May Fishing on the Gallatin River

May weather, stream flows, and summary

In most years the Gallatin River during May is too high and muddy to be a viable option. Because the river drains the Gallatin and Madison mountain ranges, when snowmelt runoff begins it usually lasts until the second week of June. However, windows of fishable conditions do occur in May on the Gallatin River. When these windows occur the fishing is often quite good—it is almost as if the fish know high and muddy flows are coming and feed accordingly. The Mother’s Day caddis hatch can occur in May on the Gallatin River, but the weather must line up perfectly for river conditions to allow for streamflows and clarity to be conducive for feeding fish. 

May on the Gallatin River is the wettest month of the year. Near Big Sky and the most productive section of the Gallatin River, over 3” inches of rain and nearly 5’’ of snowfall during the month. Hours of sunlight increase to almost 15 hours by the end of the month. Daily high temperatures climb considerably throughout the month from a cool 51 degrees F early in May to nearly 62 degrees later in the month, thus helping to speed runoff. 

The combination of warming air temperatures and increased precipitation, in most years, make the Gallatin River a high-flowing muddy mess. Most days in May see the Gallatin River unfishable, however weather watchers can predict the occurrence of small windows of fishable conditions. It is important to note the river above the confluence of the Taylor’s Fork can run clearer than the river downstream of the Taylor’s Fork. However, only a few miles of the river flow between the confluence and the boundary of Yellowstone National Park. The river inside Yellowstone National Park does not open to fishing until the Saturday of Memorial Day weekend, which in most years sees the river too high and muddy to fish. 

Because the average daily high temperature increases nearly 10 degrees throughout the month, the first two weeks of May serve up the greatest opportunity for fishable conditions. Caddis can hatch in abundance during a fishable window and the dry fly fishing can be exceptional. The ideal scenario for this to occur is daytime highs not above 70 degrees F and nighttime lows not above freezing. If this weather pattern holds for several days, the river’s streamflows will drop and clear. 

May fishing: what to expect

For fishing the Gallatin River in May it is best to expect two things—high, muddy streamflows and a little of the unexpected. If streamflows drop and clarity improves, fishing the Gallatin River for a few days before the streamflows rise again and muddy water returns is a viable option. Anglers can find very good fishing with two-fly nymphs rigs and dry fly fishing with caddis. 

Various mayfly, caddis, and stonefly nymphs fished as two-fly weighted rigs dead-drifted or slowly stripped will pick up fish. For dry fly anglers, on cloudy days emergences of Blue Winged Olive mayflies can occur, but the prospect of a strong hatch of caddis is what anglers hope for on the Gallatin River in May. 

If water temperatures climb above 50 degrees, the heaviest caddis hatches will occur. The best window of ideal water temperatures is also reliant on rising air temperatures. This also means snowmelt runoff can begin, making the opportunity to fish a caddis hatch on the Gallatin River in May an exercise in luck rather than planning. If caddis are hatching and the river is clear enough to fish, target slower banks, back-eddies, and foam lines around structure. Use a dark grey or brown-bodied caddis dry fly in sizes 12 to 16 and thank your lucky stars for being in the right place at the right time.   

Where to find May trout on the Gallatin

Finding trout on the Gallatin River in May depends on the condition of the river. If the river drops and clears and a fishable window occurs, most fish will be found near bank-side structure and foam lines created by mixing currents or other structures. In these slower waters look for rising trout. During a hatch of caddis or BWOs target slower currents, eddy lines, and seams behind rocks. For March Browns, focus on slower runs and soft water downstream of structure.

If a hatch doesn’t occur but fishable conditions happen, target slow eddys, foam lines, and any soft water near structure with subsurface nymphs. Trout will feed voraciously during these windows of clear water, but they will be in lies where they can enjoy access to food floating by without spending too much energy swimming in heavy current. 

Important May hatches

Hatches of caddis, Blue Winged Olives, and March Browns can occur on the Gallatin River in May. Appropriately named the Mother’s Day caddis hatch because it occurs around Mother’s Day, the prospect of fishing the hatch  relies on day-to-day conditions. As the water temperatures climb to 50 degrees F, caddis may hatch on sunny or cloudy days and can provide some exciting dry fly fishing. BWOs will hatch in abundance on cloudy, rainy days but on sunny days might only hatch in small numbers. Hatches of March Browns are sporadic in May, but when they do hatch, some of the river's largest trout prefer these larger—size 12 to 14—mayflies. 

Gallatin River fly box for May

Dark bodied caddis dry flies size 14 and 16

Caddis pupae size 14 and 16

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

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

Brown and yellow woolly buggers sizes 6, 4 and 2