There's a lot that goes into river velocities. What slows rivers down is primarily friction, so a wide, shallow channel will move slower because there's a lot of drag along the bottom and banks. Having debris, like plants, rocks, or boulders also creates more drag to slow the water down.

A deep river will move faster, because the depth makes a 'fast lane' for water to slip over/between the slower water along the bottom and banks.

Gravity, I reckon