May Fishing on the Big Hole River

May weather, stream flows, and summary

The Big Hole River drains the high country of the Pioneer and Beaverhead Mountains. Flowing for nearly 150 miles the Big Hole River in May can be a viable option. However, with most elevations in the Big Hole’s headwaters over 8,000 feet, snowmelt runoff in the Big Hole River watershed can begin in earnest in May. If cool and dry weather dominates the first few weeks of May, snow melt runoff can be delayed until the second half of May as well. 

In most years, May on the Big Hole sees the river mostly in full runoff mode. However, windows of fishable conditions do occur in May on the Big Hole 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 before the deluge begins. The Mother’s Day caddis hatch can occur in May on the Big Hole 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 Big Hole River is the wettest month of the year. Over 2.5” inches of rain and nearly 3’’ of snowfall during the month in the watershed. 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 53 degrees F early in May to nearly 65 degrees F later in the month, thus helping to speed runoff. 

Stream flows in May can run the gamut. Because the Big Hole River is a freestone river—there isn’t a single dam on the entire river—its stream flows are entirely the result of snowmelt runoff and a few springs. In May stream flows near Melrose range from 2,500 cubic feet per second (cfs) early in the month to well over 5,000 cfs by May 31st. At flows above 3,000 cfs only experienced boaters should float the Big Hole and walking-and-wading becomes very difficult. 

Because the average daily high temperature increases nearly 10 degrees F 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

During May on the Big Hole it is important to consider the various sections of the river. Best broken down into the following sections: North Fork to Dewey; Dewey to Melrose; Melrose to the confluence with the Beaverhead near Twin Bridges. In May each of these sections will fish a little differently than the other. 

From the North Fork to Dewey, this water will be the coldest section but it also has the best potential to have the longest duration of clear and fishable water. Dewey to Melrose has the greatest potential for Mother’s Day caddis—named because it can occur in mid-May—but if stream flows are high, floating can be dangerous and only should be done by experienced boaters. Downstream of Melrose, and especially downstream of Glen, fly fishing the lower sections in May is a rarity as stream flows are high and clarity is typically muddy for actively feeding fish. 

In general fishing the Big Hole River in May means you can expect two things—high stream flows and a little of the unexpected. If stream flows drop and clarity improves, fishing the Big Hole 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 nymph rigs and dry fly fishing with caddis. 

If water temperatures climb above 50 degrees F, 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 Big Hole River in May an exercise in luck rather than planning. 

If caddis are hatching and the river is low and clear enough to fish, target slower banks, back-eddies, and foam lines around the structure. Use a tan or grey colored caddis dry fly in sizes 12 to 16. Various mayfly, caddis, and stonefly nymphs fished as two-fly weighted rigs dead-drifted or slowly stripped will pick up fish as well. 

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 Big Hole River in May. For dry fly anglers, a little time spent watching the weather and being able to shun a few responsibilities on short notice can translate into some great fishing. 

Before runoff commences in earnest, early May can also be a good time to target large fish using streamers. Slowly stripping, dragging, or dead-drifting streamers through some of the deeper runs and pools can yield some of the largest fish of the angling season. 

Where to find May trout on the Big Hole

Finding trout on the Big Hole 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 caddis hatch is thick do not ignore foam lines and back eddies as on the Big Hole River the abundant caddis adults can stack up in these eddies and trout will follow, goring themselves in the many foam lines that are prevalent on the Big Hole River. 

If a hatch doesn’t occur but fishable conditions happen, target slow eddies, 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), and March Browns can occur on the Big Hole River in May. Appropriately named the Mother’s Day caddis hatch because it occurs around Mother’s Day, the prospect of fishing this 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 rivers largest trout prefer these larger—size 12 to 14—mayflies. 

Big Hole River fly box for May

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

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

White, yellow, or olive streamers in size 2 and 4

Brown and yellow woolly buggers sizes 2 through 6