Although the Boulder River does not have the same level of prolific hatches that some of the regions tailwaters or spring creeks do, there are some important aquatic emergences that anglers should be aware of. The Boulder is a colder fishery that drains a high elevation region that is north facing. The colder flows delay some of the hatches relative to other rivers in the region. Terrestrial fishing can also be excellent on the Boulder, especially on the lower reaches as it approaches Big Timber.
Blue Winged Olives (BWOs)
Beginning in late March and lasting through May and again in mid-September lasting well into October, these mayflies can hatch on the Boulder River on any day. Overcast days may see the strongest emergence, especially on the river downstream from the confluence of the West Boulder River near Boulder Forks access to the confluence with the Yellowstone River. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Boulder 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 Brown hatches on the Boulder River can occur in late April and early May. These mayflies can hatch on the entire length of the river, but conditions must be ideal—overcast, water temperatures between 44 and 48 degrees F, and very light winds. If these conditions on the Boulder River occur, a large emergence of March Browns can happen, making these large mayflies easy pickings for hungry trout. March Browns on the Boulder River range in size from 12 to 16, with most being size 14.
Caddis
Caddis on the Boulder River hatch from late April through early September. The strongest hatches occur in early May, June, and July. The Boulder River is home to a large variety of caddis species and knowing them all is not necessary for catching more fish. In summer—mid-June through August—caddis on the Boulder River can hatch at any time during the day. But, the evenings are the prime time for fishing caddis. Caddis nymphs on the Boulder River are active year-round and run from size 10 to 22. Late evening caddis hatches on the Boulder River can be very strong, allowing ambitious anglers a pleasant hour or so of twilight fishing. During late July the twilight hours—the few hours before sunset—the Boulder River can experience strong emergence of size 16 and 18 black caddis.
Salmonflies
Although not nearly as pronounced as on the Yellowstone River, the salmonfly hatch on the Boulder River is important to anglers. The hatch on the Boulder River usually occurs over the course of a few days. It is rare to see an abundance of salmonfly adults flying in the air, but because Boulder River trout are opportunistic, trout will rise to a salmonfly dry even if an emergence of insects is small. Salmonfly nymphs crawl to bankside structure and remove their outer shell—also known as their shuck. When this occurs an adult insect, complete with legs and wings, emerges. On the Boulder River salmonflies hatch as soon as the river begins to drop after snowmelt runoff. Water temperatures in the 55 to 58 degrees are ideal for a large emergence of salmonflies on the Boulder River.
Golden stoneflies
On the Boulder River Golden stoneflies provide plenty of action for dry fly anglers. These insects are size 8 to 12 and usually emerge slightly later than salmonflies, although on the Boulder River they can emerge concurrently with salmonflies. In most years they hatch in mid-June and last into mid-July. A Golden stonefly hatch occurs in the same sequence as a salmonfly hatch, however golden stoneflies are smaller. On the Boulder River, trout will eat Golden stonefly dry fly patterns for several weeks after the main hatch has occurred. The fast, clear water of the Boulder River forces trout to be opportunistic feeders and a well-presented size 8 to 12 dry fly is rarely passed upon by a hungry trout.
Yellow Sally stoneflies
These stoneflies may hatch concurrently with salmonflies and Golden stoneflies. Often mistaken for caddis or PMDs, the nymphs of these small stoneflies are a crucial part of a Boulder River trout’s diet. Yellow Sally stonefly nymphs become very active in mid-June, with adult insects hatching from mid-June well 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.
Pale Morning Duns
These summer-time mayflies hatch in abundance from many of the Boulder River’s riffles, shelfs, and runs. Pale Morning Dun (PMDs) begin to hatch in mid-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 on the Boulder River and they can hatch in either condition. Nymphs are available to trout year-round, but during a hatch, once on the water’s surface hatching adults require time to allow for their wings to dry before taking flight, therefore being easily available to a rising trout. Most PMDs are size 14 and 16, with size 16 being the most common.
Spruce Moths
Found primarily upstream of the Natural Bridge, these terrestrial based insects emerge as early as mid-July or as late as mid-August. When spruce moths hatch on the Boulder River, these flights of hatching spruce moths cannot be mistaken for anything else. Fluttering near pine trees and bankside bushes, Boulder River trout can be overly opportunistic during a strong spruce moth emergence. When this occurs there are plenty of dry fly fishing opportunities for anglers lucky enough to be fishing the Boulder River at that moment in time. Most spruce moths are size 14 or 16.
Trico mayflies
Although not a major hatch on the Boulder River, tricos offer small fly, technical dry fly fishing. Tricos usually hatch at sunrise in late July and throughout August. 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
On the Boulder River, because trout are opportunistic, anglers fishing terrestrials may find early success. Grasshoppers, ants, and beetles can dominate a Boulder River trout’s diet by early July. Having been blown into the river these land-dwelling insects are a common occurrence on the Boulder River. Once on the water they become easy prey for hungry trout. 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
For the walking-and-wading angler, the prospect of fishing large dry flies is not over when the first chill of fall arrives. 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 a fish feels hungry. Most October caddis are size 8 to 10.