The Amana Fish Weir site in Iowa County is well-documented site that is now "lost"... It's definitely not lost in the sense that it has been destroyed like so many other archaeological sites, but instead it is now completely buried by the shifting Iowa River! Due to drifting and flooding, the river formed a new channel and began to silt in this V-shaped weir. It's estimated that it is now covered by at least five feed of silt!
This fish weir was first documented in 1844 on a General Land Office map, but the exact age of the weir is unknown.
This image is from a 1969 postcard, copyrighted by Penfield Press in Iowa City.
Bill Whittaker wrote about this phenomenon in a 2013 issue of the Newsletter of the Iowa Archeological Society, and you can also read about the site in the The Archaeological Guide to Iowa.