How to pronounce “ontonagon river” in English

People have known about that for years and in fact when Houghton had gone to the Schoolcraft expedition they had visited a boulder that's called the Ontonagon Boulder. The OntonagonBoulder was this large piece of copper out in the woods next to the Ontonagon River.And every single person that went to the Upper Peninsula back then who's an explorer would visit this boulder and it was a rock that was so big that grown men couldn't move it.