The takeout for Little Arsenic Trail is notoriously ambiguous. The whitewater will obviously mellow out, look for the trail on river left, takeout about a 1/4 mile after when the trail climbs up and on top of a bench and is no longer next to the river. I look for a large dead tree with the top broken off, it's probably about 40' tall, it has been dead for a while and has no bark. The hike to the rim is 750 foot up in .8 miles and will take about 45 minutes. Your only other option is to paddle 9 miles of Class I. The pain is unavoidable.