Nymph fishing is a bread and better technique when fishing Montana rivers. The vast majority (over 90%) of a trouts diet comes from subsurface sources such as immature insects, crustaceans and other fish. Day in and day out nymph fishing is often the most effective technique for racking up numbers. I would estimate that at least 70% of the time in the summer nymphing puts more fish in the boat than dry fly fishing or streamer fishing. In the winter it is closer to 100%. So while hanging some nymphs under a strike indicator with a few split shot may not be the most glamorous of the fly fishing techniques; most anglers spend a significant amount of time using this method since it is so effective. Although my favorite fly fishing strategies are dry fly fishing or streamer fishing, I still end up nymph fishing quite a bit. I don't mind catching half as many trout as my buddies if I am dry fly fishing and they are nymphing, but when the ratio drops to 1:3 I usually throw in the towel and rig up some nymphs. Which nymphs to use and when is a complicated topic and probably the subject of some future posts. What I would like to focus on now is how much weight and how much leader to use when nymphing.
Tight line vs. dead drift
Most insect nymphs and larva are not good swimmers when they are drifting naturally in the current. You will read a lot about getting "natural dead drifts" when you read fishing rags or books about nymph fishing. Sometimes it is an absolute must to get a perfect dead drift. I find that when nymph fishing tail waters like the Bighorn or Missouri; or spring creeks like DePuy's it is essential to get a dead drift. When dead drifting it is imperative that the flies have zero drag from the line and leader. Volumes have been written about casting and mending techniques that produce a dead drift. Slack line casts, reach casts, curve casts and stack mends all have their place when dead drifting. Not all aquatic food sources drift naturally when dislodged in the water. Some insects like caddis flies are good swimmers when they are emerging. Other food sources like crayfish and sculpins are frequently darting along the river bottom. To imitate these food sources some motion on the flies is beneficial. There are a few ways to add motion to your flies including stripping, swinging and tight lining. Stripping adds a lot of motion and is usually used for streamer fishing and swinging is often used to imitate emerging caddis pupae (I'll save those topics for another day). Tight line nymphing is a more subtle way to add motion and it has several advantages. I prefer to tight line nymph on rivers such as the Gallatin, Madison and Yellowstone that have a large population of sculpins. The lower reaches of these rivers also have a large crayfish population. When tight line nymphing simply keep your line "tight" without trying to strip it or drag them through the water. The result is just the slightest bit of motion on the flies. This motion often catches the attention of trout and produces a more realistic imitation of sculpins and crayfish. It also produces a higher hook up rate since there is little chance a trout can spit the flies out since you are already tight. Tight line nymphing is especially effective when float fishing.
The length of your leader is an important part of successful nymph fishing. I prefer to use a shorter leader (5-7.5 feet) when float fishing after runoff when the fish are tight against the banks. Shorter leaders make the casts under the willows more accurate and result in fewer flies lost. Most of the time I default to a 9 foot leader when nymph fishing. 9 foot leaders give a nice blend of range of motion on the flies, enough depth to get where you are going and are short enough to still control flies when casting. Long leaders of 12-20' are generally used for deep nymphing when a dead drift is required on tailwaters while float fishing. When water is still high on the Bighorn or Missouri we go to very long leaders in order to get down to the fish.
Indicator height and split shot
When tight line nymphing I prefer to either use no indicator, or an indicator at the very top of the leader. Since you have a tight connection to the flies, an indicator is not important as when dead drifting (but can still be important). When dead drifting I vary indicator height and split shot to get to the depth zone that I am targeting. One of the biggest mistakes anglers can make is to always fish directly on the bottom. While dredging the bottom is sometimes required, like when fishing some tailwater runs or during cold water months it isn't always the best depth to fish at. When fish are active and on the feed they will move far for your flies. By fishing higher in the water column your flies are more visible to more trout and you will get less tangles. Split shot is more difficult to cast resulting in more birds nests and of course sometimes you snag flies on the bottom. By fishing farther off the bottom you spend more time fishing and less time untangling and re-tying. For fast water you need more leader below the indicator and more weight. If you are fishing a fast run that is 3 feet deep and you want your flies near the bottom you may need to set your indicator 6 feet above the flies with 2 or 3 split shot. For slower waters you may need to be just 1.5 times longer than the depth of the water and can use a bit less shot if you are trying to fish near the bottom.
Nymphs just below the surface
One of the best places to fish is just a few inches below the surface. One popular way to achieve this is to drop a nymph below a dry fly using a "dry dropper" rig. I also like to use double nymph rigs just below the surface, sometimes with very large nymphs. A large unweighted zonker or sculpin with a weightless small nymph trailed behind is useful for probing shallow riffles that large trout feed in. On many Montana fly fishing trips anglers skip over water that is 6"-2 feet in depth when nymph fishing which is a big mistake. Another technique is to fish small nymphs just a few inches below the surface during a hatch with a very small tuft of yarn used as an indicator about 18" above your top fly.