The converging currents meet and often create some kind of turbulence. Wider rivers often have slower, calmer currents. The gradient is the steepness of the riverbed. A rapid generally has a steep slope, fast-moving current and shallow water. A pool generally has a less steep slope, slower-moving current and deeper water.
Water moves slower near the bottom of the riverbed because of friction.
The faster water is near the surface. This is called laminar flow. The edge of the river often shows a movement called helical flow from the friction of the banks. This contact with the banks produces a spiral current. The slower water around the banks is drawn into the faster surface water toward the middle of the river.