So in my fantasy world I have a river that is in the shape of a perfect circle if viewed from above. Lets say the diameter of the interior is about 10 km, and the width would be wide at about 200 meters. So a small city could exist in and around the river. The river is magically powered such that it has a constant current that flows through it at a consistent strength. It also magically feeds itself, such that water taken from the river would be normal water, but the river would never run dry.
The river can be utilized for many things, such as powering machines with waterwheels. But that can be done on any river.
I am looking for ingenious, special uses for a river that feeds back into itself. For instance, in this river you could drop something into it, and it would eventually come back to you - something that is impossible in a normal river.
Since this is a fantasy world with magical elements, some magic exists, but it's not a heavy magical world, as such this river is a very special location and I want it to have a very special touch.
The societies in the world may have basic industrial elements like gears, waterwheels, mills, etc. But not steam-power or electricity. How could this river be utilized in a clever and special way that a normal river can not be utilized in?