Fly Fishing Red Rock River - Below Lima Reservoir

Overview

Below Lima Reservoir, the Red Rock meanders through sagebrush-and-willow country in Beaverhead County. Cottonwoods mark parts of the channel, while flats, riffle-run sequences and soft inside bends give the fishing a meadow-river character. Undercut grass-and-willow banks deserve the first casts: work the shaded edge and its seam before reaching for the middle of the run.

Lima Reservoir releases and downstream irrigation diversions shape the water. Short, controlled drifts suit clear flats and bank cover; nymphs work the riffle-runs, and streamers offer another approach along undercuts and deeper bends. When terrestrials are active, a hopper near the grass can pair with a dropper through the adjacent seam.

The Character of the Water

Start with the shape of the river, because it dictates everything about how you fish it. The Red Rock drops gently below Lima Dam, with a meandering meadow channel. You are fishing flats, long riffle-run sequences, soft inside bends, and above all undercut banks. The grass and willows grow right down to the water and hang over it, and the current has spent decades hollowing out the bank beneath them. The undercut is a useful starting point: shade, a soft seam and overhead cover give trout a place to hold.

The valley makes the contrast obvious. Away from the water it is arid, largely treeless high desert; along the river, cottonwoods stand in a hard green line below the dam. At your fishing site, look for shaded willow edges and undercut banks that shelter trout.

Fish it like a spring creek rather than a big freestone. Approach from downstream, stay low, and keep your shadow off the water; on the flats, the first cast deserves as much care as the drift. Work nearby bank water before reaching across the channel.

Trout and holding water

Brown trout, rainbow trout and mountain whitefish are the familiar quarry below Lima Reservoir. Start by fishing the cover rather than expecting a particular fish size or catch rate. Undercut edges and deep outside bends are useful places to try a streamer, while nymphs can work through the riffle-runs and soft seams.

Changing Water Conditions

Irrigation releases and diversions shape the fishing below Lima Reservoir. A reading at the reservoir outlet may not describe the lower reach: diversions can leave less water downstream. In shallow, clear water, approach quietly and make the first presentation count. In stronger current, work reachable seams and sheltered edges with nymphs or streamers. Adjust the presentation to the water in front of you.

Rigging and Flies

Keep it simple. A 9-foot 5-weight with a floating line is a useful starting outfit — dry flies on the flats, a nymph rig through the riffle-runs, and a modest streamer along the banks. Start with a 9-foot tapered leader and tippet in the 3X to 5X range, and adjust from there based on what the water is doing.

On the dry side, the workhorses are exactly what you would expect on a western meadow river: a Parachute Adams for anything mayfly-shaped, an Elk Hair Caddis for the riffles, Pale Morning Dun patterns when pale mayflies are on the water, and Light Cahill as a useful stand-in. In the warm months add Yellow Sally and Yellow Humpy imitations, Stimulators for searching, a Chernobyl Ant as an indicator dry or a target in its own right, and — critically — grasshopper patterns. This is hopper country. Sagebrush meadow, grassy undercut banks, and wind combine to can bring terrestrials onto the water when hoppers are active. Place the fly near the grass and give it time to drift along the undercut edge.

Below the surface, carry Pheasant Tails and Hare's Ears in a range of sizes, a Rainbow Warrior for pressured or clear-water situations, Pat's Rubber Legs and golden stonefly nymphs for weight and bulk in the deeper runs, and Zebra Midges for the skinny, low-water days when nothing is showing. A hopper-dropper — a big buoyant terrestrial with a beadhead nymph a couple of feet below — is a useful Red Rock rig, because it lets you fish the bank and the seam in one drift.

Streamers give you another way to work the cover. A Woolly Bugger, a Muddler Minnow, and a Sculpzilla will cover the water. Fish them tight against the undercut banks with a down-and-across swing or short strips, and vary the retrieve around the shade line and the slower water of outside bends.

Finally, the technical end. On the lower part of this stretch, Trico and midge fishing can call for precise presentations — flat water, small bugs, and trout that have all day to inspect your offering. Carry leaders out to 12 feet and tippet down to 7X before you need them. Nothing is worse than finding a pod of steady risers on a glassy flat with only a 9-foot leader and 4X in your pack.