Rivers are constantly changing. It’s up to you to be familiar with these changes.
In a river without obstructions, the slowest moving water is near the bottom and the fastest is near the surface.
Eddies are created behind an obstruction as water fills in the void created by the obstruction. The current behind an eddy is actually moving upstream. Skilled paddlers use eddies as a place to stop and rest.
Hydraulics occur as water flows over an obstruction and a slight depression forms behind it. Downstream water attempts to fill this void, creating an upstream flow toward the obstruction. A low-head dam is a perfect and deadly example of a hydraulic. Avoid hydraulics altogether.