ocks always hold trout in streams and rivers. Island rocks on the Madison River are like normal rocks on steroids and are probably the single best type of trout habit found in a river. Island rocks are special types of medium to large rocks that are surrounded by much smaller sediments like sand or gravel. Island rocks are easy to pick out because they look out of place and stick out like a sore thumb. These rocks look like an island in an otherwise nondescript section of river and draw trout like a magnet. Although large rocks are encountered with great abundance, they are not island rocks since they are surrounded by numerous other large rocks. Island rocks are usually found where the most common size of sediment is smaller making them much more unusual. Island rocks can be deposited in several different ways. On the Madison, huge car or house sized boulders were picked up by large glaciers and then dropped in random locations when the glaciers melt leaving behind “glacial erratics”. On other rivers other oversized rocks were deposited during huge floods that occur when natural dams breached in the past or when epic floods occurred during hurricane driven rainstorms. These massive floods have enough power to roll massive rocks down the river that are then locked in place for hundreds or thousands of years afterward. Finally, large rocks are sometimes deposited by heavy machinery to purposely improve trout habitat during stream improvement projects.
To understand why fishing the Madison river’s island rocks provide such outstanding habit for trout, you must first understand what effect these rocks have on both the current in the river and the sediments that make up the river bed. Current in a river can be thought of as travelling in different parallel lanes down the river just as there are multiple lanes on an expressway. On a fairly uniform section of the riverbed most of the current lanes will be travelling at the same speed. When a diversion such as a large rock is encountered current lanes need to split and travel in a longer curved path around the rock. Since the diverted lanes of current are now travelling a longer path to get downstream the speed of the water in those lanes must increase to keep up with the other lanes in the river that went a shorter distance. The disruption of the normal current flows provides a variety of different water velocities around the rock. At the upstream end of the rock water speeds up as it splits to travel around each side of the rock. Water continues to speed up and reaches the highest velocities on each side of the rock that form to slots that travel downstream. There are also areas of the water protected by the force of the current that produce slow water eddies. The largest eddy is directly behind the rock, on large car sized rocks the eddy may extend for several feet behind the rock. The currents just below the rock are moving very slow and often travel back upstream. There is a much smaller eddy that extends just at the front of the rock caused by some of the water riding up on top of the rock itself and slowing down. This pillow of soft water in front of the rock may be too small to hold fish on smaller rocks but often holds large rainbows on large car sized rocks. Several feet downstream of the rock the eddy ends and begins to blend with the two current slots forming a slow moving mixing zone of water with flat currents called a slick (due to the “slick” look to the surface of the water).
The wide range of current velocities make rocks very attractive to trout since they can access both resting and feeding areas in a very short distance around the rock. The variety of currents also helps to produce a nice blend of water depths around rocks. The combination of assorted current speeds and water depths make these locations ideal habitat for trout. High velocity currents scour out smaller sediments like gravel and sand while low velocity currents drop the sediment load that they carry. The smaller gravel and sand sediments that surround large island rocks are easily shaped and molded by the disruption in current that the obstruction provides. The zone of fast water at the front and sides of the rock dig out a “U” shaped depression that wraps around the upstream side of the rock and extends downstream in two parallel troughs on either side of the rock. The slower currents behind the rock allow sand and fine pea gravel to fall out of the current producing a shallow gravel bar that start a few feet downstream of the rock.
The wide variety of currents and water depths that surround large island rocks and the lack of other structure in the rest of the river attract trout at all water levels. Techniques for fishing these large rocks very with water levels.
High Water Conditions
When rivers are swollen from spring rains or just receding from the snowpack runoff most of the water in the river is moving to fast for fish to hold in. Trout become concentrated in the softer slow water wherever it exists. These conditions exist right against the bank in a zone that is often just a few inches, behind small points that extend into the river from the banks, downstream of islands and gravel bars and behind large rocks. One of my favorite targets while during high water on rivers like the Madison is to hit the large slicks behind large Volkswagen bus sized boulders. When float fishing these rocks come up fast and anglers have an opportunity to make one or two casts in the slick behind the rock. The most effective strategy is to cast to the opposite side of the slick keeping the rod as high as possible to lift the fly line out of the swift main current. This is generally a subsurface game unless there is a hatch. Once the flies are in the slick you get a short dead drift. We often fish a two fly nymphing combination with a streamer trailed by a more traditional nymph about 18 inches behind. Since the boat tends to be moving past the rock quickly eventually the flies get pulled out of their drift and start swinging at a downstream angle through the slick. The majority of strikes will occur just as the flies begin swinging out of their dead drift. If the fish seem to be really aggressive we often switch completely to streamers. I like to cast the streamers completely across the slick then twitch them into the opposite end of the seam and then slowly strip retrieve the fly in across the slick. Don’t mend streamers when fishing rock slicks, let the streamer go from a slow twitch to a frantic dart as the belly in the line from the main current starts to drag on the fly.
Some rivers can still safely be wade fished even when high and swollen. There are often large rocks that are close enough to the banks to access with a small amount of wading. I like to position myself at about a 45 degree angle downstream of the rock and try to get close enough to the slick so that I can high stick the fly rod to keep line out of the heavy current. I methodically work the slick and the seam where the slick and the fast slot current water mix. I like to start at the top of the slick and slowly walk or wade my way downstream fishing as I go. Just as when float fishing, many takes will occur as the flies just begin to swing out of their dead drift (when fishing nymphs).