Clark Canyon Reservoir

Clark Canyon Reservoir sits at 5,559 feet in the wide, sage-covered country south of Dillon, where the Red Rock River and Horse Prairie Creek come together behind a Bureau of Reclamation dam. At 4,815 surface acres and 94 feet at its deepest, it is a big, open sheet of high-desert water with none of the timber-and-granite drama of a mountain lake. What it has instead is fertility. The flats and shoreline shelves of this reservoir carry an extraordinary density of scuds, and the rainbows that eat them put on weight at a rate that seems implausible to anglers used to freestone trout.

The fishery is built on Eagle Lake-strain rainbow trout, planted in enormous numbers by Montana Fish, Wildlife & Parks. A single recent season saw more than 105,000 fingerlings at roughly two inches go in alongside nearly 38,000 fish at close to six inches and several thousand larger trout near nine inches. Those fish disappear into the scud beds and come back out fat, deep-bodied, and hard-fighting. Brown trout share the reservoir and add a second target, particularly for anglers throwing streamers along the rocky points and the face of the dam.

For a fly angler the calendar opens the moment the ice lets go. Chironomids emerge in staggering numbers from the soft-bottomed flats, and an indicator rig hung over 8 to 20 feet of water is the single most productive technique on the reservoir from ice-out through spring. Fish are shallow, hungry after winter, and often concentrated in the warmer arms where the tributaries come in. This is the window that produces the reservoir's reputation-making rainbows, and it is also the least crowded time of year.

Summer shifts the game to the surface. Callibaetis mayflies come off the weedy shoals through the warm months and set rainbows cruising in loose pods, sipping duns and spinners in the same maddening, unhurried way they do on Hebgen. Sight-casting a Sparkle Dun or a small parachute to a moving fish on a calm morning is the most enjoyable fishing Clark Canyon offers. Damselfly migrations toward the shoreline weeds in early summer give a second reliable target, and leeches and Woolly Buggers stripped on sink-tips near the dam and off the points cover water when nothing is showing.

The reservoir's outflow becomes the Beaverhead River, one of Montana's best-known tailwaters, and many anglers treat the two as a single destination — flats and gulpers on the lake in the morning, tailwater trout below the dam in the afternoon. Wind is the constant here. The Beaverhead Valley funnels it hard and nearly daily, and learning to fish the lee shorelines rather than fighting the chop is the difference between a good day and a frustrating one. Ice-out and fall are the peak periods, but Clark Canyon fishes well any month the water is open for an angler willing to read the wind.

Clark Canyon is a fertile, relatively shallow-shouldered high-desert reservoir with a maximum depth of 94 feet concentrated in the old river channel near the dam. The depth chart shows how much of the reservoir's area sits over comparatively shallow ground: broad shelves and gradual contours extend well out from shore in the upper arms where the Red Rock River and Horse Prairie Creek enter, then step down through a series of tightening contours toward the deep water at the dam face. That structure is the map of the fishery — the flats and their drop-off edges hold the food, and the deep channel gives fish a place to go when the surface layer warms.

The biological engine is scuds. Dense populations of freshwater shrimp live in the weed and silt of the shallows, and they are available to trout year-round, which is why Clark Canyon rainbows grow so fast and carry such deep bodies. Chironomids emerge from the soft bottoms in huge numbers, and Callibaetis mayflies and damselflies come off the weedier shoals through the summer. Water clarity is generally good but varies with wind, reservoir level, and spring runoff from the two inflows; being a working irrigation storage reservoir, water levels can swing substantially over a season and reshape which flats are worth fishing.

Wind is a defining water characteristic here as much as a weather one. Sustained valley wind builds a chop that stirs the shallows, pushes food and fish onto the lee shorelines, and can shut down the flat-water gulper game in minutes. On the other hand, that same wind lane concentrates fish predictably for anglers who follow it.

The reservoir freezes over in winter, and ice-out in early spring is the pivotal event on the fly angler's calendar rather than the ice season itself. Winter access and ice conditions vary considerably year to year on a working storage reservoir with fluctuating levels, so check current conditions and access-site status with Montana FWP or local Dillon shops before planning a winter trip. Fly anglers should focus their planning on the weeks immediately after the ice releases, when chironomid fishing is at its best.