Rivers, of course, are moving water. You may, however, come across sections with almost canal like quality. These slow moving sections can often have a silt bottom as the slow water allows waterborne silt to fall to the stream bed.
You will often see fish rising in these sections as the slow current means easy pickings for the fish. Many beginners get drawn to these parts of the river, attracted by the rising fish only to be frustrated at the lack of success. Very slow moving sections of the river are some of the hardest sections of river for beginners to fish! There are a few reasons for this. Firstly, with a smooth, ripple-free surface the fish can see you much easier. With a lack of turbulence on the water, the fish can detect every clumsy cast and vibration in the pool and are easily spooked. They also have all the time in the world to inspect their food and in turn, your fly! This means they are likely to be more suspicious of an artificial fly.
Dry fly fishing is often the best way to approach these sections of the river but small flies, low diameter tippett and stealthy, accurate casting is often required.