One possible place along the river with a slow water flow could be a meandering bend where the river widens and the current becomes gentler. Another option could be a backwater area or a shallow pool where the water flow is minimal due to obstructions like rocks or vegetation. On the other hand, a location near a fast water flow area could be at the confluence of two rivers where the merging currents create a rapid or at a waterfall where the water rushes down with force.
