Until your post (above) I always thought this was simply called the Great Falls (for which the city was named). The falls caused a significant portage for the explorers Lewis and Clark. Here's a page with a "wetter" view....
Well, thanks to CaLOldBlue, I am back to thinking that it was the other dam -- weird -- I know there was a park there (with some very scary warning signs) -- but I think Ryan Dam is more correct. Here's a page from the NPS that tends to support this notion....
Mark