Here is a day-trip that I did with my daughter last November (2016) on the Missouri River 20 miles north of Helena, MT - https://bigskywalker.com/2016/11/23/gates-of-the-mountains/

Great trip and photos. I’d like to hike up to that arch you went to. I’ve paddled below it many times but never thought about going up to it. I was down there in October last, it was really nice. If you paddle the Missouri river a lot you should join the "Missouri River Paddlers’ group on Facebook. 1800 members all along the river all the way to st. louis and beyond. Many of the group are river angels who have helped out many paddlers as they go the 2400 miles with either portages at the dams, resupply, food, laundry, information or a cold beer. There are only about 12 members in Montana, the rest elsewhere.

Thanks. I will check the FB page.