Stuart

I think that your question may be too big to answer simply. My experience is from aerodynamics but air and water behave in much the same way provided the air is subsonic. Additionally, it is very rusty because it has been more than a few years since I have sat in a classroom

With the bottom and obstructions such as rocks, close to the obstruction you will get some skin friction that will slow the water down, further away the water will try to travel faster until you get a shearing effect between the layers and behind the obstruction you will get eddy currents which will cause significant drag effects on the water flow. The difference between these 2 points in often termed near field and far field effects. Where this point occurs will be determined by water velocity, pressure, temperature and density, with the last 3 being particularly related. The equation of state and bernoullis, for the nerds is below.

Equation of State

Bernoulli's Equation

Localised flow variations of the water, even in stillwater can occur due to the heating effects of the sun or perhaps spring water injection. On a river the effects are much more dynamic. Modelling these flows over any area takes a computer the size of a house and a brain the size of a planet, both of which are beyond me.

Hope this helps

regards

Vince