The most important aspect of the retrieve for river fishing is to keep in contact with your fly and to eliminate slack in the line. Dry flies, particularly sedges, can be twitched and skidded across the surface to imitate the way sedges often run across a river. Nymphs can be fished on a dead drift, at the same speed as the current, or stripped in a little faster to represent those nymphs that are stronger swimmers. Raising or lowering the rod tip can be used to adjust the height of the nymphs in the water, or the depth at which they are fishing if you like.
On fast rivers you can fish streamers, for example, across and down and by mending the line downstream and retrieving quickly, get your fly to swim fast and aggressively in a wide loop.
When dry fly fishing it is very important not to tighten too quickly when a fish rises to your fly. If you do not allow the fish to take the fly into its mouth and start to turn down, you risk pulling the fly straight out of the fish's mouth. This results in a disappointed fisherman and an unhappy and frightened fish. When a fish makes a frantic grap or leap for your fly it is only to ease to respond in kind with an over-quick "strike". If a fish rises in a slow and stately fashion, it is much easier to give it that little bit of extra time before tighening your line.
Induced take for upstream nymphing
The induced take can be deadly when nymphing in running water. The object of the exercise is to make the trout think that the nymph that he is about to eat is going to escape. This is done by raising the tip of your rod, when your nymph is within range of a trout, just enough to make the nymph lift a few inches. How soon you have to raise your rod tip, and by how much, will depend on the speed of the current: sooner and higher for fast waters and later and less for slow water.
Something that happens on all waters is the fish that follows your fly right to the bitter end. Just as you are about to lift off and cast again you spot a fish at your feet as it makes a grab for your fly. This can be a form of induced take. Particularly on stillwaters, it is always worth pausing at the end of a retrieve to see if a fish has followed the fly. A pause or slight lift can often do the trick.
Whether you are fishing on a river or a stillwater, and whatever your choice of retrieve, you will not connect with and catch fish unless you concentrate all the time. Subtle takes may be indicated by just the slightest tweak of your line or the sensation that you have caught a bit of weed. Every time you see a slight movement or get a funny feeling holding the line, do tighten as it may be a fish. Every now and then it will be weed and a lump will fly past your head. But the next time, when you think you have caught some weed, it will be a fish.
Whether fishing stillwaters or rivers, it is all too easily to fish in a mechanical, repetitive way without thinking about what the fish are doing or what your flies are supposed to do in or on the water. If you are not catching fish, stop and have a think and try something different. Very often a different style or speed of retrieve can help put a fish on the end of the line.
