Rainbow trout are native only to the Pacific side of North America, from Alaska down through British Columbia, Washington, Oregon, California and into northern Mexico, plus a stretch of the Kamchatka coast in Russia. Every rainbow in Colorado, Montana, Utah, New Mexico and Wyoming is there because someone put it there, mostly in the late 1800s and early 1900s from California broodstock. That history matters for an angler because it explains why the rainbows on a Montana freestone and the rainbows on a Utah tailwater behave so differently: they came from different stocks, and they have had a century or more of separate selection in very different water.
What a rainbow is, and what it is not
The species is Oncorhynchus mykiss, the same genus as the Pacific salmon and the cutthroat. A rainbow that goes to sea and comes back is a steelhead, which has its own page here; this guide covers fish that live their whole lives in fresh water. Within the species there are interior forms worth knowing about. The redband rainbows of the Pit River in California and the John Day in Oregon are native interior fish that tolerate warmer water than coastal stock. The redsides of the Deschutes and McKenzie are Oregon's native river rainbows, named for the width and intensity of the lateral stripe. Rainbows spawn in spring, which is the single most useful biological fact for planning a trip: they are thin and distracted in April and May on many rivers, and at their heaviest in late summer and fall.
GBIF occurrence records place rainbow trout in more than forty states. In the West they are recorded in Wyoming, California, Oregon, New Mexico, Colorado, Washington, Utah, Idaho, Arizona, Montana and Nevada, which is every state this site covers.
Where rainbows hold in a river
Rainbows are the fish of moving water. Where brown trout settle into the slow, deep, shaded edges, rainbows sit in the current: the seam where a riffle drops into a pool, the tongue of fast water at the head of a run, the pocket behind a midstream rock, the drop-off along a gravel bar. On the Green River below Flaming Gorge the rainbows that dominate the A section hold along the drop-offs and in the seams below every riffle. On the Weber and the South Platte at Deckers, the rainbows are the fish you find in the faster water while the browns are in the pools. On the Logan and the Cache la Poudre they are the dominant species in the canyon pocket water.
A rainbow in a riffle has a fraction of a second to decide on your fly, so it is less selective than a brown examining a dry in a flat pool, and it is more often caught on a nymph, because the fast water it holds in is where the fish are looking down.
How they feed on tailwaters versus freestones
The big Western tailwaters grow rainbows on tiny food, in enormous numbers. The San Juan below Navajo Dam and Lees Ferry on the Colorado are midge and scud fisheries where a size 22 Zebra Midge is the standard fly and size 24 and 26 outfish it in clear water. The Frying Pan rainbows are fed by Mysis shrimp flushed through Ruedi Dam, which is why they carry the bulk they do and why a white or translucent size 16 to 18 pattern is the fly to carry there all year. On the Missouri below Holter and the Bighorn, the rainbows rise to Tricos at first light from July through September and the spinner fall is over by mid-morning.
Freestone rainbows eat bigger bugs and eat them with less caution. On the Madison, Gallatin and Big Hole the Salmonfly and Golden Stonefly hatches in June bring rainbows up to size 4 to 8 Stimulators and Chubby Chernobyls, and through summer a Royal Wulff or Elk Hair Caddis in size 14 to 16 covers most days. The spring creeks are the exception to every rule: the rainbows of the Henry's Fork at Last Chance and Silver Creek are wild, spring-fed fish that have seen decades of catch and release, and they will refuse a Pale Morning Dun pattern that is a size off or dragging by an inch.
Reading the season by water temperature
Rainbows feed most actively in cold-to-cool water and shut down when a river warms into the upper sixties. On desert rivers like the San Juan through Utah's canyon, that makes spring and fall the trout seasons. On the American River below Nimbus Dam, the wild rainbows are most active in winter and spring, when the Sacramento Valley climate makes a January midge day comfortable. On high freestones the calendar runs the other way: runoff blows out the Lochsa and the Logan until mid or late June, and the best rainbow fishing starts the week the water clears.
The rivers on this site that list rainbows
Seventy-five of the river pages here record rainbow trout, more than any other species. A few stand out for the specific kind of rainbow fishing they offer:
- Green River, Utah: A-section tailwater rainbows on sizes 18 through 22, with the late-spring cicada the one time they eat a size 8 dry.
- Metolius River, Oregon: a full spring creek where the rainbows demand 6x tippet and evening Pale Morning Dun emergers in July.
- Deschutes River, Oregon: redsides that are not selective and reward aggressive attractor fishing during the Salmonfly and caddis hatches.
- Provo River, Utah: lower-section rainbows fed on midges and sow bugs year-round, extremely selective, fished on 6x or finer.
The state hubs at /trout/utah/, /trout/colorado/ and /trout/montana/ list every river in each state with its species and gauge.
A working rainbow box
For a tailwater: Zebra Midge and Mercury Midge in 20 to 24, RS2 and WD-40 in 18 to 22, a scud or sow bug in 16 to 18, and a Parachute Adams in 18 to 20 for the Baetis. For a freestone: Pheasant Tail and Hare's Ear nymphs in 14 to 18, Elk Hair Caddis and X-Caddis in 14 to 16, PMD Comparadun in 16 to 18, a Stimulator in 6 to 10 for stoneflies, and a Royal Wulff for everything else. Fish the seams and drop-offs first, get the nymphs down to the bottom, and move on if a riffle produces nothing in a dozen casts. Seasons, limits and any gear restrictions come from the state wildlife agency for the river you are on, and they change; check before you go.