Basic parts of a river

Riffles...
Webster defines a riffle as a shallow extending across a streambed and causing broken water. That’s the first and best definition I could find for riffles in the average small river. Most definitions went off into terms like helicoidal flow and the Hjulström curve. All of which were really interesting to read about but left me knowing squat about the actual riffles. So anyways that "shallow extending across a streambed" in the case of our average river riffles are caused by an increase in a stream bed's slope or an obstruction. The stream is well oxygenated as the water flows over shallow water of 3 feet or less with a bottom of hand-sized rocks and gravel. The riffle will have a layering of material starting with the largest rocks on the top, followed by smaller stones, then gravel, pebbles, sand, and silt. Tucked into all the nooks and crannies will be debris such as fallen leaves and twigs. Riffles are by their definition shallow which allows good photosynthesis for plant growth. Many different kinds of bugs and insects live within riffles due to the healthy plant growth. In turn all kinds of small fish species such as darters and madtoms live in riffles feeding on these insects. The small rocks and stones also make great homes for crayfish. All of this makes riffles food factories for gamefish like smallmouth bass and white bass. Most riffles have a top end that includes small eddies at each corner, forming a triangle. This water is usually about 1or 2 feet deep. This front section of the riffle can be the best area to fish. As the water speeds up heading into the riffle it is smooth but flowing faster, this is the glide and can be a hotspot for smallies especially early and late in the day.
Runs are usually deeper than riffles, averaging something like 4 to 6 feet. Smallmouth will often use the run as holding as well as feeding water, making trips up into the riffle to feed as well as eating things washed down from above. Runs are just about my favorite feature in rivers to fish, especially when they are associated with another feature such as an island or sharp bend in the river. Runs in a bend with often cut deeply into the outside bend while piling up a rock and gravel bar on the inside of the bend that is perfect to fish from. One of the best ways to fish this is to cast a jig or grub upstream and across and let it sweep down and past you while keeping a tight line.
Pools are the deepest holes in the river. Pools with structure such as big rocks or logjams will hold many more fish than bare pools. Pools provide a haven for fish during the day and a safe winter home. Deep runs in bends can also be classified as a bend pool if the bend is dug out deep enough. These are often some of the best parts of the river to fish. If there is a lot of cover like wood in the water these are also shovelhead magnets, you can bet the gravel bar on the inside bend will have bass and catfish roaming it at night. Tails are at the ends of pools where the water rises up from the depth of the pool and gets channeled into a riffle or run. Usually there are large rocks within a tail that provides protection and current breaks for the bass. Having the deeper pool nearby is also some comfort to the trout as a means of escape. It seems to me that if bass are in the tail they are there to feed.
Flats are really long slow shallow pools. The bottom will consist of sand and gravel but mostly silt and muck. I find these on the whole to be the least productive part of the river. With the muckier the bottom the less productive a flat is. If there is lots of cover you might some bass feeding but I generally bypass long silt filled flats for the most part. A series of long flats really does concentrate the feeding fish at nearby riffles and runs though. Some flats will have weed beds extending into the water that will have some bass patrolling the edges.
The best tool modern anglers have when exploring the river are the new satellite mapping sites on the internet. Find your access point to the river and zoom in and follow it up and down till you find some riffles. Are their bends nearby? Maybe an island or the mouth of a tributary? If you take the time to find places where several features are located close to each other you are almost guaranteed to catch some fish. Locate such a spot that's a bit off the beaten path that you have to hike or float a ways to get to and you might just make a killing. There are lots of such spots along the rivers hundred mile length but the guys that know them aint talking.