I can't answer your rule of thumb question Mary but let me offer this on your current question. When a fluid flows over a surface it is slowed by friction. The slowing effect is greatest immediately next to the surface and becomes less as you move away from the surface. This is why your sail experiences stronger wind at the top than the bottom. It is also why the current is slower near the sides of the channel than the middle. The flow in the middle of the channel will be faster with increasing distance from the bottom.