnel beneath the riverbed from Nicollet Island downstream under Hennepin Island, exiting below the falls. For the next year, workers dug in the soft sandstone beneath the thin layer of limestone that forms the river's bed. By October 1869, water began seeping into the tunnel from above.
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 immediate, serious concern that the riverbed would crumble and reduce St. Anthony Falls to a long set of rapids. One witness remembered,
Proprietors of stores hastened to the falls, taking their clerks with them; bakers deserted their ovens, lumbermen were ordered from the mills, barbers left their customers unshorn; mechanics dropped their tools; lawyers shut up their books or stopped pleading in the courts; physicians abandoned their offices. Through the streets, hurrying hundreds were seen on their way to the falls.
Work started immediately to plug the tunnel and hundreds of volunteers used timbers and stones. The river easily washed these out of the tunnel. Within a few weeks, the plug held and dams were built to divert the river and stop St. Anthony Falls from being washed away. The fix was temporary and the 1870 spring floods tore up some of the new dams and swept away more of Hennepin Island. In addition, the earth support below the Summit flour mill, Moulton's planing mill and a wheat storehouse were undermined and each of these was tipped into the river.
By late 1870, the riverbed and banks were stabilized and a wooden apron capped St. Anthony Falls to stop the upstream progression of the collapsing falls. The apron buried the falls' jagged rocks and large waterfall cataract and tamed the falls' tumbling water.
The tremendous damage caused by the Hennepin Island tunnel brought lawsuits and demands for political change. A central issue at that time was whether local taxpayers should pay for repairs that would enrich riverfront industrialists. The tunnel collapse was one of the factors leading to the merger of St. Anthony with Minneapolis in 1872. To the relief of the local population, the United States Army Corps of Engineers made the repairs using federal money on the pretext that it was protecting navigation. Seven years after the tunnel collapse, by 1876, the falls were stabilized with an underground dike and low dams that largely are still in place upstream of Stone Arch Bridge. By 1880, the Army Corps of Engineers had covered the face with a sloping concrete apron, creating the artificial falls. The federal government spent $615,000 on this effort, while the two cities (St. Anthony and Minneapolis) spent $334,500.
Meeker Island Lock and Dam (demolished)