Swirls and eddies develop. Effective at keeping particles in suspension and eroding the channel. Not uniform within the stream channel. Highest in the center just below the surface where friction is least. At bends, the highest velocity shifts to the outside of the bend - an important factor in stream channel development.
Velocity is the primary factor affecting the ability of a stream to erode.
Channel roughness - a smooth channel promotes more uniform and faster flow. When discharge increases, velocity and channel cross-sectional area both increase. Constantly decreasing gradient. Steep at the headwaters becoming gentle at the mouth. Channel width and depth, and stream velocity all increase downstream.