Changing the Shape of the Falls

Changing the Shape of the Falls. . When Europeans first saw the falls, the crest was well below Hennepin Island. Natural erosion caused the line of the falls to move steadily upriver at about four feet a year. By the 1850s, the cataract was approaching the upper limit of the limestone ledge that sustained it. In the course of time, without human intervention, the falls would soon have become a rapids. , The pace of erosion increased after lumbering and milling began. Logs floating downriver crashed against the limestone and broke off great chunks. Excavating for dams and tailraces ate away at the stone, and a disastrous tunnel project nearly destroyed the falls in the 1860s. To prevent further damage, the US Army Corps of Engineers built a concrete dike under the river and placed a wooden apron over the ledge, protecting and hiding the face of the falls. The apron was later replaced with a concrete spillway which is still in place. , In the 1930s Congress authorized a massive project to improve navigation on the Upper Mississippi. Completion of the Upper Lock at St. Anthonay Falls in 1963 allowed shipping to use the river above Minneapolis. Construction of the Upper Lock altered the entire west side of the falls, eliminating Upton Island and the mill pond and cutting off access to waterpower. Two sections of the Stone Arch Bridge were replaced by a steel truss. A rocky islet known as Spirit Island was also destroyed. This landmark was the nesting ground of eagles that fed on fish below the falls and was significant in Dakota traditions. What remains of Spirit Island lies beneath the breakwater leading into the lock. , marker photo captions: , This wooden apron, finished in 1880, concluded a million-dollar government project to save the waterpower so vital to Minneapolis business. A concrete spillway later replaced the wooden apron. , Huge blocks of limestone, broken from the face of the falls, can be seen in this photo taken in 1850. , This is a diagram of the rock formations beneath the falls. Water wore away the soft sandstone beneath the limestone, and the limestone ledge periodically broke off. This continuous process of erosion caused the falls to move upriver over many years. , Spirit Island, in the foreground, as it looked in 1855. Several sawmills can be seen behind Spirit Island, built out on platforms over the falls. , Construction of the Upper Lock went forward in the late 1950s.