The Gallatin River begins high in the backcountry of Yellowstone National Park. For nearly a hundred miles this freestone river flows through mountain meadows, a deep canyon, and then through Gallatin Valley where it meets the Madison and Jefferson Rivers to create the Missouri River.
Throughout its length the Gallatin River is home to a diverse array of aquatic insects. Flowing for nearly 90-miles the Gallatin River is known for producing plenty of wild trout. Large populations of mayflies, stoneflies, and caddis inhabit the streambed of the Gallatin River. The Gallatin River’s healthy habit, both for trout and for aquatic insect life, results in a river with prolific hatches and plenty of wild trout.
Blue Winged Olives (BWOs)
These mayflies hatch in spring and fall on the Gallatin River from the headwaters all the way downstream to its confluence with the Madison and Jefferson Rivers. Beginning in late March and lasting through May and again in mid-September lasting well into October, these mayflies can hatch on any day during those two seasonal windows. Overcast days may see the strongest emergence, especially on the river around Big Sky and downstream through the Gallatin Valley. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Gallatin River is overcast, with scattered showers, and air temps in the low 50s. Spring BWOs range from size 14 to 16 and fall BWOs range in size from 16 to 22.
Western March Browns
March Browns hatch on the Gallatin River, but their emergence is very sporadic. They can hatch on the entire length of the river in mid- April and can be seen throughout May, but consistency of when or where a hatch might occur is difficult to pinpoint. Conditions must be ideal—overcast, water temperatures between 44 and 48 degrees F, and very light winds. If these conditions on the Gallatin River line-up, a large emergence of March Browns can occur, making these large mayflies easy pickings for hungry trout. March Browns on the Gallatin River range in size from 12 to 16, with most being size 14.
Caddis
Caddis on the Gallatin River hatch in great numbers from May through August, with strong hatches in early May, June, and July. One hatch in particular—the Mother’s Day caddis hatch—is well known on the Gallatin River. The Gallatin River is home to a large number of caddis species. Attempting to name them all would be cumbersome and not necessary for catching more fish. In most years the Gallatin River is fishable post-snowmelt runoff by June 20. Caddis hatches are thick immediately after runoff and last well into July. In some years, the Mother’s Day caddis hatch on the Gallatin River can provide some fantastic pre-runoff dry fly fishing. However, river conditions and the ability to fish the hatch depend on when snowmelt runoff will begin. From year to year, even day to day, because of the randomness of when snowmelt runoff will begin in earnest, fishing the Mother’s Day caddis hatch is a moving target. Ideal conditions for Mother’s Day caddis on the Gallatin River are water temperatures around 54 degrees. However, as air temperatures warm this can also coincide with the onset of snowmelt runoff. It is a double-edged sword because warm air temperatures are necessary to warm the water temperature but the warming air temperatures often commence snowmelt runoff. Because streamflow and water clarity is directly related to snowmelt runoff, the Gallatin River can come in and out of fishable shape multiple times from mid-April through late-May, so anglers must cross their fingers that the hatch hits during an “in” period.
Salmonflies
Trout on the Gallatin River will go to great lengths to gorge themselves during a salmonfly hatch. The salmonfly is the largest aquatic insect to hatch on the Gallatin River and even in very muddy water, Gallatin River trout feed aggressively on these large insects. Dependent on the end of snowmelt runoff, the salmonfly hatch on the Gallatin River can occur anytime between mid-June and early July.
On the Gallatin River from Bozeman to the Yellowstone Park Boundary, the salmonfly “hatch” occurs over the course of a few days, and in some years a few weeks. Nymphs crawl to the bankside structure and remove their outer shell—also known as their shuck. When this occurs an adult insect, complete with legs and wings, emerges. This typically occurs when water temperatures range between 54 and 58 degrees F. These adults cling to structure until it is time to fly and find a mate. Once airborne males and females will mate, females deposit eggs on the surface of the water, and males will soon die. From migration to egg laying can take as long as three to four days. Weather, sunlight, and streamflow are all factors in the quality of fishing during a salmonfly hatch. Nymphs and adults (dry flies) range in size from 4 to 8.
Being able to fish the salmonfly hatch on the Gallatin River is a matter of timing—as snowmelt runoff wanes the river must drop enough so trout can hold tight to the bankside structure to feed. Unlike many western rivers, Gallatin River trout feed voraciously even when water clarity may only be a few inches. Additionally, unlike nearly all western trout rivers, the Gallatin River salmon fly hatch is one that can be fished by wading anglers—much of the Gallatin River’s prime salmonfly habitat lies in the Custer-Gallatin National Forest.
Golden stoneflies
Immediately after—and sometimes during—the salmonfly hatch on the Gallatin River, Golden stoneflies hatch. Golden stoneflies provide plenty of action for dry fly anglers. These insects are size 8 to 12 and usually emerge slightly later than salmonflies. Beginning in mid-June and lasting into mid-July, a Golden stonefly hatch occurs in the same sequence as a salmonfly hatch, however golden stoneflies are smaller. Golden stoneflies are equally important to trout as salmonflies, they just don’t have the same celebrity status because they are the second species of stoneflies to hatch after runoff.
Yellow Sally stoneflies
These stoneflies may hatch concurrently with salmonflies and Golden stoneflies. Often mistaken for caddis or PMDs this small stonefly is a crucial part of a Gallatin River trout’s diet. Yellow Sally stonefly nymphs become active in mid-June, with adult insects hatching from mid-June well into early August. Yellow Sally nymphs range in size from 8 to 16, with size 10 and 12 being the most common. Because adult Yellow Sallies have very sporadic flying habits, trout rarely key on them making fishing dry flies difficult, but the nymphs are consumed on a daily basis by Gallatin River trout.
Pale Morning Duns
These summer-time mayflies hatch in abundance on the Gallatin River. Because the Gallatin River is full of riffles and shelfs—ideal habitat for these yellowish colored flies—any angler fishing the Gallatin River in July should have several Pale Morning Dun (PMD) patterns. PMDs begin to hatch in late June and last all through July. Primarily found in the river’s riffles and longer runs, PMDs emerge when water temperatures hit 58 degrees F. PMDs hatch on sunny or overcast days and on the Gallatin River they can hatch in either condition. Nymphs are available to trout year-round, but during a hatch, the hatching insects require time to allow for their wings to dry before taking flight, therefore being easily available to a sipping trout. Most PMDs are size 14 and 16, with size 16 being the most common.
Spruce Moths
Eagerly anticipated in summer is the Gallatin River spruce moth hatch. A land-dwelling and distant relative to caddis, spruce moths can be a significant hatch on the river in the Gallatin Canyon south of Bozeman and south of Big Sky. Emerging as early as mid-July or as late as mid-August, flights of hatching spruce moths cannot be mistaken for anything else. Fluttering near pine trees and bankside bushes, Gallatin River trout can be overly opportunistic during a strong spruce moth emergence, creating plenty of dry fly fishing opportunities for anglers lucky enough to be in the right place at the right time. Most spruce moths are size 14 or 16.
Trico mayflies
These small mayflies are primarily important for dry fly anglers who enjoy casting very tiny flies—sizes 18 to 22—to rising trout. Although not a major hatch on the Gallatin River, tricos offer something for anglers desiring small fly, technical dry fly fishing. Tricos usually hatch at sunrise and an emergence rarely lasts longer than a few hours. Adult tricos swarm in mating swirls above the water, after mating they fall to the water and die, creating “spent” mayflies, also called “spinners” by many anglers.
Terrestrials: Grasshoppers, ants, and beetles
As the heat of summer grows and hatches become more sporadic and less predictable, trout look to land-based insects for food. Grasshoppers, ants, and beetles dominate a trout’s diet by late July. Having been blown into the river these land-dwelling insects are a common occurrence on the Gallatin River. Once on the water they become easy prey for hungry trout. Gallatin River trout are not particular to the color or size of grasshoppers or ants and beetles on the river, but most grasshoppers are sizes 8 to 12 and most ants and beetles are size 14 to 18. Colorations vary but natural tones like gray and tan tend to fish better than bright colors.
October caddis
These late-season caddis are exciting to see, however, their emergence is very sporadic on the Gallatin River. October caddis can hatch as early as late September and will typically peak in late October. Compared to other hatches, an emergence of October caddis is limited to a few flying insects in a given section of the river. But an opportunistic trout may feast on one of these large caddis species if it happens to float by at that same time the fish feels hungry. Most October caddis are size 8 to 10.