Which part of a river would have animals with very muscular bodies and adaptations that let survive in turbulent water? source zone transition zone floodplain zone

The part of a river that would have animals with muscular bodies and adaptations that let survive in turbulent water is in the transition zone, the mid-transition zone to be precise. Water at the source zone possesses a lot of potential energy and as it flows from the upper reaches the potential energy is turned into kinetic energy when the course of the river begins to gradually level out and this translates into increase in velocity. By the time river water reaches the middle of the transition zone, most of the potential energy would have been turned into kinetic energy and thus water velocity would be quite high here. Animals living here would develop muscles because of constantly fighting against the strong current to avoid being swept downstream.