Big trout come from a few specific situations: a tailwater where a reservoir pumps Mysis shrimp or scuds into the river, a canyon that keeps most anglers out, or a brown trout fishery with enough water and forage to grow fish for a decade. This list is built on those situations, not on overall quality. Several of these rivers are hard to fish, a couple are hard to reach, and one is a walk-in tailwater at the end of a bad road. None of them is on our top ten for numbers, and that is the point: this is where you go when you want one fish.
The Mysis shrimp tailwaters of Colorado
Three Colorado reservoirs were stocked with Mysis shrimp, a cold-water crustacean intended as forage for lake fish. It washed through the dams instead and turned the rivers below into some of the fattest trout water in the country. Carry a size 16 to 18 white or translucent Mysis pattern on all three; it outfishes everything else subsurface, year-round.
Frying Pan River, Colorado
Below Ruedi Reservoir the Frying Pan is Gold Medal water where the rainbows are grotesquely deep-bodied from the shrimp. The largest fish hold closest to the dam, in the Toilet Bowl and around Seven Castles, and they hold in predictable lies you can watch before you cast. Beyond Mysis it is a midge and Blue-Winged Olive river from October through April, with a Red Quill hatch in spring and fall and Tricos in July and August. Browns are fewer, larger and best in October.
Taylor River, Colorado
The Taylor below Taylor Park Reservoir is the best brown trout tailwater in Colorado, and the browns at the dam outlet are the reason it is on this list. The parking area up there is small and the water is technical: these are dry fly fish that want 5X or 6X and an accurate cast, not a big pattern on heavy leader. The Lottis Creek campground area is the accessible middle, and the river ends at Almont where it joins the East to form the Gunnison.
Blue River, Colorado
The two miles of Gold Medal water below Dillon Reservoir in Silverthorne is the third Mysis tailwater, fished from a walk-wade trail within sight of the outlet stores. Browns dominate and they are as pressured and selective as any fish in the state. The move is to go when the ski crowds are on the hill: midges hatch on mild winter days and the Blue-Winged Olive fishing in fall is the best of the year. Downstream toward Kremmling along CO-9 the canyon holds fewer people and less-educated fish.
Big-water brown trout
Gunnison River, Colorado
The Black Canyon of the Gunnison is the trophy brown trout destination in this state for one reason: almost nobody fishes it. Six NPS-designated routes drop into the inner gorge, each strenuous, with a climb out of about two thousand feet. Register at the visitor center, train for it, and carry more water than seems reasonable. The Gunnison Gorge below the park, managed by the BLM, is a raft-in alternative with the same class of fish and somewhat easier logistics.
Provo River — Middle, Utah
The Heber Valley browns of the Middle Provo are the largest accessible brown trout in Utah, and "accessible" is doing work in that sentence: River Road parallels the river and Walk-In Access sites cross the private ground. The big fish sit under sod banks, so the cast has to land inches from the grass. Mother's Day caddis in May and the fall Blue-Winged Olives are when they feed most freely; the June Green Drakes bring up fish that otherwise never look at a dry.
North Platte River, Wyoming
The Miracle Mile below Pathfinder Dam is dominated by browns, and they are nocturnal feeders. Evening and after-dark fishing with streamers is how the regulars target the biggest of them. The access road from Alcova is rough dirt and high clearance helps; the water nearest the parking areas gets the pressure, so walk. Salmonflies in June and July and the fall spawn are the two daytime windows.
Truckee River, California
The Truckee between Lake Tahoe and the Nevada line is a wild brown trout river in a mountain-tailwater setting, and its Wild Trout sections hold browns that run well above what the river's size suggests. They feed most confidently on fall Blue-Winged Olives and evening caddis. Fish it in spring and fall along River Road between Truckee and Alpine Meadows; summer weekends belong to the Tahoe tourist traffic and the fishing suffers for it.
Tailwater rainbows that grow on midges and scuds
Colorado River — Lees Ferry, Arizona
Fifteen miles of the Colorado between Glen Canyon Dam and Lees Ferry hold rainbows that live almost entirely on midges and scuds, grow large in the cold clear water, and fight like fish twice their size in the current. They are educated; the local advice is to start with the size 24 and 26 midges rather than the 20s. Walk upstream from the ramp, use the Spencer Trail for the upper reach, or come in from Cathedral Wash at the bottom for the least-pressured water.
Fishing for one big trout
On all of these rivers the biggest fish are caught by people who accept fewer fish. That means fluorocarbon in 5X or 6X on the Mysis tailwaters, streamers on a sink tip in low light on the brown trout rivers, and a willingness to walk past the first half mile of any access. Several of these sections carry special regulations, including catch-and-release and artificial-only rules, and they change; check Colorado Parks and Wildlife, Utah DWR, Wyoming Game and Fish, California CDFW or Arizona Game and Fish for the section you plan to fish.