Physical requirements for river trips are generally modest. Rivers are ranked according to the difficulty of their most severe rapids:

Class 1 Easy flat water/lake and mild waves-current/river. Few or no obstructions, all easy to avoid. Some maneuvering may be required to navigate around log jams or sweepers. Risk to swimmers is slight. Self-rescue is easy.
Class 2 Straightforward rapids with wide, clear channels that are obvious without scouting. Occasional maneuvering may be required, but rocks and medium-size waves are easily avoided with some training. Swimmers are seldon injured. Group rescue is seldom necessary.
Class 3 Moderate waves and narrower channels due to obstructions such as rocks or gravel bars. Quick and accurate river reading is required with instinctive maneuvers orchestrated with a paddling partner.
Class 4 Expert whitewater canoeist. Difficult, with narrow channels obstructed by rocks, steep and narrow drops.