May Fishing on the Yellowstone River

May weather, stream flows, and summary

In most years May on the Yellowstone River is a mix of rising flows and the start of snowmelt runoff, paired with the optimistic anticipation of the Mother’s Day caddis hatch. The Mother’s Day caddis hatch—aptly named because it often occurs around Mother’s Day—is the result of several environmental factors coinciding to provide a fishable river with a strong hatch. Aside from the proverbial angling stars aligning for favorable conditions for a Mother’s Day caddis hatch, the Yellowstone River in May is often too muddy and fast to fish. But, that one year can happen when the Mother’s Day caddis hatch in May on the Yellowstone River is storied.

May on the Yellowstone River is the wettest month of the year, receiving nearly 3 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.

As April transitions to May and the air temperatures rise—the average daily high temperature in May is just over ten degrees warmer than April—water temperatures are consistently more favorable for trout than in April. However, this also means snowmelt runoff is imminent. Combine the warming air temperatures with the average monthly precipitation, and in most years the Yellowstone River during May is a muddy, 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. 

With a ten degree climb in the average daily high temperature throughout May, the possibility of fishable conditions is highest during the first two weeks of May. This is the time to hope for favorable conditions to fish a Mother’s Day caddis hatch. The ideal scenario for this to occur 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 the famous Mother’s Day caddis hatch. 

May fishing: what to expect

As the excitement surrounding the Mother’s Day caddis hatch builds, a typical day on the Yellowstone River in May begins with an honest assessment of river conditions. Stream flows and river clarity dictate if fishing is even a viable option. Regardless if the cause is warming air temperatures or accumulated precipitation, if the Yellowstone 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 Yellowstone River in May is a legitimate option. 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 Yellowstone River in May.

If water temperatures climb above 50 degrees, the heaviest 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. This also means snowmelt runoff can begin, making the opportunity to fish a caddis hatch on the Yellowstone River in May a dish best served by being in the right place at the right time with a large side-dish of luck.

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 Yellowstone 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 and thank your lucky stars for being in the right place at the right time.

Where to find May trout on the Yellowstone

Finding trout on the Yellowstone River in May depends on the condition of the river. If the river is clear and streamflows are below 6,000 cubic feet per second (CFS), trout may migrate from slower, deeper pools and then can be found in a wider variety of habitats. If the river is above 6,000 CFS trout may be found only in slower runs, eddies, and softer water downstream of riffles.

When a strong caddis hatch occurs, look for floating mats of caddis. These mats are most often found near bank-side structure and foam lines created by mixing currents or other structures. In these mats look for snouts of rising trout. During a hatch of BWOs 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. 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 may occur.

Important May hatches

The Mother’s Day caddis hatch is the primary hatch of the month, but hatches of Blue Winged Olives 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 rivers largest trout prefer these larger—size 12 to 14—mayflies.

Yellowstone 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 or brown in sizes 2 to 6