Don’t Underestimate the Undercut

Exposed undercut along Yellowstone River

Exposed undercut along Yellowstone River

Guest Blogger: Mike Cline, Bozeman, MT

One of the primary skills any angler acquires with experience is “reading water”. The ability to observe a piece of water, especially moving water, and determine the most likely places to encounter fish when a fly or lure is presented properly is an essential skill for the successful angler. Without exception, authors writing about general fishing skills always cover aspects of “reading water.” Some water is easy to read, some isn’t. The fish themselves complicate reading because they move around from place to place in most rivers as they feed and rest. You know where they should be, but they aren’t always there. Of course every river is different but the formula is pretty consistent—Fish = Dark and/or deep (protection from predators) + relief from current (resting) or a current seam (access to food). In Joseph Bates 1974 classic How to Find Fish and Make Them Strike, this formula is consistent throughout his descriptions of the best places to find trout in rivers. In my experience, one of the most favorable parts of a river to find trout is the Undercut Bank.

cline undercut 2All sorts of rivers, whether they be high mountain streams, meandering valley rivers or tail waters large or small, will along some point in their flow, have areas of undercut banks. Most typically, undercut banks occur along straight river sections adjacent to deep pools and runs or along outside bends. An undercut bank occurs when the river cuts into the bank, removing rocks and soil while leaving some portion of the bank overhanging the river. Undercut banks generally are stabilized by the presence of vegetation and roots that hold the topsoil intact. Seriously undercut banks may be a foot or more deep. For the fish, an undercut bank provides a most favorable location to live and feed. Obviously, undercut banks are dark and provide perfect overhead cover to protect fish from predators such as Ospreys and Herons. They also provide respite from fast current as with any current, as it encounters the edges of the stream—bottom and sides—it slows. Apart from a great place to rest, undercut banks are also ideal feeding locations as they usually occur along areas of significant current. The trout residing in the undercut, secure from predators, only has to move a few inches to be in position to capture food in the current seam.

Undercut above and below collapsed bank on Firehole River

Undercut above and below collapsed bank on Firehole River

The Firehole River in Yellowstone is loaded with miles of classic undercut banks which are generally very easy to fish. My largest fish of any trip almost always comes from areas of undercut banks. The photograph on the left shows why undercuts can be so productive. As the river flows through Goose Lake Meadow a strong undercut formed along the Western edge of the river. At some point, the undercut became so deep the bank collapsed and formed this area of slack water. When PMDs and BWOs are about in good numbers, they will congregate in this slack water and fish will move out of the undercuts above and below to feed. However, when there is no hatch, there is almost always a hungry trout holding in undercuts. Islands in rivers can also be a great place to find productive undercuts. On the Firehole, islands such as the one in the photo below will have very deep undercuts at the upstream end of the island where the current divides. A well placed streamer or soft hackle swung around the point of the island can be very productive.

Islands can have a deep undercut at their upstream end

Islands can have a deep undercut at their upstream end

One of my favorite undercut banks is found along the Yellowstone River just upstream from Emigrant. In a fairly straight ¼ mile section the river’s flow pushes up against the western bank. The bank is lined with willows and grass. In early Spring and late Fall, the river is low and exposes the entire bank. Fish orient along a 2-3 deep feet cobble ledge several feet from the water’s edge. However, in early Summer as the river recedes from runoff, the water reaches to the top of the bank at the base of the willows. Now fish are orienting to the extensive undercuts along this bank. Hoppers or buggers/crayfish under indicators fished tight to the bank usually results in several nice browns every trip. As the river recedes, more and more of the undercut is exposed, but the browns are reluctant to leave its security. More than once, I’ve spooked a large trout resting in just inches of water at the edge of the undercut. The photo at the top of the post is just one spot along this Yellowstone River undercut which lies a foot or more below the top of the bank. Fishing this section many times over the course of a season has taught me that once you know where these undercuts are, they are productive spots even when partially exposed or completely covered. Trout, especially browns, will hold in these dark spaces as long as possible.

cline undercut 5The most productive way to fish undercuts to my mind is swinging streamers or other wet flies (soft hackles) while moving downstream. If you can safely reach the opposite bank with your casts, then placing flies within inches of the undercut slightly upstream of your position and swinging them along the edge of the undercut, you will catch fish. Too often, I watched other anglers make casts that just weren’t close enough to the bank. One foot can be too far. Unless the trout are being overly aggressive, they can be reluctant to leave the security of the undercut. Of course if you can safely wade upstream along an undercut, dry flies and hoppers can be productive as well as large streamers, nymphs or crayfish under a short indicator. However, trying to place floating flies tight to the bank while standing on the bank is difficult and unless you are extremely stealthy, walking along an undercut bank probably spooks more fish than you would think.

cline undercut 6Undercuts aren’t always as obvious as the rest of the productive lies in a river. Sometimes while reading the water, they seem like footnotes or fine print that can easily be overlooked or ignored. My experience on a short section of the Gibbon River above Gibbon Falls illustrates why undercuts should not be underestimated. Eight to 10 inch browns dominate this stretch of river. They are fairly prolific and eagerly rise to hoppers, caddis and other attractor dries. It is a fun place to fish, but wouldn’t be where you’d head for larger fish. The photo shows a typical deep run on the Gibbon. Fish can be found throughout the run. But the far bank, although significantly shallower than the middle of the run, is a deceptively deep undercut bank providing up to a foot of overhang for fish to hold in. This 13-14” brown, a large fish for this section of the Gibbon succumbed to a soft hackle placed at the edge of this undercut. Almost all the larger fish I catch in this section come from the undercuts, not the main, deeper parts of the run.

So as you fish your favorite rivers, find the undercuts and put your flies close, really close, to those dark, safe holding spots. Once you know where they are whether in low or high water, don’t underestimate their ability to deliver better fish who relish the safety of deep, dark spots along the edges of your favorite river.

One thought on “Don’t Underestimate the Undercut

  1. Michael Vorhis

    Terrific article Mike. We all try to get flies to go an inch from undercut “caverns” (despite the hinging that indicators can cause), and close is not easy.

    But I was most affected by your very first photo; it illustrates quite clearly why drifting the fly closer than close, actually under the bank and into the “cavern” itself, can potentially be a really bad idea. Unless we want to catch tree roots, that is. I think really close might be close enough. : )

    Again, excellent food for thought; thanks.


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