Trout eat under the surface far more often than on it, and the indicator nymph rig is the tool most anglers use to reach them. It is also the rig most people fish badly, because the indicator hides the two things that matter, depth and drift, behind a bobber that looks fine even when the flies are riding three feet above the fish.
The rig, and why each piece is there
A standard indicator setup: 9 foot leader tapered to 4X, an indicator, then tippet to a heavier fly, then 12 to 18 inches of finer tippet to a smaller dropper. Split shot goes on the leader above the first fly, or you skip the shot and use a weighted point fly. Every part adjusts.
The indicator is a depth tool and a strike tool, in that order. Yarn indicators are the most sensitive and land softly, which matters on low clear water. Foam or plastic bobber types float more weight and are easier to see in chop. Use the smallest one that will suspend your rig; if it goes under on its own it is too small, and if you cannot see a take it is too buoyant for the weight below it.
Two flies is standard: a point fly that carries weight and a size 18 to 22 dropper that does not. On a tailwater like the San Juan below Navajo Dam that might be a red San Juan Worm or a gray scud on point with a size 22 Zebra Midge trailing. On a freestone like the Gallatin in the canyon, a Pat's Rubber Legs or golden stone nymph in 8 to 10 on point and a Pheasant Tail behind it.
Depth is the setting you adjust most
The rule most people learn is indicator at one and a half to two times the water depth. It is a starting point, not a setting. Current speed pulls the rig downstream of the indicator, so in fast water the flies ride shallower than the leader length suggests, and in slow water they hang closer to straight down.
The actual test is contact with the bottom. If you never tick rocks, you are too shallow. If you hang up every third drift, you are too deep or too heavy. Start shallow and lengthen or add weight until you touch bottom occasionally, then back off slightly. Moving the indicator six inches changes the fishing more than a fly change does.
Adjust for the water in front of you. The deep buckets of the Lower Provo in the canyon may want the indicator at seven feet with two BB shot; the riffle above wants three feet and one small shot. Anglers who leave the rig set all day catch fish in one water type only.
Weight: shot, beads, and putty
Split shot is the fastest way to change weight and the easiest to lose. Put it 8 to 12 inches above the first fly with a tippet ring or knot below it to stop it sliding. Tungsten beads on the fly sink faster for their size and never slide, but commit you to a weight for that fly. Tungsten putty is quiet and molds to any size, and it comes off in the cold. Whatever you use, the flies should reach the fish inside the first third of the drift; if they are still sinking as they pass, add weight or cast farther upstream.
Reading the drift
The indicator should move at the speed of the current it sits in, or slightly slower, because the flies below are in slower water near the bottom. If the indicator races ahead of the bubbles beside it, the line is dragging the rig; mend upstream. If it stalls or wobbles, the flies are ticking bottom or a fish has them. Set on any hesitation. Hooksets are free.
Cast upstream at about 45 degrees, let the rig sink, and follow the indicator with the rod tip. Mend as soon as the line lands, before the current gets hold of it, then leave it alone. Fish the drift all the way past you and let the flies swing up at the end; the rising fly imitates an emerging insect and takes fish, especially with a caddis pupa or soft hackle in the rig.
Work lanes methodically. Start close, extend a foot or two each cast, and cover the seam between fast and slow water with several drifts before moving. A drift that is a foot off is a drift the fish does not see.
Tight-line as a contrast
There is a second way to nymph that drops the indicator and keeps the line off the water, holding a straight connection from rod tip to flies. You feel the take rather than see it, and the flies get to the bottom faster because nothing on the surface holds them up. That method has its own page: Euro nymphing. Here is the short version of when to choose each.
The indicator rig wins when you need distance, when you are fishing a long run or a slow pool where the flies drift a long way, when you are in a drift boat, and in wind. Tight-lining wins in pocket water close to your feet and in fast broken water where an indicator gets pulled every direction. Many good anglers carry both and switch mid-day.
Fly choice matters less than you think, until it matters a lot
Most days a size 16 Pheasant Tail, a Hare's Ear, a Prince, and a Zebra Midge in 18 to 22 will cover it, because they look like enough things. Match size and rough profile first: a small dark thing for midges and Baetis, a bigger buggy thing for stoneflies and caddis larvae.
Tailwaters with a dominant food source are the exception. The Frying Pan below Ruedi feeds its rainbows on Mysis shrimp, and a white size 16 to 18 Mysis pattern out-fishes generic nymphs there. The Bighorn and the Missouri run on sow bugs and scuds in 14 to 18 year round. Check the river page for what actually lives there.
Setting up on new water
Estimate depth in the deepest lane. Set the indicator at about that depth and a half, put on a weighted point fly and a small dropper, and make three drifts down the near seam. No bottom contact, add a shot. Still nothing, slide the indicator up a foot. Once you tick bottom, back off one shot and fish. Move the rig, not just the fly, every time the water changes.