I"ve been running my flatbottom boats and originally a fairly narrow 12' v-hull on the Fraser for over a decade with a 9.9 mariner. Used the 12' first for a few years , then the 11' 100 lb. flyfisher, and for the last 8 years a 10' welded flatbottom that probably weighs in at 135+ lbs.. The area of the river I primarily fish at is between Herrling Is. and Hunter Creek.
I've always felt confident running the river but I learned how to read water early on with my dad and cousin on the Skeena. Don't think I've hit the prop since my very first year on the river while trying to navigate a very shallow channel between Herrling Island and the freeway side of the mainland.
Personally I wouldn't want anything less than a 9.9 just for the sake of being able to move quickly up the river. Even with my 10'er it would be painfully slow with anything less!