Don’t Cross The Thalweg


When swinging wet flies, closer is often better.

I followed Poul Bech, retired British Columbia fisheries biologist, through a long and beautiful run in the upper reaches of a famous steelhead river, far above a dam, where the river’s inhabitants had reverted to resident trout. I got a whipping, and with it an education. Poul’s job, over his long career, was to catch native steelhead for spawning in B.C. hatcheries, a program designed to prevent the cookie-cutter “product” of similar systems in some Western states.

Poul caught those steelhead on hook and line, often—though not always—fly-fishing, and over time accumulated a body of experience not many of the rest of us are ever going to get. He drew on all that knowledge to swing a wet fly ahead of me down that run. Trout after trout, mostly rainbows with the occasional pretty cutthroat, came to his casts. Obviously there were enough fish in the river that I should have hooked some leftovers, but no more than one or two chased down my own swung wet. I finally ceased casting and watched Poul at play, the smartest thing you can do whenever somebody else is enjoying lots of success and you’re suffering its opposite.

The river was substantial, that particular run requiring a long cast to cross. Poul fished short casts, so I’d been reaching out to cover the more distant water that he didn’t, to show my own wet fly to fish that failed to get a look at his. It should have worked, but it didn’t. Poul consistently cast short of 50 feet, far from straining an experienced steelheader’s arm or gear. Each cast was angled at 45 or even 60 degrees down with the current, rather than straight across it or even up into it (as I’d been doing). His swings, as a consequence, were slow, and he added frequent mends to put the brakes on any acceleration his wet fly tried to get going. Takes were often and soft, much the way a summer steelhead accepts a swung fly, with no more than a slight tug.

After we both finished fishing the run, I asked Poul why he’d been doing so well while I’d been doing so poorly. His answer drove me to the dictionary. “You’re casting across the thalweg,” he said. “When you fish wet flies for trout, steelhead or salmon, it’s a mistake to cross the thalweg if you can avoid it. It’s a concept that originated in Atlantic salmon fishing.”

“What’s the thalweg?” I was forced to ask. I’d never heard the term.

“It’s the deepest part of the river, in cross section,” Poul told me. “In fishing terms, it’s the location of the main current.”

Thalweg 2

Keep your casts to your side of a river’s main current, and you’ll get soft, natural wetfly presentations, and maybe cradle more trout like this.

Thalweg is a German word. Thal means valley; weg means way. The thalweg is the deepest line down a valley that a creek, stream or river carves on its way to an ocean or wherever else it’s aimed. It’s the current tongue in a pool. It’s the central or side current in a run. Riffles often spread out wide, with fairly even depths, but there will almost always be a point, in all types of moving water, where the flow is deepest and strongest. The reason is simple enough: The most vigorous current erodes the greatest depth.

Enough definition. Let’s look at consequences.

If you cast short of the thalweg, or driving central current, as Poul did, then you’ll get a swing with an even, or even a decreasing, speed. Your fly (or flies if you’re fishing two or three of them where it’s legal) ambles along as natural insects might move. Whether you’re after trout, steelhead or Atlantic salmon, your enticement dangles in front of the fish for a longer period of time, moving toward them and then away from them at far less than escape velocity.

If you cast across the thalweg, as I did, your fly or flies land in slower water on the far side while your line crosses that faster main current. The result is fairly obvious: A downstream belly forms in the line, the fly begins to race, and it doesn’t ever slow down sufficiently until it’s reached the end of its swing. If that cast across the main current is long, then no amount of mending will slow the racing fly, which will look like little in nature.

The simple formula for not crossing the thalweg is precisely what Poul was following: Cast just short of the main current; angle the cast downstream; fish a slow swing; mend often; be patient at the end of the swing; take a step; repeat the drill. If the current is central to the stream, whether in a riffle, run or pool, fish the full length of the near side first, then if it’s possible wade across the river, move back to the head of the water you want to cover, and fish the far side of the thalweg with the same simple formula. You’ll catch more fish than you would if you tried to cover both sides of the current from one side of the river.

Avoiding casting across the thalweg is a useful idea that goes beyond wetfly fishing. Though nymphs are now fished most often with indicator-and-shot, it’s a poorly kept secret that nymphs on the swing are still capable of doing devastation to lots of trout. If you rig with one heavy and one light, you can even achieve some depth that you can’t with unweighted wets. Streamer fishing for trout is most closely related, in terms of methods, to the pursuit of steelhead and salmon. Not casting streamers across the thalweg is a good way to keep them from racing in an unnatural manner.

Though we usually think of fishing wet flies with floating lines, it’s often advantageous to cast them on intermediates, to get them down that critical foot or so below the surface. With nymphs and streamers, in water more than three to four feet deep, it can be wise to go with fast-sinking tips, or even full-sinking lines. No matter the setup, casting without crossing the central current allows a more life-like drift and swing.

The concept of the thalweg is perhaps most important in rivers of medium to large size, where the water spreads out sufficiently to get a decent swing on the inside of the main current. Where the current is central, you can fish one side, then wade across to fish the other. Where the current is pushed against a cliff or an outside bend, the thalweg will be driven there as well. The entire sweep of current on the inside will be open to perfect presentations, while the far side, against the wall, will have no room for any sort of swing.

If the current is located on one side or other of the river or stream, instead of being central, it can be profitable to cross the thalweg with your cast, then to high-stick out a drift in such a way that the current can’t get a purchase on your line. This is most likely to work on small streams, but can be employed on medium streams and even small rivers if the water is less than brutal, letting you wade out close to that central current.

I’ve used the high-sticking method to fish back eddies tucked on the far sides of some swift and nasty currents, usually with small soft-hackles that look at least a little like some insect that happens to be hatching. But it only works if you can lift your line off that fast main current. If the current gets a grip on your line, it’s going to race your fly right out of there, and no fish is going to be able to catch up with it.

On a few riffles and runs, the thalweg is spread out, almost impossible to locate, the water the same depth and speed over a wide width of current. In that case the solution is simple: Don’t worry about the thalweg, because trout might be anywhere out there, and your casts, because they don’t cross currents of different speeds, will not cause your flies to race.

On most trout waters, the thalweg is well-defined, easy to see, and not difficult to deal with once you learn to recognize its importance. In fact, I recommend you forget the term but remember the concept: Avoid lofting long casts across main currents and your fishing is suddenly a lot easier. It is likely to become a lot more productive as well.

Dave Hughes is author of Pocketguide to Western Hatches and Wet Flies.
Photos by Dave Huges

Dave Hughes
About Dave Hughes 6 Articles
Dave Hughes is the author of Reading Trout Water and the recently released second edition of Wet Flies.

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