Montana is famous for its great fishing streams and many undammed rivers. They attract newcomers to live on their banks, but then, as rivers do, comes a flood. Riprap often follows to the great detriment of the stream.
Here is a long feature article on the practice. It is especially a problem on the Yellowstone River, the longest undammed river in the West.
Wild Rivers and Riprap: The Case of the Yellowstone. By Hal Herring. New West.