Where trout sit in a riffle

You get to the water with an hour of light. The riffle at the top of your home run is knee-deep gravel, sixty yards of it, and it looks the same end to end — so you start at the head, because that's where the water comes in and that's what you were taught. You fish the seams. You catch six-inch fish. Every one of them, all evening, and by the time the light goes you've decided that's just what the riffle holds and next trip you'll walk past it to the pool.

That conclusion costs you the best water in the run. Two things put you there. You spent the hour on the first ten yards of sixty, and your nymph rode six inches off the bottom the whole time — which felt right, because six inches up is where you could see the food going past.

Nothing about that looks like a mistake while you're doing it. Which is why it pays to be exact about where a trout in a riffle actually sits.

Where do trout hold in a riffle

Most of a riffle is water a trout can't afford to sit in. Confine decent-sized young cutthroat to riffle habitat and they don't just grow slowly — the larger juveniles lost weight in riffles while the same age class gained in pools. The current bills more than the food pays back. Small fish clear that bar easily, which is why your riffle is full of six-inchers and the better fish is in the run below it.

The limit is speed, and it's tighter than it looks. At around a foot per second a trout catches most of what it goes for. Take it to two feet per second and capture success drops to about 10% — it can't pick items out in time, and getting back to its station costs more than it can hold. The fastest chute isn't lying to you about the food. It just isn't a place to eat from.

So you're not looking for food. You're looking for the few spots where the bill drops far enough for a decent fish to turn a profit, and those spots are dips in the bed. When holding positions were mapped fish by fish, every trout sat in a depression — and 40% were on the upstream slope, against 27% down at the deepest point. That upstream lip is the spot. The flow has already lifted off the bed as it comes over the crest, the fish is out from under the worst of it, and it's looking uphill at everything the riffle is sending down.

They're also lower than you picture them. 85% of noses within 5 cm of the bed, bellies averaging 2.3 cm off the rocks and most often flat against them. Not hovering mid-column. On the gravel, looking up a couple of inches at what comes past.

One more thing separates a lip a fish uses from one it ignores, and it isn't depth. It's how much the water over it churns — and that's the part most of us have backwards.

What does that look like from the bank?

The thing the drawing looks wrong about, the first time you see it, is which of those tongues is the rough one.

Isn't the fast water the rough water?

Fast looks rough. Stand in a riffle and the loud broken water is what you brace against, while the smooth tongue sliding down the middle looks like the hardest place a fish could possibly sit. It's the other way round. Turbulence goes up as speed comes down. That smooth fast tongue is the most orderly water in the riffle. The ragged slow margins — the churn off the back of a gravel bar, the boil where flow trips over cobble and loses its shape — are the water that shoves a fish off balance and makes it work just to stay upright.

Trout choose on it. In the same mapping of daytime positions, occupied sites had lower turbulence than comparable unoccupied ones, and turbulence intensity ran inversely to current speed. Two spots that look identical from the bank are not identical to the fish, and it takes the calmer one.

So slow doesn't mean restful. It depends whether it's smooth slow or ragged slow, and you can see which from the bank once you're looking. Smooth slow water has a surface that slides. Ragged slow water has a surface that keeps rearranging itself.

This also puts a ceiling on the sit-in-the-calm-and-dart-out plan. Leaving a velocity refuge to grab something raises cost per attack about 65% and cuts the odds of getting it. A fish would rather the food arrived where it already is, which is exactly what the upstream lip of a dip gives it: slow water at nose height, little churn, and a clear view uphill. The old surface rule of thumb, holding water running about 1–2 ft/s, is describing the same place from above.

That's which foot of a riffle a fish can live in. It says nothing about which end of sixty yards deserves your hour, and the food decides that one.

Should I fish the top of the riffle or the bottom?

A riffle makes food and then hands it downstream. Per patch of bottom it produces about 3.5x the drift of pools, and the caddis larvae and mayfly nymphs coming off it run roughly three times the size of pool bugs.

It builds as it travels. Total drift rises with riffle length, so water arriving at the tail of a long riffle carries what the whole riffle has shed, not just the last ten feet of it. And it isn't recycled downstream: few invertebrates reach the next riffle, because the pool between them is a sink. The bottom third of a long riffle is the richest water in the piece. It's also the part you wade through on your way to the head.

I fished the head of mine all morning for a season's worth of six-inchers. Then one evening, half packed up, I threw a couple of lazy casts into the flat bottom end where it tips into the run. Same depth, no seam, no rock, nothing to look at. Hooked a fish twice the size of anything I'd touched up top. Next trip I checked my depth properly and found I'd been running the rig six inches off the bottom the whole season.

Both fixes fit in one trip. Start at the tail and work up, so you cover the richest water before you push silt and shadow through it. Add weight until you're ticking gravel, then stop. And know what you're up against in daylight: mid-teen fish only at low light in the riffle itself, the rest of the day spent in the deep run carved below it.

Fishing it in that order raises the same two questions every time: how you know you're really on the gravel, and whether an ankle-deep riffle is worth the walk.

Take it to your own riffle

Next time out, give one riffle a full pass in this order and see what changes.

• Walk to the tail first. Fish the flat twenty yards above where it tips into the run before you go anywhere near the head.
• Add weight until the rig ticks gravel and hangs up now and then. That's the depth. Six inches up is a different piece of water.
• Read the surface for dips: darker patches, slicked-over glides in the chop. Put the fly on the upstream edge of each one, not in the middle.
• Compare smooth fast against ragged slow. Fish the smooth first.

A few more questions people ask about riffles

How do I know the fly is on the bottom? You touch it. Add weight until you're ticking gravel and hanging up every dozen casts or so, then leave it alone. A drift with no contact at all is a drift you can't judge.

Is an ankle-deep riffle worth fishing? The sheet of it, not much. The dips in it, yes. Adult fish want roughly a foot over their backs, and holding water inside a riffle tends to run 1–3 ft deep. A pothole a few inches to a couple of feet deeper than the bed around it is enough.

Is the riffle only worth fishing at dawn and dusk? Not for everything. Brown trout yearlings moved into riffles during daylight in summer and early fall, so the middle of the day gives you honest small and medium fish. The better ones use it on the edges of the light.

Upstream or downstream? Upstream, short casts, zig-zagging across the bed from the tail up. You keep your fly in front of fish that are facing uphill, and you're not walking through water you haven't fished yet.