In general, when running in a straight line the current is strongest in the middle and weaker near the banks.
The friction from the banks slows the current down and causes it to spiral. The surface flow can push a swimmer away from the bank therefore never assume that a swimmer is safe until they are on the bank. However it is get ashore against a helical current than when the main current sets into the bank.
In some man made structures with smooth sides the main current may flow right up to the sides.