The Riffle
The area of the stream or river that has an extreme bottom gradient is called the Riffle. The water flowing through this area is generally faster flowing because of the degree of gradient and at times can be very rough. The waters depth here is also somewhat shallower than the other areas of the flowage. This current speed is due to the fact that the water can't compress the rough substructure of heavier gravel, rocks and other objects present on the riverbed. These Riffled areas are, noted by their irregular, rough, broken and much faster surface flows.
The rocks, boulders, heavy aggregate and other structures that protrude out of or into the water's surface, as well as, those situated just below its surface cause this effect.
The Riffled area of a stream or river is, natures way of cleaning house. This area of real rough and turbulent water allows gases and other microscopic materials to be thrust to the surface where the gases will be released into the air and the microscopic materials will be forced downstream and deposited along the banking. This same action of getting rid of unwanted gases and materials, also allows the blending of much needed oxygen back into the water system. This activity encourages the growth and establishment of the subsurface environment that allows new, fresh and healthy aquatic vegetation and habitat that will foster microorganisms. These same organisms will encourage the presence and growth of various aquatic insects, crustaceans, and other life forms that ultimately provide 80% of a fishes diet.
The Riffled area of a stream or river is one of the most productive areas to fish. Trout and Salmon will actively cruise these areas looking for clinging or crawling nymphs. Just down-stream from the heavy turbulence, the water velocity starts to slow and deepen. Fish will hold in these less turbulent waters to feed on the dislodged aquatic life forms carried along by the currents. The stronger and more aggressive fish will actually cruise these areas looking for food dislodged by the rough waters of the Riffled area. The presentation of your offering will depend on the use of wet or dry imitations. The best way to position yourself is to observe this area from an elevated point. You want to see the water to determine possible holding areas, feeding lanes, velocity seams and other physical characteristics that will determine the best possibility of luring a waiting fish to your imitation.
The Run
The section of the river or stream directly down-stream from the Riffle, is the Run. The water at this point will become somewhat deeper and the velocity slows down until the flow reaches a Pool. The width of the river or stream along this area can either widen or narrow but the depth will always increase as the flow moves towards the Pool area. The rough water of the Riffle becomes much calmer in this part of the flowage because of the reduced speed and increased water depth. Near the Pool area there will be some backpressure that can cause the water to appear rolling or boiling. You will also note a change in the watercolor due to the increased depth.
The Run also is extremely important to the angler because the fish will find holding areas down-stream of the Riffled areas, waiting for the dislodged debris and life forms drifting along in the water column. The fish will actively protect their Run territory by chasing off the smaller fish, so the remaining fish tend to be larger and very active in this prime location.
The area of the stream or river downstream from the Riffle and Run is called a Pool. This Characteristic is caused by the actions of the two previously mentioned prime areas of the flowage. These Pools are areas of the water that tend to be both wider and deeper. The deeper high side will have a faster current then the slower and shallower low side. This slower and shallower side will also become the waters dumping ground for very fine gravel, silt and mud. This concentration of sediment often promotes aquatic plant life and in turn will become very active with aquatic insect life as well. These Flat areas become attractive to cruising fish during low light conditions, when these aquatic insects will hatch. Here the current can do strange things, such as, eddies, back-eddies or the appearance of the current coming to a dead stop. The Pool will also have a channel where the fastest water will create a groove from the head of the Pool to the tail of the Pool. Fish will cruise this channel looking for feeding opportunities. Pools have one final characteristic and that's called the Tail of the Pool. Here the current starts to pick up speed and the water columns depth increases. This is a prime feeding station for fish. This characteristic precedes the next set of Riffles, where the whole process begins again. Fish also use this area for both mating and spawning activities.
This one area of the waters flowage is the most important area for fish. It is also an angler's most difficult challenge. The Tail of the Pool has many velocity changes; current directions and the fish can easily seek cover. With all the different current speeds, the absolute need to maintain a drag free drift is very difficult to control. The largest mistake made is by over mending and this will drive the fish from your imitation. These Pools should be fished from the sides and not by wading into them.
Remember that no matter where water flows across the earth, there will always be Riffles, Runs and Pools with Flats and Tails. If you can have a clear understanding of these characteristics and how the fish will use these features to survive, you will always be able to place your offering to a feeding fish.