We can better understand where to fish by examining the different areas within a river. These areas are created by the current eroding the valley floor. Because these are features are characteristics of the water flow in a river we call these hydrological features.
General hydrological features
First we begin with a River that is spreading out onto a shallow section, called a flat. Past floods has created a bed of sand and the rivers spreads widely over the sandy bottom. At some point, the gradient (the fall of the river valley over distance), forces the water to cut into that downstream section of the sand bed. As sand is washed away the river flows through the gravel forming a riffle. The downward motion of the river gives the river greater eroding power and downstream of this current is bent into a focused powerful flow against a group of large boulders, forming a rapid. The river now flows hard into the left bank creating a hole of maximal depth called a pool. Having eroded the left bank, the water has lost much of its energy and will deposit some of the eroded material in a long tail of light current. This is sometimes called a run. Often this progression will repeat itself with the tail of run, spreading out into a shallow flat.
We can understand fishing In a river better when we consider what kinds of habitat each of these areas offers.
The flat offers a habitat best suited to invertebrates that flourish in a silty to muddy bottom. With minimal current and a minimum depth, small invertebrates dominate the ecology. These kinds of animals often burrow in the mud, or offer flat profiles as to avoid being swept into deeper water. Occasionally small fish and crawfish will make forays into these areas but exposure to the thin water can be dangerous; birds patrol the shallow flats. Bigger waters offer deeper flats (which encourage greater use by the minnows) but even in these habitats the lack of hiding places limits the population of animals utilizing this habitat. Game fish are rarely successfully targeted on flats. Here there ability to detect the fishermen is greatest. Also there is normally a lack of cover and current that makes it more difficult to target likely places.
Once the gradient has picked up water velocity and starts to erode the silt and sand. The river falls into a riffle. As both oxygenation and current increase, Vertebrates such as small fish and fry utilize small resting places in the rubble. Getting around can be tough (For a small fish), but frequently food is near a maximum between the nymphs that live in the rubble and others drifting in the current. Fishing is problematic in riffles as well. Many game fish only make short forays into riffles, and the insect life is insufficient calories for the effort a predator would need to hunt in them.
If gradient is sufficient and the river narrows, the river may fall into bouts of large turbulent flow over boulders and other large sections of bedrock. Rapids offer few homes to small invertebrates. They also usually are a less hospitable home to small minnows, with turbulent deep currents. They do make good homes however for the several large groups of fish. Those fish that feed on drifting insects will take position themselves in front of or behind large boulders to take advantage of the current break while being able to dart in heavy currents around them.
While many game fish only occasionally venture into the turbulent rapids; Bass and Trout are well known to use rapids. Both have a opportunistic diet that adapts well to drift feeding behind large boulders. Fishing in rapids means utilizing the heavy current to sweep an attractive imitation through many possible feeding stations. Mobility can be difficult; with wading approaches being difficult and approaches by a boat impossible.
As the water velocity increases due to rapids and riffles, The river will erode a deep spot. This deep spot is known as a pool. With both depth and water velocity the pool is an attractive place for fish of all kinds. There are four different habitats within a pool and are used differently by forage and game fish.
The upper lip
The uppermost riffle or rapid will usually end in an abrupt deepening called an upper lip. The upper lip offers a small area of reduced flow immediately below the current and therefore with the most concentrated amount of drifting invertebrates. Forage fish will often hunt in the area, concealed by air-born predators. At times active game fish will then also roam this area for minnows.
The deepest part of the hole is generally rather poor in nutrients and drift. Often times game fish lie here when Inactive. Rarely do minnows and other fish use this area. Fish also use the area during the winter season.
Rarely does a river flow strait. Normally it meanders across the landscape. In the course of this back and forth motion; pools are usually located into elongated patches near one bank. The outside edge of the river often rapidly falls towards the depths. Over Time, erosion has occurred above and below the waterline, uncovering large boulders and toppling trees into the water. With the security of depth and cover the outside bend has abundant forage fish and offers a prime location for game fish.
Once the river current flows to one side of the riverbed, a seam between the faster current and the back current develops. Sometimes large eddies develop funneling water from the tail of the pool towards its head. Current seams and particularly eddies can concentrate drifting food. small forage fish will use the current seam to pick through the drift. if the inside seam is shallow, game fish will only make occasional forays into the inside seam (or use it during night). In other rivers, high water will straiten currents and lead to considerable erosion on the inside seams. Eddying currents and depth can make inside seams prime locations for many game fish.
As the river erodes the depth of a pool, it pushes this material further downstream. The character of this area greatly depends on the gradient and bedrock of the river. Most small rivers offer moderate gradient yet abundant boulders; creating a large field of boulders and basketball sized rocks just downstream of the pools. This area would be considered a run; a relatively shallow but diverse area of boulders. The shallow water encourages insects and minnows, and The boulders create current breaks and turbulence for predatory game fish. These locations are prime, but often fish are more spread than upstream in the hole.