Why is wetted perimeter the reason velocity increases?

the wetted perimeter is the amount of water in contact with the river's bed and banks. if the wetted perimeter is large, the velocity would decrease because alot of water is in contact with the bed and banks to over come friction ( disruption of water flow) hence velocity decreases . if the wetted perimeter is small, then less water is in contact with the bed and banks so less friction and so the velocity would be high (nothing to prevent the flow of water)