April Fishing on the Gallatin River

April weather, stream flows, and summary

The Gallatin River has several personalities. It begins as a high mountain meadow stream in Yellowstone National Park, changes to a pocket-water canyon river near Big Sky, and then finishes as a riffle-pool-run valley freestone river outside of Bozeman. In April, the fishing is as varied as the 90+ plus miles of river. Because the Gallatin River drains the northwest corner of Yellowstone National Park, then flows near Big Sky and the Gallatin Canyon before it enters Gallatin Valley, April fly fishing can mean winter-like conditions in one section and spring-like conditions in another. 

Weather throughout April on the Gallatin River can be a variety of snow showers, sunshine, and rain showers—and all can occur on the same day. The river near Big Sky may feel more like winter until the middle of the month, while the river in Gallatin Valley may feature hints of spring—buds on the cottonwoods, a sporadic hatch of skwala stoneflies, and some early Mother’s Day caddis. Armed with quality gear to protect from the weather, anglers fishing the Gallatin River in April could experience some of the best fishing of the year. 

April on the Gallatin River is one of the wetter months of the year, with an average of 7 days of measure-able rain or snow. The average daily high temperature is 56 degrees F, but this varies from the beginning of the month to the end of the month, as the end of April can see an average daily high temperature in the high 60 degrees F. 

Early to mid-April is the most consistent fishing window of the month because flows remain stable and clear. As the days of the month tick away, warmer weather means warmer water temperatures which means more actively feeding fish. However, warmer air temperatures can also mean spring snowmelt runoff can commence causing a muddy river.  

The ideal scenarios to maintain consistently good fishing on the Gallatin River in the second half of April are daytime highs not above seventy degrees and nighttime lows not above freezing. If this weather pattern holds for several days, anglers can experience some exceptional fishing opportunities, including the start of the well-known Mother’s Day caddis hatch. 

April fishing: what to expect

April on the Gallatin River is best fished with two-fly nymph rigs. Because nymphs are active year-round on the Gallatin River, and in April water temperatures are typically on the rise more than on the drop, nymphs become that much more active. Choose beadhead or weighted nymphs that imitate mayflies, caddis, or stoneflies. Anglers desiring to fish streamers on the Gallatin River in April will find some opportunity, but the Gallatin River in April is best fished with subsurface nymphs. A slowly stripped or lightly drug streamer may entice some fish in slower, deeper water. 

If hatches of Blue Winged Olives (BWOs), March Browns, or caddis are to occur, they will emerge around mid-day. BWOs will hatch on a cloudy, overcast or rainy day. March Browns, sporadic at best, may hatch midday as well. 

For two-fly nymph rigs choose a stonefly pattern for one of the flies and then a smaller nymph such as a Pheasant Tail or Prince Nymph as the second. A size 8 or 10 is best for the stonefly nymph and the size 12 or 14 is the best for the second fly. Because hatches of BWOs, March Browns, or caddis are possible in April, it is wise to carry a variety of dry fly patterns. For streamers, black and olive are the go-to color choices and most anglers fish a smaller size compared to choices for the Yellowstone or Madison Rivers. The Gallatin River is home to plenty of trout, but on average they run smaller than other rivers. The exception can be some of the deeper runs of the river once it enters Gallatin Valley—here some true lunkers lurk. 

There is no need for early starts on the Gallatin River in April. Trout will become active as the water temperatures warm up. As the saying goes, there is never bad weather, only bad gear. On the Gallatin River in April sun, rain, and snow can all happen on the same day. And many days the worst weather creates the best fishing conditions so be prepared to be flexible and fish in a variety of changing weather conditions. 

Where to find April trout on the Gallatin

April on the Gallatin River sees its trout holding and feeding in a variety of habitats. If a hatch is prevalent, trout will follow the food. However, water temperatures are still cold—rarely rising above 52 degrees before runoff commences. Because of the predominance of cold water, Gallatin River trout will most often be found in more winter-like lies. 

Focus on the rivers' slower and deeper waters—drop-offs near shelfs, places like inside bends, eddy-lines and foam seams around large rocks or other structure. These “softer” waters allow for trout to expend very little energy while having access to available food. 

If hatch a occurs, look for trout to move into feeding lies. In April, hatches of BWOs will most likely make up the bulk of the hatches. Target slower water near bankside structure or the tailouts of longer, slower runs. If caddis hatch, fish may move to some faster water, but because the water temperatures are still cold, expect to find fish in slower water. 

Rainbow trout and some rainbow-Westslope cutthroat trout hybrids may still be spawning on the Gallatin River in April, so avoid targeting spawning trout or disrupting their spawning areas.

Important April hatches

Hatches of BWOs, March Browns, and early season stoneflies are the most prevalent and consistent. The Mother’s Day caddis often starts to appear at the very end of the month, but this can also coincide with runoff. BWOs can hatch on sunny and cloudy days, but a cloudy, slightly rainy day can create a large emergence. March browns are not as prolific as BW0s, but the larger size 10-12 mayfly can entice bigger trout to the surface even during a sporadic hatch. Like BWOs, March browns will hatch in greater abundance with overcast skies. As the month progresses and local weather becomes more spring-like than winter-like, caddis can hatch in prolific numbers. Most caddis will be size 14 and 16 and are dark bodied. However, as the warmer weather ensues and month’s end nears, the risk of off-colored water and rising flows increases, thus decreasing the clarity of the water and onset of spring runoff.  

Gallatin River fly box for April

BWO dry flies size 16 and 18

BWO emergers size 16 and 18

BWO nymphs size 16 and 18

March brown dry flies size 12 to 16

Dark bodied caddis dry flies size 14 and 16

Caddis pupae size 14 and 16

Skwala dry flies (dark olive stonefly) size 10 and 12

Stonefly nymphs in brown and black sizes 10, 8 and 6

Sculpin and streamer patterns—white and black—sizes 6, 4 and 2

Egg patterns size 12

Brown and yellow woolly buggers sizes 6, 4 and 2