Reading a pool: head, body, tailout

August, low and warm. There's a big flat pool you skip every time, because there's nothing in it to read — no current tongue, no seam, surface like glass, every rock on the bottom in plain sight.

So you fish the pool above it instead. A good tongue coming in, a clean seam, a foam line you could set your watch by. You give it the afternoon and get nothing, in water that looks exactly like the pictures.

Walking out at dusk you cut across the flat pool because it's the shorter line, and halfway over you push up a fish that leaves a wake like a boat. Then another one.

Here's the move almost everyone makes, and it looks entirely right: you fish the pool that looks like the diagram. Head, body, tail, deep is better, work the seam. That map is drawn from the bank, and nearly everything that decides whether a pool is worth your evening is invisible from there.

Start where the map tells you to start, at the top of the pool. The reason that water is good is not the reason you were given.

Why does the top of the pool matter more than the rest of it?

Food arrives at the top of a pool. That's true, and it's the smaller half of it. The bigger half is that nothing has eaten it yet.

Work a pool out on paper — count what each feeding fish pulls out of the drifting food, allow for how that food spreads sideways and how new food enters at the top — and prey depletion predicted where fish sat in two pools of a mountain stream. The model needed one assumption: every fish takes the most profitable position its rank in the pecking order allows. The shape of the pool lays out the positions and ranks them. The fish sort themselves into that order.

Which makes a pool less a scatter of good lies than a queue. The fish at the front eats first. Every fish behind it is eating what got past the one in front, and the further back you go, the thinner that gets.

Two things follow. Your first cast into a pool belongs at the top of it, not in the middle where the water looks nicest. And when you wade in at the head to get a better angle on the body, you haven't spooked a fish — you've spooked the one that matters, and shuffled everybody below it. The pool goes quiet from the top down.

Worth knowing where this gets less certain: whether a whole pool strips the food and passes leftovers downstream is pools as drift sinks: mixed evidence. Fish thin out the food around their own feeding spots. That part holds. Don't assume the bottom of the pool is starved.

That bottom end is where most evenings come apart anyway. It's the water everyone calls soft, and the fish in it are gone before the fly lands.

Why do the fish at the tail of the pool leave before my fly lands?

The tail of a pool — the last stretch before it shallows out and tips into the next riffle — gets described as slow, soft, forgiving water. Down where the fish live, it often isn't. Three-dimensional modelling of a natural pool-and-riffle sequence found high near-bed velocity at pool exits in two of the four pools, plus mid-pool sections where water down against the gravel moved at least as fast as in the riffle next door. The surface is slow. The bottom foot is already speeding up toward the drop.

So a fish sitting there is doing something specific. It's holding over gravel where the flow is accelerating underneath it, with about a foot over their backs and nowhere to disappear to. That's the thinnest cover in the pool, and a fish with one exit will take it.

It's also why those fish appear when they do. Tailout trout feed in low light — dusk, dawn, heavy cloud — moving a foot or two to take something and dropping straight back. In flat afternoon light that water can look empty and be full.

Which means you fish it from below, staying low, and the first cast is the one that counts. A leader that lands in a heap on glassy shin-deep water gives you away as loudly as wading does. If yours piles up instead of straightening, that's worth sorting before you start changing flies.

All of that assumes the pool is worth standing below in the first place. In high summer most of them aren't, and the one that is looks nothing like this.

Why did the pool that looked dead have all the fish in it?

"Deeper water is cooler" is the standard summer reason for fishing pools, and it's true of a minority of them. In one summer survey of a coastal stream, pools that genuinely separated into a cold layer on the bottom made up under 9% of pools surveyed. The rest were much the same temperature top to bottom. Deep, and just wet.

The fish know the difference. Where those pools were mapped across a river, 76% of 449 adult steelhead were sitting in the large stratified ones. In summer, pools are not interchangeable. Roughly one in ten is a refuge and the others are water you walk past.

The difference is worth real degrees. Bottom water in those pools ran up to 7°C cooler than surface water in the same pool, and as much as 11°C cooler than the main river. That is the gap between a fish that feeds and a fish that sulks.

Here's the tell, and it's backwards from everything you've been taught. A cold layer only forms where the water is barely moving: residence times of 2.5 to 9.3 hours, against under an hour in the pools that stayed mixed. Depth on its own didn't do it either — pools over 3 metres deep only stratified once flow dropped below about a cubic metre a second. So the summer pool worth the walk is the big, still, glassy one with no current tongue running through it. The one that looks lifeless. Fish it slow and deep, and treat the surface like it will report you, because it will.

Flow is doing all of this. Which is why the pool you learn in August is not the pool you'll be looking at when the river comes up.

Read one pool from the bottom up

Take a pool you've fished a dozen times and work it in one careful pass, in order. Start below the tail and fish that last shallow stretch before you walk through it. Then the body. Then put your best cast at the very top, and stay out of the water up there for as long as you can stand it.

One pass in that order tells you more about a pool than a season of starting where the current looks best. When you've done it, walk up to the riffle above — where trout sit in a riffle runs on the same logic in water you can read from the bank.

Do that pass at an ordinary flow first. When the river rises, the pool rearranges itself, and not in the direction most people assume.

Questions people ask about fishing pools

Where do trout go in a pool when the river's up?

Not the deep middle. In the same three-dimensional modelling, as discharge rose, bed shear stress rose in the pools — with the biggest increases in the deepest parts — while it fell at every mid-riffle section. The fast near-bed zone slides downstream and the small soft pockets along the edges shrink and merge. A pool at high water is a simpler pool, with the liveable spots pushed to the margins and the bottom end. The riffle above is getting gentler down where fish sit, so don't write it off.

Why are the bigger fish in the pool and the small ones in the riffle?

Cost of living. In an enclosure experiment with juvenile cutthroat, the youngest fish grew in both water types, but larger juveniles lost weight in riffles while gaining in pools. Energy accounting alone explains it.

Should I fish the head or the tail first?

Whichever you reach first walking upstream, which is normally the tail. Fish it from below before you stand in it, then work up to the top. The order you take a pool in matters more than the fly you take it with.