Guest Blogger: Mike Cline, Bozeman, Montana

In railroad parlance, being side-tracked means you’ve been shunted off the main line onto a siding so that the fast trains can get by. As a noun, sidetrack is commonly known as a “minor path or track”. Although on the river, we anglers know it is easy to get “sidetracked” by all manner of things. Wildlife, tangles, weather, bugs, agonizing over fly selection, cold fingers, you name it. When your fly isn’t in or on the water, you’ve been side-tracked. However, getting side-tracked by an actual sidetrack can be a good thing.

A large majority of our larger Montana rivers have natural sidetracks that can be taken advantage of in the right circumstances. In fact, in my angling experience, rivers in the Southeast and Atlantic coast (as do most large to medium sized rivers anywhere) have sidetracks. In my experience, river sidetracks take on two forms, both equally valuable to the angler. The most obvious is the small natural channel that leaves the mainstem and flows some distance before returning to the main river—the true “side channel.”. The other is the natural trench or trough that lies adjacent to the main flow, but is separated by shallow areas. In both these cases, the sidetracks can be identified by the presence of an island of sorts that separates the minor flow from the mainstem. In rivers where the flow regime varies seasonally from runoff, the flow in these sidetracks varies as well. From an angling standpoint, sidetracks should be approached just as you would a small stream, because in fact, that’s exactly what they are.

The productive sidetrack has two defining characteristics which the inquisitive angler should seek out. One is flow. The channel or trough must have sufficient flow to keep the water cool and provide a constant stream of food for the fish. Typically, in the Rocky Mountain west, flows in these sidetracks will vary from non-existent in winter and early spring to peak flows during the height of runoff. Flows then tend to decline throughout the summer into fall. These sidetracks can become productive as flows rise in advance of runoff, but their most productive period is the first few weeks after runoff begins to clear which I’ll explain a bit later. The second characteristic the angler should search out is depth. Sidetracks should have areas of sufficient depth to allow fish security. As with most small streams, there is always a lot of unproductive shallow water and fish tend to congregate in deep pools and runs. Find sidetracks with good flow and areas of depth and you’ll generally find fish. But unlike most small streams, the big river sidetracks have advantages anglers should relish—bigger fish.

In the early spring, rivers are at their lowest flows. Many sidetracks may have no flow at all or so minimal that no self-respecting trout would venture near it. As the rivers rise fish may venture into a side track, but typically flows rise so fast during early runoff that anglers may not have much opportunity to seek out these productive small streams. The contrary is true during the post-runoff period. During runoff, fish will have sought out shelter and refuge along edges, many of which will morph into sidetracks once flows subside. As the flows clear, the sidetracks become productive little streams holding lots of bigger fish, far more than you’d expect in an isolated stream of the same size. Depending of how fast flows drop into summer, these fish may remain in the sidetrack for weeks. Once flows decline to a certain point, these larger fish will migrate back into the mainstem of the river.

Recent experience on both the Madison and Big Hole rivers, validated, at least for me why seeking out sidetracks is productive. I was hosting another angler from Florida. We were fishing the Valley Garden section of the Madison. At a sharp bend in the river, a side channel took about 10% of the flow but was separated from the mainstem by a very large expanse of shallow gravel and weed. Flows were still up at 2000CFS. At the more typical summer flows of 1000CFS, there would be minimal flows into the channel and the large weed flat would be exposed. Even so, for most of the channel’s length—700 feet—there was little deep water. But at the head of the channel—some 150 feet from the mainstem, the current pushed tight up against the bank creating a deep stream less than three feet wide but more than two feet deep. As the current left the bank, a small pool was created before the flow spread out across a shallow riffle. A well-placed cast by my friend nailed a fat 17” brown. Because flows were up, numerous small side tracks throughout this section of the Madison produced nice fish over the course of the morning.

Unfortunately, I’ve seen many anglers completely ignore these sidetracks, instead focusing all their attention on the mainstem. I am sure they are passing a lot of fish. Even drift boat anglers rarely slow down long enough to tackle these small sidetracks that are not all that uncommon but hard to reach during early summer flows. However, my favorite sidetrack is the true “side channel” as these are places generally untouched by the angling public.

The channel at the top of the aerial image of the Big Hole is about ¾ mile long. Because access across private land is impossible without permission, this sidetrack probably doesn’t see another angler all season. Drift Boat anglers likely just pass it off as an unproductive backwater that would take far too long to explore on foot. The long straight stretches are very shallow and don’t hold fish. Yet at every significant turn a deep pool or run is formed—five or six of them over the ¾ mile length. At 975 CFS, when I last fished it, this side channel had a lot less water than conveyed by the image which is a few years old. Flows had been upwards of 1800 CFS in the few weeks previous. As the Big Hole drops to its more typical summer flows of 500-750 CFS fish will move out. In the next image, one can see that even at 975 CFS, this side channel had limited time left to maintain sufficient flows to hold fish. Yet at 975 CFS, those holes held fish. In the hour and a half, it took me to fish this sidetrack, at least 15 fish came to hand with three that exceeded 18 inches. Those big fish most likely entered the channel during runoff and would remain there even as flows cleared if there was sufficient flow to provide shelter and food.

Unless there is lots of room to maneuver sidetracks should be fished just like you’d fish a small stream—moving upstream slowly, drifting nymphs or dries through the deeper pools and runs. Many sidetracks form deep undercuts at sharp turns or pool tail outs and this is where the bigger fish will reside in these tight quarters. Every river is different. Anglers seeking out sidetracks should review aerial photos to identify potentially productive side channels. Anglers should visit their favorite rivers at different flows to look for those instream sidetracks that might dissappear as flows subside. Most importantly when you encounter an obvious sidetrack whether it be instream or side channel don’t hesitate to explore it as you would any “small stream.” Look for those defining characteristics—sufficient flow and areas of depth. Don’t pass them up, no matter how far they might be from the mainstem. They might hold the best fish of the day—get side tracked.


  1. Sometimes I wonder about fish migration, and how they know. For example, in periods of drought, fish can hunker down in holes that have some depth and still some water; if the drought continues they eventually abandon such places, seeking deeper holes…and I guess they’d do so at night. But how do they know there are better lies? What if they get caught downstream, unable to get back to the hole they were in but unable to find anything else deep enough?

    Or as you say, fish in “side-track” areas (alternate channels) eventually migrate back to the main channel as the alternate lie loses its depth or flow. But what if they get out, fully committed, and there’s nothing?

    And yet I never see quantities of trout stranded in shallow water. Maybe they all get picked off by raccoons and hawks when they do that, or maybe they rarely get stranded.

    If they don’t tend to get stranded in shallows, how do they avoid it? How do they know there’s better water, and in what direction? I often wonder if, during their movements when water is plentiful, they might instinctively accumulate a kind of “map” of the portions of the river they’ve swum, complete with depths and inlet/outlet flows and such…so that they have a 3D plan of sorts for when conditions deteriorate?

    Antlered mammals have migration routes stored somewhere deep in their brains, and those routes still adapt when necessary. Trout brains are only about the size of a pea, but still they’re probably big enough to contain some kind of 3D awareness of the options around them. Obviously they can’t climb a tree to look, so some kind of awareness has to be available to them. I don’t think it could all be just trial and error, or we’d see a lot more evidence of stranding.

    I suppose such ability could be aided by sonic data (for example if they could “hear” fin motion in a certain direction, signaling the existence of other fish surviving thataway) or perhaps thermal data of some kind. Just speculating wildly here.

    There’s so much of their abilities and adaptabilities that we don’t understand.

    – Mike

    1. Mike,

      I think when it comes to side channels where the flow regime is seasonal, the real keys for the fish are flow, water temperature and availability of food. When any one of those conditions are not optimum, the fish will seek a new location. Seasonal flow regimes in medium and larger rivers don’t change instantaneously, but go through slow, but steady transitions. A side channel never empties overnight. As flows lower in the mainstream, the flow eventually reaches a level where it no longer spills over the “dam” that defines the head of the channel. But that usually occurs over a period weeks. At some point the flow into the channel will subside to the point that water temps increase and sufficient food is no long flowing into the channel. I would think fish can sense this and would move. But where? As long as water is flowing into the channel, it will be flowing out the other end providing a easy escape to main stem. There is no “dam” at the lower end of the channel. This must be instinctive for the fish. While flow and temperature are obvious keys, I think food is the main key. In side channels that go seasonally dry, there is a very limited source of typical aquatic insect life. You don’t get great hatches of stoneflies, caddis or mayflies in side channels that go dry. That food in nymph or pupa form has to flow into the channel from the main stem. When the flow reaches levels where food is no longer abundant, I suspect the fish just go with the flow and head back to the main stem.

      Although I’ve seen the occasional trout (and lots of fry) stranded in isolated pools, I attribute that to “bad luck”. It would not make sense biologically for it to happen with any magnitude given probability of very low survival.

      1. Interesting assessment. Myself, I would have assumed water temperature would be the primary move-on factor, unless oxygenation was proving an issue first–basically water quality. That would just have been my own guess though. I’d have based it on the assumption that fish would move out of a side hole even if there was sufficient terrestrial food there. But maybe not.

        It’s still a mystery to me how they’d know where to go…but they always seem to choose the best direction and end up in the best hole or channel, even if it happens to be upstream. if a side channel has one inlet and one outlet, perhaps there might not be a lot of choices…as you describe. And perhaps the default would be “downstream,” although that might not be a good default if water temp is the reason to vacate.

        I for one don’t know…but one way or another they must have a sense for the ‘when’ and the ‘where.’

        Anyway, interesting points about “side-tracking.” Thanks Mike.

        – Mike

Leave a Reply

Your email address will not be published. Required fields are marked *