May Fishing on the East Gallatin River

May weather, stream flows, and summary

The East Gallatin River during May is often too high and muddy for reasonable fishing opportunities. Because the river drains the Bridger and Gallatin mountain ranges, when snowmelt runoff begins it usually lasts until the second or third weeks of June. Windows of fishable conditions can occur in May on the East Gallatin River, but the weather must line up perfectly for river conditions to allow for stream flows and clarity to be conducive for feeding fish. 

May on the East Gallatin River is the wettest month of the year. Over 2” inches of rain and nearly 2’’ of snow fall 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 60 degrees F early in May to nearly 75 degrees later in the month, thus helping to speed runoff.

Because the average daily high temperature increases nearly 15 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 stream flows will drop and clear. 

May fishing: what to expect

Because the weather during May is the driving factor on whether or not the East Gallatin River is fishable, it is best to plan for the unexpected, yet expect high and muddy stream flows. If stream flows drop and clarity improves, fishing the East Gallatin River for a few days before the stream flows 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. Choose size 8 or 10 stonefly nymphs for the first fly and size 14 or 16 beadhead caddis or Pheasant Tail nymphs for the second. 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 East 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 East Gallatin River null and void. If caddis are hatching and the river is clear enough to fish, target slower banks, back-eddies, and foam lines around structure. Use a grey or tan-bodied caddis dry fly in sizes 12 to 16. 

Where to find May trout on the East Gallatin

Finding trout on the East 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 or foam lines created by mixing currents or other structures. In these slower waters look for rising trout. During a hatch of caddis or Blue Winged Olive (BWOs) mayflies 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 (BWOs) mayflies, and March Browns can occur on the East Gallatin River in May. Appropriately named the Mother’s Day caddis hatch because it occurs around Mother’s Day, the prospect of fishing it 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 rivers largest trout prefer these larger—size 12 to 14—mayflies. 

East Gallatin River fly box for May

Grey or light-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

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

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

Streams and wooly buggers in black, brown, or brown/yellow in sizes 6, 4 and 2