Find the thalweg for better trouting in tiny headwaters
[This article is an excerpt from The Orvis Guide to Small Stream Fishing (Universe, 2011). The Editor.]
When I'm asked to do a seminar or a podcast, a common request is a program on reading the water. I think some anglers hope that somewhere in the quagmire of fly-fishing books and articles is a standard manual on reading the water, a long-forgotten volume on the secrets to this black art. If you're looking for specific rules, such as, "When a big, flat rock is above a round rock, trout will always be found on the left side of the round rock," I think you will live an unfulfilled life. Reading the water is a dynamic skill that changes with water levels, kinds of food, amount of fishing pressure, and the quantity and species of predators that threaten a trout population. It also depends greatly on an invisible force that is perhaps the most important aspect of reading the water: fluid hydraulics.
Water, Friction, and Turbulence
Water flow is either classified as laminar or turbulent. True laminar flow, where the water flows straight downstream without any diversion in direction, is virtually nonexistent in trout streams. Any tiny projection in the bank, on the streambed, or any change in direction causes friction, which changes the direction of the flow and slows its downstream progress.
It's this turbulence that forms slow pockets in the current where trout can rest and feed. The larger the irregularities and their density, the greater the turbulence and the more places for a trout to live. So when looking over a small stream, those with more rocks on the bottom, or more logs or large rocks along the bank, will potentially hold more trout.
There is a point where turbulence gets too strong and trout have trouble holding in the current because they can't maintain their position. When you see smooth whirling cells of water that change shape constantly and boil from the bottom, making large bulges in the surface, the water is too turbulent.
In extreme cases—especially in very fast current with hydraulic jump (places that exhibit a very quick change in slope, releasing potential energy and causing extreme turbulent mixing)—standing waves might form. In this water, a trout's food gets pushed aside unpredictably and fish don't like food snatched away just when they think they're about to get a mouthful. Besides, they have trouble holding their position when being pushed from different directions at once.
Better turbulence is found when the water shows a gentle riffle that moves in a predictable direction—it might be the edge of a whirlpool that moves upstream, a finger of current that is suddenly pushed sideways, or a gentle, dimpled run that flows at a stately pace downstream. As long as the flow is predictable, trout are content.
Friction causes turbulence behind objects and along their sides, but a less intuitive aspect is that turbulence also develops in front of objects. When current hits an obstruction dead-on, it bounces back and the loss of energy is transferred back a longer distance than you might think, forming a cushion or pillow in front of a rock or log. In addition, the turbulence that forms in front of an object digs out the gravel or sand in front of a rock and forms a deeper pocket, which provides additional protection from the current.
Often the spots where turbulence is just right are immediately apparent to humans peering through the surface. Pockets around large rocks, places where a shallow riffle suddenly dumps into a deep pool, and logs that stretch out into the current are obvious places to find trout, as they offer protection from the current and a place to hide when danger looms.
However, because actual hydraulics are invisible to us, we only see their obvious manifestations on the surface. We can't see, for instance, what happens when the turbulence between a set of underwater rocks converges, which is why a trout will occupy one place on the bottom that looks just okay, and not another spot in the pool that looks terrific.
We just can't see what's going on, so we have to rely upon those few gross changes we can see on the surface—plus a lot of experience and guesswork. Reading the water is not an exact science.
What Trout Need
I get annoyed when I write about trout fishing because I find myself overusing waffling adverbs like "sometimes" and "often." But trout are wild creatures and encompass three genera (Salmo, Onchorynchus, and Salvelinus), and inhabit streams of all shapes and sizes. They even have individual personalities, and a trout on one side of a tiny pool might behave differently than his buddy on the other side of a rock. So giving you concrete rules for reading the water is foolish. I think it's better to understand what trout need, how they feed, and how currents affect them.
Of course I'll give you some hints, but I think they'll make more sense if you spend some time learning details that you can use on any small trout stream, anywhere in the world.
In most large trout streams there are two kinds of feeding behavior. Most trout are drift feeders, which means they station themselves in a safe, comfortable place in a stream and pluck food from the current as it drifts by. But a few individuals in each stretch of water, usually the bigger ones—fish longer than 14 inches (or perhaps 20 inches in food-rich streams where the fish grow bigger on average)—develop a hunting, ambushing strategy where they feed less often, but cruise through a pool hunting for bigger prey like baitfish, crayfish, mice, or frogs. They act more like pike than trout.
You might find these hunters eating insects during a heavy hatch, but if food in the current is not abundant they'll rest under heavy cover for days at a time, only leaving the security of a logjam or deep pool after dark or when a rainstorm clouds the water and makes hunting baitfish easier. In small streams we seldom find these bigger hunters.
Baitfish and crayfish are not abundant in many small streams, so we should concentrate on drift-feeding trout when planning our strategy for them.
Drift-feeding trout like to lie in water that flows at about one foot per second and lies adjacent to water that is slightly faster. Move your finger in front of your face a foot while you say "one-thousand-one." This piece of one-foot-per-second water does not have to be very large—it can be just slightly wider and longer than a trout's body, so a little depression in a riffle or a slight projection where the current slams against a rocky bank may be enough to give a trout a comfortable place to lie and feed.
Trout are not territorial in the way behavioral scientists describe this behavior, because a territorial animal defends a specific location, and although a trout may spend 90 percent of its time in one spot, many of them have three or four different places within a pool. A trout may lie on the edge of the fast current when food is scarce and it needs the visual protection from predators that the wrinkled, distorted surface of fast water gives it.
When insects are hatching, this same fish might slide back to the tail of the pool, where it can see floating insects better against the smooth surface, and where the current (and thus drifting food) is constricted both horizontally and vertically. When frightened by an angler or otter, this same fish will always have a bolt-hole like a logjam or deep pocket beneath a piece of flat rock where it can swim quickly to get out of danger. This fish will not defend a territory, but if it is larger than other fish or more aggressive, it will defend its immediate location, especially in front of it. Trout that move up behind this trout, or are visually isolated from the trout by a rock or bubbles in the current, will be ignored and you can often find pockets of fish in one place feeding quite close together. The biggest one will almost always take the upstream position.
In general, the greater the number of rocks or other obstacles in a pool, the greater the number of trout you'll find living and feeding there.
The Importance of Cover and Shade
When I give a seminar on reading the water, I always ask, "What is the most important need of a trout, assuming water temperature and oxygen are within a comfortable range?" And the most common answer is always, "Cover!" But that's not correct.
The most important need is getting enough food while lying in a current that won't burn all the calories the fish is consuming while feeding. Trout will forgo secure protection if it's not right beside a good place to feed, because all the protection in the world won't help if they starve.
Now there are many places where trout can find that one-foot-per-second flow alongside faster current and still be hidden next to a log or large rock, and these are often known as "prime lies," as opposed to "feeding lies" or "protection lies." (A feeding lie is one that a trout uses only for eating, returning to a more protected location after it finishes gorging. A protection lie is that logjam in the bottom of a deep pool where a trout hides when disturbed or when it is not eating.)
A prime lie affords everything a trout needs without having to move, and these are the places most anglers concentrate on, for good reason. Prime lies often (there goes that indecisive word again) hold the largest fish in a pool. But for every prime lie in a pool, there are a half dozen lies that are perfect feeding lies.
The problem is that fish in feeding lies are often more spooky, less secure, than fish feeding out in the open because the current there is perfect. Additionally, a trout in a prime lie will many times be visually isolated from an approaching angler because the rock or log that provides shelter often hides your approach.
I shouldn't imply that cover is not important to small-stream trout; it's essential. If you compare two stretches of a small stream, one with wide-open gravel and no streamside brush and one pocked with boulders and logs, the stretch with the greater density of cover will hold more fish.
Trout in small streams are, by definition, almost always living in shallow water, and where trout in rivers have deep pools and large expanses of riffles where distortion on the surface hides the fish from predators, trout in small streams don't have the same luxury. Otters can swim across a small stream with one wiggle of their torso. Raccoons can swipe at trout without getting their feet wet. Kingfishers can see to the bottom of every pool.
Shade is another form of cover. Less light getting into the water means trout shadows are not as visible, and if the foliage causing the shade is low to the water it prevents avian predators like ospreys and kingfishers from diving on trout from a perch above.
I think many anglers overemphasize the importance of shade. It changes throughout the day, and a trout is reluctant to move into shade that does not also offer a steady food supply. Although there is no doubt that trout feed more aggressively on cloudy days, in the morning before the sun hits the water, and in the evening after the sun is off the water, I think this is more a response to insect abundance, as most aquatic insects hatch more readily in low light than in bright sunlight. Trout can and will feed in bright sunlight if they are not disturbed and food is present. So pay attention to shaded areas of a small stream, but also keep an eye on current threads and where food is drifting. Shade does amplify the possibility of trout being in a spot if the main current threads pass through the shade.
But when fishing a small stream, don't just look right next to cover to find fish. Use the presence or absence of cover to find the right part of a small stream to fish. Then use your knowledge of how water flows through a tiny pool to look for places they'll be eating.
Finding the Thalweg
Stream ecologists and hydrologists use the term "thalweg" to describe the deepest part of the channel that runs through a pool, which usually hosts the fastest current and carries the most debris—and food. Anglers call this the "bubble line" because you can trace its passage as it winds through a pool by the line of bubbles or foam that betrays its progress.
Because most small streams have relatively slow current as compared to bigger rivers, and their food supply is also sparse, finding this line is one of the most important tools in learning to read the water. You may see a fishy-looking log on one side of a pool that you're sure will hold trout, but if the main thread of current misses the log by five feet, you might find trout hiding there when they are frightened, but probably not when they are feeding. One way to eliminate spots in a pool where you won't find trout is to look for leaves and other lightweight debris on the bottom. If the current is slow enough to accumulate lightweight debris, it's probably too slow to bring sufficient food to a hungry trout.
Now we get down to the details of what I'll bet you thought reading the water in small streams was all about: locating fish in relation to rocks and logs and undercut banks. You should take heart in that reading the water in a small stream is much easier than in a big river.
Trout distribution in moving water is never uniform. Some places will be full of trout, some will have a few widely scattered individuals, and many places in trout streams are completely barren of fish — or they might hold 3-inch young-of-the-year fish only. In a small stream, the distance between a spot that holds fish and one that doesn't might be only a foot, and because trout in small streams don't get the same bountiful food supply as fish in bigger rivers, they are more inclined to move a foot or so for a fly, whereas trout in bigger rivers, where they are well fed, might not move more than a few inches for a fly. In a big river you might have to walk a half mile between spots that hold fish!
But that does not mean you can get sloppy in your stream-reading skills. While it's possible to cover a tiny pool with just a few casts, by casting blindly to cover all the water you risk dumping your fly line on top of a trout, which is sure to scare it and keep it from feeding for a few minutes to a few hours, depending on how much you frighten it. By making precise casts only to places where you suspect trout will be lying, you will have many more opportunities to put just your fly and tippet over a feeding fish. In addition, drag is often a big problem in small streams, especially in the tails of pools or where conflicting currents snatch a fly out of the current it is drifting in. Again, by making precise casts you will get a short, drag-free, effective float over places where trout are feeding.
Depth, Bends, & Rocks
Depth is usually a limiting factor in where trout live in small streams. The deeper pools and pockets will usually hold the most fish. A big plunge pool beneath a waterfall is an obvious place where you'll find this depth, but some small streams have few well-defined pools and you have to look closer.
In one small stream I was fishing on my lunch hour while researching this book, I caught a respectable 11-inch brown trout from a shallow run. Something told me right where he would be and I was right, but I stopped fishing for a few minutes and reflected on why I just knew a good fish would be lying in that spot, as it was not immediately next to the two large rocks in the run.
I realized that the fish was holding in the one spot in the pool where the bottom was blurry, where I could not see sharp contrast between the edges of the rocks. It was the deepest slot in the pool, and I now use this method in very shallow water to help me find the deepest water. I find the spot where the bottom just begins to get blurry, knowing that I'll be fishing the deepest part.
Everyone wants to learn about finding trout in relation to rocks in the streambed. Trout hang in the slow current behind a rock, as long as it is not so turbulent that the currents swirl in all directions, and as long as the rock is not too large, over a few feet across. The reason trout will not be found as often behind very large rocks is that they block all the drifting food from the place immediately behind them, and although the current speed right behind the rock may be very comfortable for a trout, if that rock strains all the food off to either side a trout living there will starve. In the case of large rocks, trout will more often be found along its sides or directly in front of it.
In fact, especially in small streams, the spot in front of a rock is almost always a better spot than behind a rock. Because food is at a premium, trout take up positions where they will encounter the greatest amount of drifting food, and they also like to be able to see the food coming toward them while keeping an eye out for predators. The view is much better in front of a rock, and the front of a rock is a gourmet deli compared to the soup kitchen behind it. Don't just look for trout in relation to single rocks. Look for places where a series of rocks produces the ideal spot with depth, uniform current, and a place to dart when danger threatens from above.
Logs that stretch across a stream give you the same results. This makes presentation difficult because most small-stream anglers prefer to work upstream, and getting a fly above a log and obtaining a decent drift, if the brush will even let you cast in front of a log, is not fun. But I assure you that more trout will feed above the log—or at its outside point if it does not stretch across the river channel completely—than behind it. Logs that lie along the bank or in midstream parallel to the current are much easier to fish and to read—you know trout will be lying along the log, and will favor the side of the log closest to the thalweg because that's where the food will be.
Many small creeks, like meadow streams, boggy, lowland streams, or spring creeks, may have few rocks or logs to help you read the water. In these streams, the river channel itself provides most of the clues. Look for a narrowing of the stream channel, which speeds up the water and forms riffles. Riffles provide cover for fish because the distorted surface prevents fish from being seen, and riffles are the food producers in a stream. Where a riffle ends, you'll find a pool, or if not a pool at least a slowing and deepening of the channel. The lip at the downstream end of a riffle provides a perfect place for trout to rest and feed.
Bends in a lowland river will also stimulate a quickening in the current, whether it is manifested in riffles or not. At a bend, on the outside (or concave) part of the stream channel, the faster current is pushed up against the bank and will form deeper undercuts that shelter trout and bring food right to them.
As the current lessens below the bend, you will find a shelf that stretches across the stream where the slower current has deposited gravel, silt, or sand. The decrease in depth here acts just like the cushion in front of a rock, and trout will often lie on the boundary between the shallow and deep water, resting comfortably, but ready to dart into the deeper water or undercuts just upstream at the slightest hint of danger.
I live on a small stream that runs through large and productive cornfields in a narrow valley. Over the centuries, the farmers in the valley have straightened the stream channel so that it doesn't stray into their valuable cropland and instead runs straight against one side of the valley. My property is one of the few places where the stream has been allowed to regain some of its natural meander, and because the stream takes a severe bend in front of my house, it is one of the most productive places on the creek. Just below my house is a long, straight piece of stream that runs for about a half mile before running around another bend. I have tried that straight piece of water again and again, and although a few small pools form where the stream makes a slight bend, it isn't until the next bend that the fishing gets decent.
Once I leave my property I'll walk a half mile to fish the next bend instead of wasting my time in the water just downstream of my house.
Because you sometimes cannot see all the rocks and logs in the water, plus you can't always tell which ones are better than others, it may be better to fish places in a pool rather than looking at specific obstructions on the bottom. A pool in a stream is any piece of water initiated by a riffle or waterfall, followed by a slower, deeper midsection, and ending in a smooth, shallow tail where the current is concentrated. The middle and head of a pool are the easiest places to fish and the ones most people cast to immediately. The middle of the pool has the deepest water, and you already know that depth is important in a shallow stream. The head of the pool has the fastest water, so you can make more mistakes with your casting because a bad cast is veiled by the riffled water and fish have to make a quick decision where the water runs swiftly.
But the tail of a pool, the place most people cast over to get to the middle or head, often holds the most and biggest fish in a small pool. It is a trickier place to cast because the water is accelerating and drag often takes over immediately. The water is shallower than the middle so the fish are often the spookiest ones in the pool, but in the tail the water is smooth, so it's easier to spot drifting food, and the current is narrowed both vertically and horizontally, so fish get more opportunities to feed.
Many times fish that hide in the deeper middle of the pool, where food is scarce, drop down into the tail to feed, and where the tail of a pool has big rocks or a log or a shelf for protection, trout may live there all the time. If you pass up the tail you're going to miss some of the best opportunities of the day.
In the middle of the pool, look for that main thread of current and then look for the larger rocks that will provide a good place for a trout to rest and feed. If you can't see the rocks on the bottom because of light conditions or water depth, look for the bump on the surface that betrays the presence of a rock beneath it. Just remember, though, the bump will be about a foot or two downstream of the position of the rock in this shallow water, and trout might be lying in front of the rock. So cast your fly at least two feet above the surface bump.
At the head of a pool, the water in the main current may be too fast for trout, so look along the edges of the fastest water where the surface smoothes out a bit. The seams on either side are almost always the hotspots in a small stream. However, in meadow streams, where the current is slow enough that the center of the main flow is just a gentle riffle rather than a confused torrent, you may find trout right in the main flow as well as along the edges.
You should also look for tiny flows off the main current at the head—many times big rocks at the head of a pool concentrate most of the flow down the center or off to one side, but secondary flows of just the right speed might hide small, deep pockets with just the right flow. I know I often focus so much on the main current that I miss these secondary flows until I wade up through the pool to approach the next one, look off to one side, and then mentally kick myself for not taking off the blinders. By that time I've spooked any fish in these hidden pockets, but I try to remember them for the next trip.
You'll sometimes find stretches of water in a small stream that offer few distinct pools. The water runs over fields of boulders, but never settles down enough to form a classic pool. This is some of the most exciting water to fish because trout can be almost anywhere, and because the current is uniformly disturbed the fish are easy to approach and you can relax a little without worrying about your every step. You will probably still see a thalweg, but it will often be split up into several threads, or there may be side pockets that offer almost as much depth and flow as the main channel.
In swift pocket water, one of the best ways to locate trout is to look for dark lenses of smooth water between the areas of white foamy water. White water is probably too swift for trout, and the fish have problems seeing food (and your fly!) in the lathered turbulence. A slick, darker surface tells you the water there is slower and deeper than the foam around it, and trout lie in these hidden slots.
In pocket water you can go crazy trying to fish every spot that looks fishy, so you have to look at the whole streambed instead of concentrating on a single rock. Look for places where two current threads join, or where a series of rocks forms a miniature pool, in a deep slot between rocks, or where the stream takes a bend and concentrates flow and depth on one side.
Tom Rosenbauer is vice president of marketing with The Orvis Company of Manchester Vermont. He is the author of many important books including the best-selling The Orvis Fly-Fishing Guide which was revised and updated in 2007.