The East Gallatin River’s source lies in the mountains surrounding Bozeman, Montana. It is formed when Rocky Creek, Bridger Creek, and Sourdough Creek converge on Bozeman’s northside. With stream flows originating high in the Bridger and Gallatin Mountains, yet flowing through the largely populated Gallatin Valley, the East Gallatin River is home a surprising array of aquatic insects.
The East Gallatin is year-round fishery, but from November through March it is mostly fishing weighted nymphs with midge patterns and the very rare late-season or early-season Blue Winged Olive hatch. From April through October, the East Gallatin is home to strong hatches of mayflies, caddis, a few stoneflies, very good terrestrial fishing, some tricos, and a few October caddis.
Midges
On the East Gallatin River midges range in size from 16 to 26, with most midges sizes 18 to 22. A trout on the East Gallatin River may consume several hundred midges, but it happens nearly all subsurface. Midges can hatch year-round and trout feed on midges subsurface throughout the year. Any subsurface tandem nymph rig on the Gallatin River should always include a pattern that imitates a midge, such as a Zebra Midge, Ray Charles, or Soft Hackle Sowbug.
Blue Winged Olives (BWOs)
These mayflies hatch in spring and fall on the East Gallatin River. They can hatch as early as March 15 and last through May 15 and again in mid-September and last well into October. Overcast days may see the strongest emergence, especially on the river around Cherry River Fishing Access north of Bozeman. Sunny days can experience very strong hatches as well, but the ideal BWO day on the East 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.
Skwala stoneflies
This hatch pales in comparison to the stronger stoneflies emergences of the main Gallatin River. On the East Gallatin River skwalas emerge in random fits and spurts as flying adults are rarely seen. Unlike hatches of salmonflies and Golden stoneflies on well-known rivers like the Madison or Big Hole Rivers where hundreds of adults can be seen fluttering in the air, a skwala emergence is weak on the East Gallatin River. But because these large insects hatch in mid- or late-March, 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 East Gallatin River, but their emergence is very unpredictable and often coincides with runoff which occurs earlier on the East Gallatin than other rivers in the regino. Conditions must be ideal—overcast, water temperatures between 44 and 48 degrees. They can hatch on the entire length of the river in late-March and can be seen throughout the month, but consistency of when or where a hatch might occur is difficult to learn. F, and very light winds. March Browns on the East Gallatin River range in size from 14 to 16.
Caddis
Caddis make up the majority of the hatches on the East Gallatin River. Beginning in late April and with potential for strong hatches any day during the summer, caddis on the East Gallatin Rive are important. Like many other freestone rivers the potential for a Mother’s Day caddis hatch exists, however because that timeframe can also line-up with snowmelt runoff fishable conditions are always touch-and-go. If snowmelt runoff us delayed or air temps and water temperatures align—daytime highs not above 70 degrees F and water temps in the mid-50 degrees F—the East Gallatin River can experience a strong Mother’s Day caddis hatch.
If a Mother’s Day caddis hatch does occur, after it, runoff usually begins in earnest. Snow melt runoff can last a few weeks and most years the East Gallatin River is fishable post-snowmelt sometime between June 15 and 25. Caddis hatches are thick immediately after runoff and last well into July.
Yellow Sally stoneflies
On the East Gallatin River, Yellow Sally stoneflies will hatch after runnoff. Although they are not as abundant as caddis and pale morning duns, they are still relevant. These stoneflies hatch during the few weeks after runoff. 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.
Pale Morning Duns (PMDs)
These summer-time mayflies can hatch in abundance on the East Gallatin River as long as stream flows and water temperatures allow. Hatches of PMDs will certainly occur in late June and early July but in some years by late-July stream flows are low and water temperatures rise above 65 degrees, causing PMD hatches to wane. 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 East 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 on the East Gallatin River are size 14 and 16, with size 16 being the most common.
Trico mayflies
For anglers desiring easy access to some very technical dry fly fishing, the East Gallatin River provide strong hatches of trico mayflies. Because the East Gallatin River is a walk-and-wade fishery, intrepid anglers willing to walk a lot can enjoy plenty of river and plenty of tricos. These small mayflies are for anglers who enjoy casting very tiny flies—sizes 18 to 22—to rising trout. 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 East Gallatin River flowing through abundant ranchlands and hay fields, summer season fishing with terrestrials is consistent. 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 East Gallatin River. Once on the water they become easy prey for hungry trout. East Gallatin River trout are not particular to 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
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 float by at that same time the fish feels hungry. Most October caddis are size 8 to 10.