If a deep river is faster why doesnt the larger wetted perimeter increase friction and slow down speed of river?

As a river becomes deeper, the ratio of the wetted perimeter to the cross-sectional area of the river decreases.

This means that the frictional resistance per unit area of the riverbed decreases as the river becomes deeper.

In other words, the deeper a river is, the less friction it experiences per unit area of its bed.

Additionally, deeper rivers tend to have smoother beds, which also reduces friction. So while the larger wetted perimeter of a deeper river does increase friction, the other factors mentioned above offset this effect and allow deeper rivers to flow faster.