|20150720 3491, replacement dam and bypass from IL-47 bridge|
|Photo from DNR|
A video in DamSafety explains a roller better than I can. It then explains how people can be rescued from a roller. The erosion of the riverbed downstream of the Glen D. Palmer Dam was so bad that it was replaced in 1960-61. But they still did not pay attention to dissipating the energy of the water so it also suffered from riverbed erosion and produced a roller. By 1978 there had been enough drownings that they placed riprap in the scour hole to stop the roller. But by 1991 the riprap had been scoured out and 26 more people had lost their lives at the dam.
On the south side of the river, a 11,000 foot long bypass chute was designed for whitewater sport. A Denil fish ladder was constructed on the north side. (DNR)
Below are upstream and downstream views from that bridge. You can see how they added rocks to create whitewater action. And you can see the IL-47 bridge in the background of the downstream view.