Perhaps the biggest small-stream secrets are undercuts, those dark places where water carves at its imprisoning banks, or pools almost jelly-like against tight corners. Such dark recesses hide many things, including fish. If approached with forethought, undercut secrets can be revealed, and sometimes they will change entirely the way in which you view small streams.
My personal undercut revelation came when I was in my single digit years. It was during one of many July days spent on the Vermejo Park Ranch property in northern New Mexico (USA). Our family was fishing Costilla Number One creek and I had wandered off toward some inviting-looking cut-banks that I had seen in the distance. At my father’s prodding, I had switched over to a nymph and split shot rig, rather than my usual bushy dry fly. A few minutes of cross-field trudging and a quick bit of rock-hopping brought me to a grassy elbow in the creek. The far side was deeply sliced, overhung, foam-flecked and blackish-brown with depth. My youthful imagination, fired-up by being alone, made it seem almost foreboding.
The first pitch with my nymph brought a rambunctious brookie in the ten-inch class. It was a tiny treasure to be savored and quickly erased any trepidation that may have lingered in my psyche.
My second cast drifted farther than the first, with the nymph hunkering down and the currents drawing it back into unseen recesses. The indicator quivered and I lifted. For a moment there was nothing but a bent rod curving along a taut line. I had snagged one of my most-despised undercut nemeses, a root ball. Then the root ball began to swim. Within seconds I realized that I was attached to something much bigger then I had ever expected to be attached to in such small water. When I at last brought the obstinate root ball to my shaking hands, I discovered that it had been transmogrified into an 18-inch rainbow. The seemingly out-of-place fish had come up from the reservoir below and secreted itself in that undercut, awaiting an eager young boy and his faux nymph.
The magic of the day was not over, though. My fish was soon to be eclipsed by another Costilla undercut rainbow that fell to my father. His prize was a colossal one, the “holy grail” of my fly-fishing youth: a trout that measured a real 20 inches. I had taken fish in the Vermejo lakes that taped (oh-so-carefully, precisely and hopefully) at 19 1/2 inches, but never 20. I was stunned, and never again would I approach small streams and simple undercuts in a presumptuous and lackadaisical manner.
Undercuts can seem a difficult nut to crack. After all, to fully explore them, you have to get back under the cut. One trick to achieving that is to use the currents to present the fly—a sinking fly—for you.
While one can deliver a fly from a variety of angles, I'll concentrate on a down-current approach here. The idea is to cast down- or down-and-across current into the current tongue that feeds the undercut. The tip of the fly rod is not lowered to the water upon delivery, but is instead angled high. The high angle of the rod keeps a portion of line off the water’s surface, and holds the fly near the water’s surface, allowing you to align the fly carefully in the currents. The rod tip is then lowered, allowing the imitation to sink and move down-current into the recesses of the undercut. Once the fly has dropped into the dark, it can be actively worked using the tip of the fly rod and/or line retrieval.
When searching for secrets in undercuts, you may find many things. Some may be no more exciting than an old root ball, but maybe, just maybe, you will find your own small-stream “holy grail.”