What Are Those Big Concrete Pyramids Down by the River?

Before Keyhole Dam was built in 1952, the old bridge built in 1928 across the Belle Fourche River was frequently threatened by flooding during high water, as the river flooded every spring and often changed course dramatically. In 1929-1930, a new channel was cut for the river and 39 massive concrete pyramids, or tetrahedrons, were installed to control the flooding river. In 1933, 33 more tetrahedrons were added. During floods, they helped prevent erosion and kept the river from changing its path by supporting the banks. The new bridge and dam later made them unnecessary, leaving them as a curiosity from an earlier time.

Hulett, Wyoming · United States