Editing Saint Anthony Falls (section)

===1869 collapse of the Eastman tunnel===
{{main|Eastman tunnel}}
[[Image:Pillsbury and Phoenix mills.jpg|thumb|left|The falls in the early 20th century]]
The early dams built to harness the waterpower exposed the limestone to freezing and thawing forces, narrowed the channel, and increased damage from floods. A report in 1868 found that only {{Convert|1100|ft}} of the limestone remained upstream, and if it were eroded away, the falls would turn into a rapids that would no longer be useful for waterpower.{{sfn|Kane|1987|p=}} Meanwhile, as the result of a lawsuit, the St. Anthony Falls Water Power Company approved a plan for the firm of [[William W. Eastman]] and [[John L. Merriam]] to build a tailrace tunnel from below the falls, under Hennepin Island, under the riverbed to Nicollet Island. Nicollet Island is {{convert|700|ft|m}} or more above the falls. The tailrace tunnel would allow waterpower development at the Island. This plan met with disaster on October 5, 1869, when the limestone cap was breached.<ref name="corp" />{{sfn|Kane|1987|pp=63–79}}

The leak turned into a torrent of water coming out the tunnel. The water blasted Hennepin Island, causing a {{convert|150|ft|m|adj=on}} section to collapse into the tunnel. Believing that the mills and all the other industries around the falls would be ruined, hundreds of people rushed to view the impending disaster. Groups of volunteers started shoring up the gap by throwing trees and timber into the river, but that was ineffective. They then built a huge raft of timbers from the milling operations on Nicollet Island. This worked briefly, but also proved ineffective. A number of workers worked for months to build a dam that would funnel water away from the tunnel. The next year, an engineer from [[Lowell, Massachusetts]], recommended completing a wooden apron, sealing the tunnel, and building low dams above the falls to avoid exposing the limestone to the weather.{{sfn|Kane|1987|p=63-79}}

The fix for the tunnel disaster was a concrete dike constructed by the [[United States Army Corps of Engineers]]. The dike was just above the falls and Hennepin Island, from right under the limestone cap down as much as {{convert|40|ft|m}}, and {{convert|1,850|ft|m}} long across the entire river channel. The dike, completed 1876, cut off the tunnel and any possible future bypass channels. A separate problem was damage to the falls and its upstream progression. To stop the damage the Corps built a protective wood timber apron completed 1880. The Corps also built two low dams, completed by 1880, on top of the limestone cap to keep the cap wet.<ref name="corp" /> The federal government spent $615,000 on this effort, while the two cities spent $334,500.<ref name="Pennefeather">{{cite book|title=Mill City: A Visual History of the Minneapolis Mill District|last=Pennefeather|first=Shannon M.|year=2003|publisher=Minnesota Historical Society|location=St. Paul, Minnesota}}</ref><ref name="corp" />
{{see also|Mills District, Minneapolis|Pillsbury "A" Mill|Washburn "A" Mill|Mill Ruins Park|Northwestern Consolidated Milling Company|North Star Woolen Mill|History of Minneapolis, Minnesota}}