Every one of the 92 rivers on this site carries midges in its hatch chart, and on the two big desert tailwaters, the San Juan below Navajo Dam and the Colorado at Lees Ferry, the matrix reads "peak" in all twelve months. No other insect on the site does that. Midges keep a river fishable from November through March, when the mayflies and caddis have quit, and on dam-controlled water they are often the main course year-round. This page is about river midging: small flies, cold water, and trout eating a bug you can barely see. Stillwater chironomid fishing is a different sport with different flies and rigging, and it has its own page.
What a midge is, and why "size 22" is the whole story
Midges are true flies, order Diptera, family Chironomidae. They look like mosquitoes that don't bite. Thousands of species live in North American trout streams, and in a fishing sense they behave like one insect: a wormlike larva in the silt and weed, a pupa that rises to the surface with a bulge of gas under its skin, and a winged adult that skitters on the film for a few seconds before flying off to mate. In rivers the whole animal is tiny. The patterns listed on the rivers here run from size 20 down to 26. If you are used to a size 16 Adams, tie on a 22 and look at it for a minute. It is small. That is the point.
The three stages and what each one asks of your rig
Larva
The larva is a thin segmented worm, often red from hemoglobin (the "bloodworm"), sometimes cream, olive or black. Trout eat them when a flow change knocks larvae loose, which on tailwaters is every release change. Patterns are dead simple: a Zebra Midge is thread and wire, and the "Midge Larva #22-26" listed for Lees Ferry is barely more than that. Fish larvae deep, under an indicator or on a tight line.
Pupa
This is the stage that matters most. The pupa leaves the bottom, hangs in the water column, then drifts up to the surface, and trout pick them off the whole way. Gas under the pupal skin gives it a glassy look, which is why so many river patterns have a flash rib or a bead: the Mercury Midge, Disco Midge, WD-40, Black Beauty, the Jujubee Midge on the Missouri and South Platte, and the Top Secret Midge on the Green. Fish pupae from mid-depth up. In a two-fly rig, put a larva on the point and a pupa a foot above it.
Adult and cluster
Single adults are hard to imitate and see. What trout key on is the mating cluster, a raft of several adults tangled together on the surface. A Griffith's Gnat in 18 through 22 is a cluster imitation, not a single bug, which is why it works when it looks nothing like the insect. The Henry's Fork and Silver Creek pages list a CDC Midge Cluster #18-20 for the same reason. On calm afternoons, cluster fishing on the flats of the Missouri or the Middle Provo is real dry-fly fishing to rising trout in January.
Tailwaters: why the dam makes the midge
Bottom-release dams put out water at a steady temperature all winter, along with the fine sediment and organic material midge larvae thrive in. That is why the rivers with the longest midge peaks here are almost all tailwaters: the San Juan and Lees Ferry (twelve months), the Salt and Upper Sacramento (October through May), and a long list peaking October through April: the Bighorn, Missouri, South Platte, Frying Pan, Blue, Taylor, Gunnison, Owens, Truckee, East Walker, Strawberry, Ogden, and Weber. The Provo and Middle Provo peak October through March, and their fly lists add a Thread Midge in 22-24, a body of thread and nothing else.
Freestones list midges too, as a winter fallback. The Montana freestones (Madison, Big Hole, Yellowstone) peak November through March on a Zebra Midge and an Adams.
Winter midging: reading the day
Midges hatch in the warmest part of a winter day, late morning through mid-afternoon on most of these rivers. Before the hatch, fish larvae and pupae deep and slow in the soft water at the tail of pools and the inside of bends, where trout hold in winter. As the hatch builds, watch the slicks and foam lines for the first noses. Those fish are eating pupae in the film, and a pupa on a greased leader, or dropped a foot behind a Griffith's Gnat, catches them. Cloudy, still days are best; wind breaks up the clusters. On tailwaters, flows matter more than weather: a bump in release knocks larvae loose and turns fish on, a big cut pushes fish off the shallow flats. Check the USGS gauge on the river page before you drive.
Rigging for size 22 and smaller
- Tippet: 6X subsurface, 6X or 7X for dries on the flats of the Missouri, Provo, or San Juan.
- Indicator: a pinch of yarn. A big bobber drowns tiny flies on the drop.
- Weight: one small shot above a two-fly rig, or none when fishing pupae near the surface.
- Hook set: lift, don't snap. A size 24 on 7X breaks on a hard strike.
- Vision: a hi-vis post on the cluster pattern, or a larger dry a foot ahead of the midge as a sighter.
A river-by-river starter box
| River | Peak midge months | Patterns listed on the river page |
|---|---|---|
| San Juan, NM | All year | Zebra Midge #22-26, Disco Midge #22-24, WD-40 #22-24, Miracle Nymph, Thread Midge #24-26, UV Midge |
| Lees Ferry, AZ | All year | Zebra Midge #20-26, Mercury Blood Midge #22-24, Midge Larva #22-26, Black Beauty #22-24 |
| South Platte, CO | Oct-Apr | Zebra Midge #20-26, Jujubee Midge #20-24, Mercury Blood Midge, Poison Tung #22-24 |
| Missouri, MT | Oct-Apr | Zebra Midge #20-24, Jujubee Midge, Mercury Midge #22-24, Disco Midge #20-22 |
| Green, UT | Nov-Mar | Zebra Midge #20-22, WD-40, Griffith's Gnat #20-22, Top Secret Midge #22 |
Before you go
Carry black, red, and cream Zebra Midges in 20 through 24, a flashy pupa (Mercury or Disco) in 22, and a dozen Griffith's Gnats in 18 through 22. Most of these tailwaters have special regulations below the dam, so check the state fish and game agency before you rig up. If your winter plans lean toward a reservoir rather than the river below it, the chironomid page covers that: the same insect at three times the size on a different rig.