In rivers and streams most of what a [trout eats] is delivered by currents. In any river or stream there are usually several currents of varying strength and width across any section of the river.
Where some of these currents are 'compressed' into one strong current, the maximum amount of food will be concentrated in the smallest area.
Compression can be caused by bottom and bank formation, rocks, boulders, fallen logs etc.
[Prime lies] are those lies where there is the best shelter close to the most compressed currents (shown as darker blue in the diagram above. The two 'red' trout are in the prime lies and can feed to the left or right of 'their' boulder, while being sheltered from the current by that same boulder.
So why are the lies behind other boulders in the diagram not prime lies? It is back to compression. An obstruction lying in the middle of one current produces only a little compression. In the prime lies shown below the currents are compressed between two boulders, or boulder and bank, concentrating feed items.
Riffles can be as shallow as a few centimeters in small streams, thigh-deep in smaller rivers, and in big riffles (rapids) meters deep between the rocks and boulders. Usually the water moving through riffles is the fastest moving water in the river or stream.
Lies in riffles are mostly found in the calmer water behind obstructions such as rocks and boulders. The obstruction provides shelter from the swift current. In the fast swirling water around the obstruction trout are difficult to see and thus safety from predators is maintained.
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Fish are not always behind obstructions.
There is a 'cushion' of slack water in front of obstructions; it is what forms the 'bow-wave'. Trout often hold in this cushion.
But fish will lie alongside the edges of drop-offs, shingle banks and boulder banks which lie almost parallel to the main water flow. It needs only very little change in underwater contours to produce a change in water direction, and provide a sheltering lie for a trout.
The current provides food in riffles. Nymphs and bugs are carried by the current and close to the trout, which only has to move a few centimeters to engulf its prey. The fast current also maintains temperature and oxygenation at optimum levels.
It would seem from reading this that riffle water should provide the ideal habitat for most trout, and often this is true. Riffle water does often hold the big numbers of trout, but these trout – as a wild generalisation – usually tend to be the smaller fish in the river or stream.
The reason for this is that the ‘pocket’ of calm water behind an obstruction is often relatively small and movement – without being subject to the strong current – is restricted. In addition, food for a fish holding in a riffle lie is restricted to that being carried by the current.
I saw a very pertinent illustration of this last point in the lower Horomanga River, near Rotorua (Central North Island, NZ), a few years ago. I was shown a riffle running hard alongside a steep bank. In the middle of this riffle was a flat slab of rock about three metres square lying in mid-stream.
The current flowing over the back edge of the rock had scooped out a hole about the size of a bathtub in length, width, and depth behind the rock. Immediately to the left of this hole was a shallow shingle ‘beach’, no deeper than 20 centimetres at its deepest, dry shingle at its shallowest.
The week before, my companion that day, a guide, had spotted a big fish in the ‘bathtub’ lying just under the lip of the slab of rock. He asked his client to cast a nymph up ahead of the slab. As the nymph drifted over the rock and crossed over the lip a big brute of a trout rose up and grabbed it.
Immediately the fish felt the prick of the hook it bolted, straight out of the ‘bath’ and onto the shingle bank where it beached itself, unable to move. The whole performance lasted less than a couple of seconds.
The trout was a true brute – 16 pounds of rainbow. The angler posed quickly and proudly with the fish for photos, then released it, to swim straight back into the bathtub.
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