May Fishing on the Boulder River

May weather, stream flows, and summary

In most years May on the Boulder River is a mix of rising flows and the start of snowmelt runoff. Many anglers forgo the Boulder River entirely and focus on nearby spring creeks, private lakes, or venture to the Madison, Missouri, or Bighorn Rivers. DePuy’s, Armstrong’s, and Nelson’s spring creeks are less than an hour away and Burn’s Lake is less than thirty minutes away. 

However, small windows of fishing opportunities can occur. If they do occur the possibility of a strong Mother’s Day caddis hatch is an angler’s best bet for fly fishing the Boulder River in May. 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 fishing opportunities, including hatches of caddis or Blue Winged Olive mayflies. 

May on the Boulder River is the wettest month of the year, receiving nearly 4 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. With slightly over 15 hours of sunlight, the angling day is long. 

Because the river drains the snowfields of the high altitudes of the Beartooth Plateau, once snowmelt runoff begins, the river often doesn’t come back into form until mid-June. Combine the warming air temperatures with the average monthly precipitation, and in most years the Boulder River during May is a rising torrent with un-fishable conditions. However, despite the river being unfishable more-often-than-not during May, slightly predictable timeframes of fishable conditions can occur.  

May fishing: what to expect

Because the Boulder River can be defined as two different sections—above and below Natural Bridge—fly fishing the Boulder River in May can also be multi-faceted. The river above Natural Bridge can remain clear but stream flows will be high. Fishing in these high stream flows is a possibility. Once the river leaves the Boulder River canyon downstream of Natural Bridge, flows from the East and West Boulder River’s increase stream flows to levels that are high enough where angling typically just doesn’t make sense or is even considered safe.

Regardless of above or below Natural Bridge if the cause of rising stream flows are warming air temperatures or accumulated precipitation, if the Boulder River has rising stream flows on any given day in May, the fishing action is likely to be poor. 

But, if stream flows are dropping and the weather becomes cold and dry for at least two to three days, the Boulder River in May can be a legitimate option, especially above Natural Bridge. The window of fishable conditions may only last a few days, but during this short window hatches of caddis, March Browns, and Blue Winged Olives can serve up plenty of hungry trout. 

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 the star of the show on the Boulder 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 Boulder 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.

It is also important to note only experienced wading anglers and boaters should fish the Boulder River in May. The river’s large boulder substrate paired with its many whitewater sections and fast currents make the Boulder River best fished by strong, experienced wading anglers and veteran floating boaters. 

Where to find May trout on the Boulder

Finding trout on the Boulder River in May depends on the condition of the river. If the river is clear and stream flows are below 1,500 cubic feet per second (cfs), trout may migrate from slower, deeper pools and can be found in a wide variety of habitats. If the river is above 1,500 cfs trout may be found only in slower runs, eddies, and softer water downstream of riffles. If the river is above 2,000 choose another river, stream, spring creek, or lake. 

When a strong caddis hatch occurs, 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 Blue Winged Olive 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

On the Boulder River, May can harbor several strong hatches. However, stream flows dictate the ability to fish any of the river’s hatches. Hatches of caddis, Blue Winged Olives (BWOs) and March Browns can occur. 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 river's largest trout prefer these larger—size 12 to 14—mayflies. 

Boulder River fly box for May

Caddis pupae size 12 to 16

Caddis CDC emergers size 12 to 16

Caddis dry flies with dark grey, black or brown bodies 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