Fish are most likely to be just about where you expect them.
They will hold in places where water currents bring them a steady supply of food.
One place to look for is the area where one water current merges with another of a different speed, color or turbidity or cloudiness.
Also you may find fish where the water is cooler or warmer than the main water mass.
Fish also move around between different seasons.
In early spring, the darker bottom shallows may hold more fish than the colder open water.
In the summer, fish may congregate just downstream from the confluence of a small, cold water brook.
Look for spring seeps, but be aware: fish in this shallow, clear water are going to be extremely skittish.
They need to be in order to survive.
Another area to look for fish is the place where the current might cause food to collect.
Lines of foam and bubbles on the water's surface will reveal the nuances of tricky currents and also carry insects in their flow.
Fish sit beneath foam lines and foam piles and eat the insects as they float by.
Foam lines are an excellence place to cast a fly.
Fish also like overhead protection, so look for them under trees, cut banks and other places most people can't or won't cast to.
The downstream of a riffle, where the water deepens and flattens out, is also worthy to check out.
Usually these areas are well oxygenated, and fish there get first dibs on any food coming off the riffle.
The tail of a pool is another place to find fish.
Check out the water just above where the water increases speed and perhaps starts another riffle.
Always thoroughly check out any location where the channel moves from one side of the river to the other.
Also look for pocket water and quieter water; in these spots where a fish might hold on the downstream sides of rocks or logs.