The Big Hole River in southwest Montana is the state’s second longest free-flowing, undammed river. Beginning in the high country between Dillon and Missoula, the Big Hole River boasts healthy populations of wild and native fish along with a renowned tradition of producing big brown trout. The Big Hole is famous for its prolific salmonfly hatch in June, but there are also several other emergences worthy of interst.
Throughout its length the Big Hole River is home to a diverse array of aquatic insects. Flowing for nearly 130-miles the Big Hole River is home to a broad range of trout habitat. Throughout the river’s course exist large populations of mayflies, stoneflies, and caddis. The Big Hole River’s healthy habitat, both for trout and for aquatic insect life, result in a river with prolific hatches and plenty of wild trout.
Blue Winged Olives (BWOs)
These mayflies hatch in spring and fall on the Big Hole River from the headwaters all the way downstream to its confluence with the Beaverhead River. 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 Melrose and downstream of Glen. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Big Hole 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.
Skwala stoneflies
On the Big Hole River skwala stoneflies are the first large insect to hatch. The emergence of skwalas may feel more like legend than fact, but these size 8 to 12 insects are not to be fully ignored. On the Big Hole River skwalas emerge in random fits and spurts as flying adults are rarely seen. Unlike salmonfly and Golden stonefly hatches on well-known freestones where hundreds of adults can be seen fluttering in the air, a skwala emergence is subtle with only a few adults spotted airborne. But because these large insects hatch in early April, opportunistic trout may rise to a well-presented dry fly. Most adults are size 8 or 10.
Western March Browns
March Browns hatch on the Big Hole River, but their emergence is unpredictable. They can hatch on the entire length of the river in early- April and can be seen throughout the month, 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. March Browns on the Big Hole River range in size from 14 to 16.
Caddis
The Big Hole River is home to a large amount of caddis species. Attempting to name them all would be cumbersome and not necessary for catching more fish. Perhaps the most anticipated caddis hatch of the angling season is the Mother’s Day caddis hatch. Ideal conditions for a Mother’s Day caddis hatch on the Big Hole 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 raise water temperatures, but the warming air temperatures often commence snowmelt runoff. Because streamflow and water clarity is directly related to snowmelt runoff, the Big Hole 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.
After the Mother’s Day caddis hatch, runoff usually begins in earnest. Snow melt runoff can last a few weeks and most years the Big Hole River is fishable post-snowmelt sometime between June 10 and June 15. Caddis hatches are thick immediately after runoff and last well into July.
Salmonflies
The salmonfly is the largest insect to hatch on the Big Hole River. It is fitting this large insect kicks-off the summer angling season on the Big Hole—during the first few days of the hatch, Big Hole River trout feed aggressively on these large insects. Dependent on the end of snowmelt runoff, the salmonfly hatch on the Big Hole River can occur anytime during the first two weeks of June.
On the Big Hole River from Wisdom downstream to Notch Bottom, the salmonfly “hatch” occurs over the course of a few days, and in some years a few weeks. About a week or so before any insects have actually hatched from the river, 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 Big Hole 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, Big Hole River trout feed voraciously even when water clarity may only be a few inches.
Golden stoneflies
Immediately after (and sometimes during) the salmonfly hatch on the Big Hole 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 early 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 Big Hole River trout’s diet. Yellow Sally stonefly nymphs become active in early-June, with adult insects hatching from mid-June and into July. 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 Big Hole River trout.
Pale Morning Duns
Because the Big Hole River is full of riffles and shelfs—ideal habitat for Pale Morning Dun mayflies—any angler fishing the Big Hole 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 Big Hole 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 on the Big Hole River are size 14 and 16, with size 16 being the most common.
Spruce Moths
Although sporadic and not an aquatic based insect, spruce moths are consumed by Big Hole River trout. A land-dwelling and distant relative to caddis, spruce moths can be a significant hatch on the river in the Big Hole upstream of Melrose. Emerging in August, flights of hatching spruce moths cannot be mistaken for anything else. Fluttering near pine trees and bankside bushes, Big Hole 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 Big Hole River and mostly occurring on the river below Divide, 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
With the Big Hole River flowing through miles and miles of ranchlands and hay fields, the late summer fishing with terrestrials is some of the most reliable in southwest Montana. 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 Big Hole River. Once on the water they become easy prey for hungry trout. Big Hole River trout are not particular to the color or size of the grasshoppers, ants or 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 Big Hole 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.