This can be defined mathematically:
Where is the length of each surface in contact with the aqueous body.
In open channel flow, the wetted perimeter is simply how much water is in contact with the river channel. The greater the wetted perimeter, the more friction there is between the water, bed and banks and, therefore, the slower the water flow. In a practical experiment, one is able to measure the wetted perimeter with a tape measure weighted down to the river bed to get the true measurement.