The Falls Evolve

The river here flows over land that was once a seabed laid down 500 million years ago, with hard Platteville limestone above a narrow band of Glenwood shale and a thick bed of soft, easily eroded Saint Peter Sandstone, while soil left much later by glaciers covered the top layer. Although the area was ice-free by at least 14,000 years ago, runoff from receding glaciers etched the future course of the Mississippi, and a massive cataract in present-day Saint Paul advanced upstream as its weight fractured the limestone and eroded the underlying sandstone. For 10,000 years, the waterfall moved about 4 to 5 feet annually, but as the limestone layer thinned and industry hastened its destruction, the annual change accelerated to as much as 35 feet in the 1850s and 100 feet in the 1860s, bringing the falls close to extinction as deterioration neared the edge of the limestone layer above Hennepin Island. Accounts by visitors over time reflected both substantial changes in the falls and exaggeration about their height and force. Industrial works also reshaped and damaged the falls: by 1858 a horseshoe dam directed the river's flow to the mills, and during floods excess water, logs, and ice battered the fragile limestone ledge, so that by 1870 the crest had moved 300 feet upstream. Waterpower companies tried to protect the falls with a timber apron in 1866 and again in 1869, but floods destroyed both efforts, and after the Eastman tunnel collapsed later in 1869, the U.S. Army Corps of Engineers led stabilization work from 1870 to 1884 until Congress stopped funding it. The apron was rebuilt again in the 1890s, patched with timber and concrete until a record-breaking flood in 1952, and replaced by the existing sand-filled, reinforced-concrete apron completed by 1955.

Minneapolis, Minnesota · United States