Water in rivers exerts a very powerful force against any fixed object. This force once in place remains constant unlike the ocean where the force is released with each tidal surge.
Even calm looking rivers are very powerful, do not be deceived!
The downstream flow of water is called Laminar Flow.
Water flows in a straight line, changing direction at points of high resistance. The line of greatest depth and strongest laminar flow will be on the outside of the corners. The water on the inside of the bend is slower moving.
Within New Zealand there is a recognized grading system for rivers. This classification is based upon difficulty, loss of height, amount and speed of water, and the number of hazards. Grade 1 is the easiest, Grade 6 is the hardest.