For hatchery brats...if the water is low and clear I believe they tend to linger in holding spots throughout the river that provide relative safety where as in medium to higher water I believe that they are racing for the hatchery. I move around more in low water. In higher water I find a good spot and wait there for 'em. With the later I usually have more company.
For nates I believe the fish also linger in holes in low water and fly in moderate to higher water. Just a matter of finding the resting spots or lucking into a moving fish.