The current is the main flow of water in the river. It is possible to have multiple unique currents in the same section of the river. They are determined by what obstacles are in and around the river/riverbed, and each obstacle will have an effect on the current and main river flow.

If a river is wide and does not have many obstacles in it you may have difficulty detecting the direction of the current.

You can determine the direction of a current by throwing some leaves in the water and watching which way they go.