May Fishing on the Stillwater River

April weather, stream flows, and summary

May on the Stillwater River typically has two chapters. The first chapter is about fishing before the clock runs out—in other words, before snowmelt runoff commences. In most years the first two weeks of May serve up a river with stream flows that are low and clear enough for fishable conditions. Anticipation is high because strong hatches of caddis or Blue Winged Olive (BWOs) mayflies can occur. 

The second chapter occurs in the second half of May, when the river rises with snowmelt runoff and becomes too fast and muddy to fish. During this second chapter, anglers must seek other options to fish, like the Bighorn River, one of many private lakes, or perhaps one of the spring creeks in Paradise Valley—Nelson’s, DePuy’s, or Armstrong’s. 

When fishable conditions occur on the Stillwater River in May, the fishing is often quite good—it is almost as if the fish know high and muddy flows are coming and feed accordingly. While BWOs can hatch in early May—and often do hatch in a strong emergence—many Stillwater River veteran anglers keep their fingers crossed and five weights rigged in hopes of a river in fishable conditions when the caddis hatch occurs.  

The ideal scenario to create a few days of fishable conditions is 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 fly fishing with caddis dry flies. 

May on the Stillwater River is the wettest month of the year, receiving nearly 4.5 inches of accumulated precipitation. Temperatures range from daily highs in the low 60 degrees F in early May to almost 80 degrees F near the end of the month. 

Because the river drains the high elevation snowfields of the Beartooth Plateau, once snowmelt runoff begins, the river often doesn’t come back into form until late-June. Combine the warming air temperatures with the average monthly precipitation and the recipe for the Stillwater River to be a rising, muddy, and fast torrent is all there…the question is just a matter of when the torrent will begin. In most years, by the second week of May it begins in earnest. 

April fishing: what to expect

During May on the Stillwater River the river can be clear and a delightful river to fish, or it can be a muddy and raging torrent. From week to week, or even day to day, conditions on the Stillwater River can change…and expect them to. Because the Stillwater River is a mountain freestone with flows that originate from the high mountains of the Absaroka-Beartooth Wilderness, conditions in May change regularly. 

In most years the first week weeks of May see the river in pre-run off mode. This means flows are gradually rising between 800 cfs and 1,500 cfs. Anglers can expect a few days of consistently good fishing but the prospect of rising stream flows exists every day. A typical day fly fishing the Stillwater River in May truly is anything but typical. 

If the river is below 1,500 cfs, on any given day, anglers can have success with various mayfly, caddis, and stonefly nymphs fished as two-fly weighted rigs dead-drifted or slowly stripped will pick up fish. Choose patterns in size 12 to 16 and fish them on leaders no longer than 9-feet long. 

For dry fly anglers, on cloudy days emergences of Blue Winged Olive (BWOs) mayflies can occur. For these choose a favorite, high-floating parachute pattern in size 14 to 16. The potential for BWOs exists in early May, but the prospect of a strong hatch of caddis is the star of the show on the Stillwater River in May. 

If water temperatures climb above 50 degrees, the heaviest caddis hatches will occur. Because this hatch is so dependent on daily weather patterns, planning to fish this hatch is a nearly impossible feat. The target window of ideal water temperatures is also reliant on rising air temperatures, but rising air temperatures mean snowmelt runoff increases. 

When fishable conditions are favorable, anglers will want to fish subsurface until numerous caddis are present on the water’s surface. Most caddis hatches in May on the Stillwater River begin with a blizzard of bugs flying in the air. But during a strong hatch, it isn’t until midday or afternoon that the surface will be blanketed with caddis. When this occurs, 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.

When stream flows are above 1,500 cfs, only experienced wading anglers and boaters should fish the Stillwater River in May. Despite the name, the river is anything but still. The make-up of the streambed is such that the river’s large boulders and sections of pocket water make walking-and-wade angling challenging for even the most physically fit angler.  

Where to find May trout on the Stillwater

If trout are to be found on the Stillwater River in May, uncontrollable outside factors must be favorable…aka, the weather must cooperate. If the river is clear and stream flows are below 2,000 cubic feet per second (cfs), trout may migrate from slower, deeper pools and can be found in a wide variety of habitats.

The challenge on the Stillwater River in late May is that stream flows are rarely below 2,000 cfs. Already a fast-flowing river, so when flows increase and the current speed increases, finding trout becomes even harder. 

If the river is above 2,500 cfs, trout may be found only in slower runs, eddies, and softer water downstream of riffles. If the river is above 3,000 choose another river, stream, spring creek, because flows are just too high and fast for a reasonable chance at catching any fish, plus at flows above 3,000 cfs only expert boaters should be on the Stillwater River. 

If early May stays cool and a strong Blue Winged Olive (BWOs) or caddis hatch occurs before the river begins its snowmelt runoff, look for trout rising in the various pocket-water feeding lies—in-front of rocks, behind rocks, and any soft water created by any type of structure. During a hatch of BWO mayflies, target slower currents, eddy lines, and seams behind rocks. For March Browns, focus on slower runs and soft water downstream of structure.

As each day can bring unique weather, the fishing action on a day-to-day basis is unique as well. If stream flows allow for fishing, before noon most fish will be found in slower, deeper water and then as a hatch begins the fish will adjust their locations based on the available insects, particularly if a caddis hatch occurs. 

Important May hatches

Because during May on the Stillwater River, stream flows dictate whether the river is in fishable condition or too high and muddy to be a reasonable option. If stream flows and clarity do not cause the river to rise too much, hatches of caddis, Blue Winged Olives (BWOs), and March Browns can be thick and productive. 

Named the Mother’s Day caddis hatch because it can occur 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 to trout rising to caddis. Or, if the river rises and becomes too fast, high, and muddy to fish, the hatch passes without the ability to be on the water to fish.

On cooler days, BWOs can hatch in abundance when it is cloudy or rainy. On sunny days the hatch may only occur in small numbers. Another species of mayfly, the March Brown, can have sporadic hatches in May, but when they do hatch, some of the rivers largest trout prefer these larger—size 12 to 14—mayflies. 

Stillwater River fly box for May

Caddis pupae size 12 to 16

Caddis CDC emergers size 12 to 16

Caddis dry flies in size 12 to 16; 

BWO dry flies size 14 to 18

BWO emergers size 16

BWO nymphs size 16

March Brown dry flies size 14 to 16

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

Sculpin patterns in sizes 2 to 6

Streamers in olive, black, brown/yellow in sizes 2 to 6