Improve Your Skill at Reading Water for Fly Fishing: Riffles
Updated: Feb 22
Riffles are one of the tougher water types on a river to read and decode in order to locate trout. The clues to where trout hold in riffles are typically subtle and anglers pass by many of these likely trout holding spots. However, as you'll see in the article and illustrations below, there are a few clues that you can scan for when fly fishing the riffles. Little by little, utilizing these insights will help you to identify "trouty-looking" riffles from riffles largely absent of fish.
What Exactly Are Riffles?
Along a river, riffles form when the slope (gradient) of the river bottom increases. As the water flows over a steeper gradient it shallows out and its velocity increases. Typically, there is also a lack of large boulders and most of the substrate (river bottom) is made up of small stones and pebbles. There may be boulders or submerged large rocks, but not quite to the extent that we see in pocket water.
On the surface, riffles look very choppy and chaotic with bubbles and tiny waves all over the place. The depth will usually be a few inches to 2-3ft of water. Take a look at the picture below, it shows a raft floating through the tailout of a run heading towards a section of riffles. In the run, the water surface is fairly smooth with one channel of rougher surface current (the main current). Downstream of the raft, there is a section of riffled water. Here, the rough surface water stretches from bank to bank and it is much shallower than the run upstream.
In the next image, the raft has now floated into the riffles. You can see that the surface is very choppy. There are a few larger submerged rocks, but there is a lack of large boulders to really break up the current to form ideal pocket water. Downstream, this section of riffles ends at the head of a pool.
Why and When to Fly Fish the Riffles
Riffles are mainly considered a summer-time water type (i.e. anglers primarily focus on fly fishing the riffles June to early fall while using a single dry fly or dry-dropper rig). The reason for this is that trout will move into riffles when water temperatures have warmed up enough. As the water temperature increases, a trout's metabolism increases as well. Once water temperatures have creeped up into the mid-50s (Fahrenheit) trout will begin to move into the riffles in order to feed more heavily.
Aquatic insects are more abundant in riffles because of the greater amount of dissolved oxygen, protection (all those nooks and crannies between stones), and food (not only other aquatic insects but also aquatic vegetation that relies on abundant sunlight). With all of those aquatic insects living in riffles, it is a great place for a trout to station itself to get a meal.
The water temperature really dictates when you should start considering fly fishing the riffles. Trout rivers all have their own unique characteristics, but normally when water temperatures reach 55F trout really seek out riffles. Having a thermometer with you is really important so you can check the water temperature. At times during the summer, the water temperature could be chilly in the morning (upper 40s), but as the day progresses and the sun heats up the river, the water temp could easily get into the mid-50s. Again, having your thermometer and checking the temps throughout the day can clue you in on these changes.
Increases in water temperature also spur aquatic insects to start moving up through the water column to hatch. During this time, they are more susceptible to trout. You can therefore think of a trout's rise and fall of metabolism being in conjunction with insect movement and hatches.
What to Look For
Reading riffles can be disorienting because the water all looks uniformly choppy and chaotic. However, there are certain components that we can train our eye to spot which will help identify where trout might be holding. A single one of these components may not hold trout, but start to add two or three of them together and you will begin to hook into a few fish.
The first component to good trout holding water in riffles are depressions. A depression is any spot where the water is deeper, and this is typically identified by darker water color and, at times, slower surface current. In riffles, depressions are usually smaller than what you might think of, and I will often call them divots when guiding anglers. These depressions (or divots) may only be large enough to hold a single trout, that is the size we are talking about. Other times, these depressions may be elongated, traveling for a few yards, and able to accommodate a number of fish. Depressions are one of the best indicators I use when trying to find trout amongst the riffles.
Depressions can be formed by larger rocks or collection of rocks that deflect the river's current, but many will just be subtle slots where the current has scoured away areas of the river bottom.
While depressions can provide additional security for a trout they more importantly offer a refuge from the fast current of the river. For trout to efficiently feed in riffles without expending excessive energy, they have to position themselves in areas that are out of the main current. Depressions are a great spot for a trout to tuck in and be out of the fast current, then all they have to do is tip their mouths up to take in an incoming morsel of food drifting by.
With all of this in-mind, let's look at a couple illustrations that show depressions in riffled areas.
The first illustration (Figure 1, click to enlarge), shows a profile view of a small depression, barely enough space for a single trout to station itself in. We can see in the figure arrows that denote the speed and direction of the current. Flowing higher in the water column is the fast surface current, and as the current reaches further down towards the river bottom it slows due to friction with the substrate (i.e. the rocks, pebbles, etc.). When the current passes over the depression, much of the faster current stays above the divot creating a cushion of relatively calm water in the depression. Our trout has taken advantage of this and has positioned itself further towards the head of the depression where the flow of current is least.
There is also one other thing to notice. As the surface current passes over the depression it slows down as well because some of the current is pulled down into the divot. Standing on the bank, we can see this change in slower velocity on the surface and make an assumption that there might be some depression underneath the water. The change in velocity can be very subtle and hard to discern, so that is why we also look to match it with darker colored water which indicates depth.
In the second illustration (Figure 2), we can see the overhead view of a few depressions. With all of these depressions, it is easiest to recognize them by the change in water color, in other words the darker the water color the deeper the depression. We also can pinpoint depressions where the surface current appears to slow down in relation to the surround water.
(Be sure to also check out: A Guide to Urban Fly Fishing)
Our trout in Figure 1 is located at Spot A. Another depression (Spot B) is longer and is able to hold multiple trout. In Spot C, the depression is too shallow (characterized by water color not as dark as the other spots) and is not deep enough to be worthwhile for a fish to stay in. The last depression (Spot D) is formed by completely submerged rocks, and in the rocks' slipstream (downstream) some the substrate has been scoured away forming our divot.
Training your eye to recognize depressions in riffles will take experience, but there are two quick tips that I can provide to help shorten your learning curve. The first is to have a good pair of polarized sunglasses which will help to cut out glare so you can see into the current and pick out those darker water spots.
The second tip is to get out in those riffles and wade around. As you wade, take note of spots where the water becomes deeper, then walk back on to the bank and look at that deep spot you just stood in. Look at and study it for a moment as well as the surrounding water taking notice of the change in water color and speed. Doing this will quickly train your eye to see beyond the turbulent surface water and read what is going on below.
The second component to "trouty" spots in riffled water we will look at are bubble lines. A bubble line is very useful indicator for an angler since it shows where current (and food) are being funneled. Bubble lines can be short (a couple feet) or span for many yards down a river, and they can be wide (a foot or more) or narrow (a few inches).
I think bubble lines are more valuable indictors in other water types (such as pools, glides, and runs for example). In places like pools and glides, there is a lack of available food drifting by, so bubble lines importantly point out where food is being concentrated. In riffles, on the other hand, there is a higher abundance of aquatic insects so food is tumbling by all over the place. With this in mind, while I look for bubble lines I don't focus on them nearly as much as I do with trying to spot depressions.
Just like looking for depressions amongst the chaotic surface chop of riffles, finding bubble lines can be tough to pick out at first. There are a lot of bubbles and churning surface water in riffles and we need to look for where bubbles calm down a little and begin to flow in a uniformed direction.
In the illustration below (Figure 3) we see a lot of bubbles on the surface of the water due to the riffles. However, there are two spots where bubble lines form more nicely. In Spot A, there is a piece of the bank that juts out and deflects the just enough of the current to form a better bubble line that flows right over a depression. In Spot B, a collection of submerged rocks/boulders helps to slightly constrict the current to also form a bubble line over another depression.
Transitional zones are where distinct water types (pools, runs, riffles, glides, etc.) begin to converge with one another. With riffles in particular, transitional zones can occur where riffles enter into a run, or where a glide shallows out, picks up velocity and transitions into riffles. Transitional zones are general areas to focus your attention because many trout will move into riffles to feed and then swim back to a nearby pool, glide, or deep run for security or just to rest.
Transitional zones are also great places to target when water temps barely rise up enough to energize a trout's metabolism and spur it to move into riffles to feed more heavily.
Early summer and early fall are two times that come to mind with this. During those periods, water temperatures can fluctuate throughout the day starting with cooler temps in the morning (upper 40s to low 50s), peaking in the afternoon (mid 50s to low 60s), then cooling back down into the evening. Since the water temperatures are waxing and waning, a trout will move from slower bodies of water (pools and glides) when the water temps are low and into riffles when the temperatures climb. During early summer, if you start your day off fly fishing the riffles when water temps are in the upper 40s to low 50s you may only have marginal success, but as the day progresses and water temperatures rise you can have better results by focusing your efforts on the transitional zones between riffles and pools/glides.
In the illustration below (Figure 4), there is a length of riffled water that converges into deep run. The trout in the run (Spot A) are either feeding or resting, but water temps have begun to climb up into the upper 50s. The warmer water temperature has energized the metabolism enough that a few trout have moved just up into the riffles to find a more substantial meal (Spot B). Warmer water temps may have also spurred some aquatic insect species to begin working their way up and out of the rocks in the riffles in order to hatch, and this provides an even greater chance for an easy meal. If you take note of water temps throughout the day you could be able to capitalize on this trout movement.
Our second illustration (Figure 5) shows the tailout of a pool and the head of some riffles. For this example, the water temperature in the afternoon is around 53F bumping up from 45F from that morning. Let's say we are working our way up the riffles and towards the pool. Since the water temp has just bumped up to the low 50s, I wouldn't expect the riffles to be stacked with trout but I would begin to focus some of my time in that area. As we continue fishing up to the pool, that transitional zone would warrant more of our attention.
The pool upstream of the riffle offers both security and a place to rest for the trout. As water temps cool back down in the evening, the fish might seek out spots where they can conserve their energy (like the pool). The closer they are to slower bodies of water, the easier it is for them to move back and forth to feed and then rest.
If we had limited success in the riffles, I would slow things down and take more time in the transitional zone between the riffles and the pool. We could really focus on targeting any depressions and bubble lines making sure to not rush past that zone and into the pool, because we could be missing out on some nice trout.
If you struggle to locate trout in the middle of riffles shift your focus to the banks. Look for spots where the main current flows closer to one side of the river or the other. Wherever the main current flows there is a better chance that it will also scour away deeper slots along the bank which can be holding spots for trout. There can also be parts of the bank that erode away and form an undercut section (see Spot A in Figure 6).
Overhanging tree limbs are additional cover for trout (Spot B in Figure 6). Any sort of shadow, whether it be from trees, bridges, large boulders, low sun angle, etc., can be a magnet for trout because it helps to break up their silhouette so avian predators have a harder time spotting them from the air.
These bankside lies, especially in lower water level months (like August), can be productive spots to prospect for trout because the water level may be too low throughout the rest of a riffled section. However, don't be drawn immediately to these bankside lies and skip out on the rest of the riffles, be sure to scan the riffles for depressions and bubble lines for likely trout holding spots.
Boulders, Logs, and Other Objects
Larger objects in the water like boulders and logs help to break up the current and provide trout holding spots. If the object is also large enough, it can provide security as well. Many of these objects may not be visible above the surface because they are completely submerged, yet completely submerged boulders and logs still help to scour away the river bottom in their wake which creates those depressions and divots that trout can station in.
In Figure 7, there are a couple trout tucked up close to a submerged log whose length is running parallel with the current (Spot A). Even though the log is laying parallel to the current, it still provides enough of a cushion for trout to hold close to and feed from.
In Spot B, there is a large rock that is also completely submerged and has created a nice trough downstream of it for a trout to hold in. Also, over at Spot C, there is some aquatic vegetation below the surface. This aquatic vegetation creates enough friction to slow the current down, it can provide security, and it may be chock full of aquatic insects like scuds and certain caddis and mayfly species.
Riffles are a fantastic place to target while fly fishing during the summer months. It is technical water that many anglers pass over in order to get to the pools and runs. However, some of the larger trout station themselves in the riffles in order to feed before the warm weather is through. Picking apart and working on reading the water can help you to appreciate and enjoy this technical water type and land a few nice trout along the way.
Be sure to also check out more articles from the Reading Water Series:
Reading Pocket Water Part 1 here: How to Fly Fish Pocket Water: Targeting Trout Around A Single Boulder
Reading Pocket Water Part 2 here: How to Fly Fish Pocket Water: Rock Gardens
Reading Pocket Water Part 3: How to Fly Fish Pocket Water: Casting, Mending, and Line Management
(And check out: 7 Useful Knots to Know for Fly Fishing)
Lastly, if you live in or are visiting Spokane, I provide guided fly fishing trips and instructional lessons around Spokane to include float trips on the Spokane River and stillwater trips across various Eastern Washington lakes. I guide with Fly Fish Spokane, and more information/trip rates can be found at the website: FlyFishSpokane.com