The Harps used a cave to attack flatboats much like in the movie. The movie does indicate that the crew of a flatboat was killed, but I did not think this was real. Anyway, the reality was way brutal.
Yes it is in part a true story. The pirates did use this cave to attack the keel boats coming down the Ohio river. The cave was also used for a time as a trading post. This cave is very real, located in the Cave-In-Rock State Park, at Cave-In-Rock Illinois. On the banks of the Ohio River. This park is visited by 1,000s every year. The movie was shot for the most part on location. The Keel boats used in the movie was in fact built by the residents of Cave-In-Rock.All of the Indians in the movie plus a few others were all local folks. I know this all to be true because I am from there and I also was a extra ( Indian) in the film. Thought you migh like to know.
Just out of curiosity: Was the story of the Harpes also the inspiration for the HOW THE WEST WAS WON segment where Jimmy Stewart tangles with Walter Brennan and his band of cutthroats? It never occurred to me until I read these postings.