A trout is trying to solve one equation all day: eat as much as possible while spending as little energy as possible and not getting eaten. Every place a fish holds in a river is a solution to that equation, and once you can see the river in those terms, the good water stops being a mystery. This page is about the water. Casting and flies are other pages.
The three things a trout needs
Food, delivered by current. Rest, meaning current slow enough to hold in without constant swimming. Cover, from overhead predators and from anglers. The best lies have all three within a foot of each other: a fish sits in slow water next to fast water, with the fast water bringing food past its nose, and something over or beside it that breaks the view from above.
Add oxygen and temperature. Trout need cold, oxygenated water, and in summer the churning, shaded parts of a river hold fish that would have been in a slow pool in April. The Firehole is the extreme case: geothermal inflow warms it so much that midday fishing in July and August is often pointless, and the fish stack up near cold tributary mouths.
Riffles and runs
Riffles are the shallow, broken, fast sections where the river tumbles over gravel and cobble. They are the grocery store. Mayfly and caddis nymphs and stonefly larvae live in riffle rocks more than anywhere else, and the turbulence keeps the water oxygenated. Riffles look too shallow and too fast to hold fish, and they hold fish all the time, especially in summer and early morning, tucked behind individual rocks or in depressions where the current slows at the bottom. Look for any spot where the surface is slightly smoother than the water beside it; something on the bottom is slowing the current there.
Below a riffle the river deepens and the surface smooths into a run: moderate depth, steady current, usually a defined deep lane on one side. The run is where most trout spend most of their time. It has depth for cover, food coming from the riffle above, and current soft enough to hold in near the bottom. The deep lane, the edge where the run meets slower water, and the head where the riffle drops in are the three spots to fish first.
Pools and tailouts
A pool is deep and slow. It has a head, where the run or riffle enters and the food arrives; a body, the deep slow middle; and a tailout, where the bottom rises and the water accelerates toward the next riffle. Fish hold at the head in the current line during hatches, rest in the body during the day and in winter, and move to the tailout at dusk to feed on whatever the pool is funneling out. Big pools on rivers like the Deschutes or the North Umpqua are also where steelhead rest, which is why swung-fly fishing is organized around them.
The tailout is the most overlooked water in a pool. It is shallow, it looks empty, and the fish in it are the easiest to spook because they can see you against the sky. But it concentrates everything drifting out of the pool into a narrowing lane, and in the evening a tailout on the Middle Provo or the Henry's Fork will have fish rising in a row across it. Approach from below, stay low, and fish it before you walk up through it.
Seams and foam lines
A seam is the line where two currents of different speed meet: fast against slow, main current against an eddy, the edge of a riffle against the bank. Seams are the single most reliable place to find feeding trout, because the fish holds on the slow side, spends almost nothing to stay there, and picks food off the fast side as it goes by. On the surface a seam shows as a line of bubbles, a change in texture, or a strip of smoother water beside rougher water.
Foam lines are seams you can see from a distance. Foam collects where currents converge and so does everything else that floats: spent mayflies, drowned hoppers, emerging caddis. A foam line down a run on the Missouri is a feeding lane with fish under it.
Structure: rocks, banks, logs, and drop-offs
Every object in the current creates a pocket of slow water around it, and fish use those pockets.
- Boulders. Fish hold in the cushion of slow water on the upstream face and in the eddy behind. The upstream cushion is the spot most anglers miss.
- Undercut banks. A slow, shaded, overhead-covered shelf. This is the classic big brown trout lie on meadow rivers like the Big Hole near Wisdom. Fish hold under the bank and eat off the seam beside it.
- Logs and root wads. Cover and current break together. Fish sit downstream of the wood and beneath it.
- Drop-offs and shelves. Fish hold at the lip on the deep side and move up onto the flat to feed. Clear tailwaters like the Green below Flaming Gorge let you watch this happen.
- Confluences. A tributary brings food and often colder water. In August the mouth of a cold creek can hold every fish in a quarter mile.
Steep, boulder-filled pocket water like the Gallatin canyon is a run broken into a hundred small pieces. Each rock has a cushion, a pocket behind it, and a seam on each side, and each of those is a separate lie with a fish in it. Fish it close and methodically, one pocket at a time.
How water type changes the reading
On a freestone river the features move every spring with runoff, so you read the river fresh each season. On a tailwater the flows are dam-controlled, the structure is stable, and the fish hold in the same lies year after year; regulars on the South Platte in Cheesman Canyon fish named rocks. On a spring creek like Silver Creek the current is slow and the structure is weed beds, so the seams are the channels between weeds and the fish hold at the weed edges.
Walk a run before you fish it
Stand at the tailout and look up. Find the riffle at the head, the deep lane through the run, the seam on each side of that lane, the biggest rock, and the foam line. Decide where you would sit if you were a trout with no interest in wasting energy. Fish that spot first, from below, before your shadow crosses the rest of it.