Henry's Fork Hatch Overview

by: Mike Lawson

Salmonfly

The first major aquatic insect hatch of the summer on most western rivers and streams is the salmonfly hatch. Anglers throughout the country anxiously anticipate the emergence of this giant stonefly (often reaching 1-1/2 to 2 inches long).

The salmonfly hatch starts immediately after the run-off on most western rivers. On the Henry's Fork, they usually emerge during the last week in May. The emergence usually starts on the lower sections of the stream and moves up the river a few miles each day. The hatch is usually over by the end of the first week in June.

Golden Stones

A smaller version of the salmonfly, the golden stonefly, starts to emerge a few days after the giant stoneflies. These stoneflies aren't as concentrated as the larger salmonflies and they hatch over a longer period of time.

Caddisflies

While caddisflies are not as notorious as the salmonflies or some mayfly species, they may be the most important group of aquatic insects on many rivers and streams. The trout rising on the flat-water stretches of the Henry's Fork, Firehole and other Yellowstone waters in early June are usually feeding on caddisflies.

Rivers that do not have serious run-off problems have excellent caddis fishing during June and July. The Firehole River in the park fishes better in early June than at any other time of the year. There are usually some great hatches of small grey caddisflies during the evening hours. The caddis hatches can be phenomenal on the Henry's Fork throughout June and early July.

Pale Morning Dun (Ephemerella inermis)

The Pale Morning Dun may be the most important western mayfly. These size 16 - 18 mayflies are an elusive olive-yellow cast. The males are much darker than the females and have a reddish tan cast.

Duns emerge about 10 AM. Spinners fall at dusk and in the morning hours. Mid June through early July is the peak but varies from river to river. Pale Morning Duns are the most important hatch of June on the Henry's Fork.

Western Green Drake (Ephemerella grandis)

The Green Drake is one of the largest mayflies found in the west. Even the most cunning brown trout will lose its caution when there is a good hatch of Green Drakes on the water.

The hatch usually dribbles along with a few flies emerging each morning for the first four or five days. Then, on a cloudy cool day the duns will start emerging about noon and continue until 5 or 6 in the afternoon, with the duns literally covering every square foot of the water.

The Green Drake hatch usually lasts for 10 days to two weeks on most streams in the Yellowstone area. Once the trout have had a few days to feed on these large morsels they will seek out the Green Drakes even if the hatch is very sparse. It was once thought that spinners were not significant. We've learned otherwise, however. The spinners fall on the water to lay their eggs early on bright clear mornings. 7:00 AM is the best time to find big trout sipping the spinners. On the Henry's Fork, the bulk of the Green Drake emergence generally occurs between June 15 and June 30.

Brown Drake (Ephemera simulans)

These large mayflies are slightly bigger than the Green Drakes. They aren't common on many rivers, however, because their habitat requirements are very specific. The nymphs are burrowers and need slow moving water with a silty bottom. Stretches of the Harriman State Park on the Henry's Fork support tremendous populations of Brown Drakes.

In late June, the duns start to emerge in the evening hours and continue until after dark. The best hatches occur on warm, humid, muggy evenings. The spinners come back to the water to lay their eggs about the same time the duns start to emerge. This means the surface can be covered with emerging duns and spent spinners. The trout can become very selective when this occurs. One trout may be taking emerging nymphs, another crippled duns, another fully emerged duns, and another showing its preference to the spent spinners.

Grey Drake (Siphlonurus occidentalis)

During mid-June, the duns crawl out on vegetation to emerge during late morning and early afternoon. These large mayflies are very unpredictable but are important on streams like the Henry's Fork, Yellowstone, South Fork of the Madison and other slow moving streams in the Yellowstone region. The nymphs live best in swamps along the margins of the stream. Wet years that produce a lot of standing water usually produce the best Grey Drake hatches. During drought years the hatch may be non existent.

Because the duns have to crawl out of the water to emerge, the duns are usually not important. I've had some great fishing, however, when there was enough of a breeze on the water to keep the duns blowing across the surface. The nymphs are very fast swimming and will produce violent strikes when teased across a trout's holding position.

The spinners fall at dusk and are often very concentrated. Imagine all of the Grey Drakes that live in the swamps and sloughs of the surrounding area all coming to the river at the same time to lay their eggs. On good years the spinners can literally blanket the water. I've had some of the best dry fly fishing of my career on the Henry's Fork and the South Fork of the Madison when this happened.

Flav (Ephemerella flavilinea)

Flavs are very widespread and emerge on most streams of the Yellowstone region during the month of July. The duns emerge in the afternoon and often continue until the early evening hours. The intensity of the hatch is often very unpredictable. One day the duns may carpet the water with several flies per square foot of surface area. The next day, although conditions appear to be the same, the hatch may be very sparse and sporadic.

Trout can become very selective when the Flavs are on the water. They seem to prefer them over anything else that is on the water. Many anglers are drubbed into thinking the trout are feeding on caddis when they are actually seeking out the Flavs that are more difficult to see than the brighter caddisflies.

The spinner fall is also very important. They come back to the water to lay their eggs in the morning and evening hours, depending on the conditions. Since the Flavs usually emerge during the heat of the summer, the spinners will usually return to the river during the coolest time of the day. The trout will really get after them when these large spinners are on the water.

Flav patterns are tied in size 14 or 16. Favorite patterns: Pheasant Tail Nymph, Slate / Olive No-Hackle, Flav Thorax, Olive Sparkle Dun, Flav Cripple, Flav Emerger, Olive PT Emerger, Rusty Spinner.

Blue Winged Olive (Baetis)

Blue Winged Olives consist of a large family of mayflies called Baetis . They are mostly small mayflies ranging from size 18 down to 24. This family of mayflies is the most common and widespread of all mayflies. Most species hatch in the early spring and late autumn. Anglers who visit the Yellowstone region in late March and early April can count on seeing plenty of Blue Winged Olive Mayflies emerging at mid day on the streams they fish.

There are several species that are also important during the early part of the summer, however. Many anglers make the mistake of not seeing these tiny mayflies on the water during a hatch of Pale Morning Duns or Green Drakes. The trout can become very selective on these tiny mayflies. Anglers should be prepared with some size 20 - 24 Blue Winged Olive patterns. I've had days when these tiny flies made the difference with my success.

Favorite patterns: P. T. Nymph, Baetis No-Hackle, Baetis Thorax, Baetis Sparkle Dun, Olive PT Emerger, Baetis Floating Nymph. Terestrials: In addition to the aquatic insects that I have mentioned, you should also have a few terrestrial imitations in your fly boxes. Hoppers usually don't mature until later in the summer but I've seen beetles and ants on the water as early as May. I like to carry a few patterns to imitate these terrestrial insects in size 14 - 18. I frequently use a beetle or ant as a searching pattern when the trout are rising and I'm not certain what they are taking. I have confidence that they will have a hard time refusing a terrestrial if it is presented correctly.

Remember, each stream has a population of specific aquatic insects. What hatches on one stream may not be present on another. It is impossible to cover all of the streams in and around Yellowstone and southwest Montana. My best advice is to stop in our fly shop and ask specific questions. We have a good idea of what insects you should expect to find on the water during the time of your planned fishing.