May Fishing on the BigHorn River

May weather, stream flows, and summary

Of all the rivers in Montana during the month of May, the Bighorn River is the most consistent. With flows regulated by Yellowtail Dam and no major tributaries to cause muddy water, the river remains clear while many rivers in Montana are muddy due to snowmelt runoff. With fish counts well above 4,000 trout per mile—and most of the between 12- and 16-inches—the Bighorn River is a unique fishery in Montana. With characteristics closer to a giant spring creek than a freestone, fly fishing the Bighorn River has a rich history and a loyal following. 

May on the Bighorn River may see hatches of Blue Winged Olives (BWOs) and midges. Because water temperatures often remain cold—rarely topping 52 degrees—the prospect of BWOs lingers throughout the month. Because the river has a massive population of scuds and sowbugs, anglers desiring plenty of action regardless of method, should fish weighted two-fly nymph rigs and will keep their rod bent throughout the day. Plus, with abundant brown trout, dedicated streamer anglers will find plenty of fish to chase with a well-presented streamer. 

During May, weather on the Bighorn River is often pleasant, yet with the possibility of rain. Average daily highs in early May are in the mid 60 degrees F, but by month’s end increase to the low 70 degrees F. Average daily precipitation also increases throughout the month, with the first two weeks of May seeing nearly half the potential of measurable precipitation compared to the last two weeks. It is rare to have snow in May, but there is a good probability for overcast skies. 

Fly fishing on the Bighorn River in Montana consists of almost 30 miles of flowing water downstream of the afterbay at Yellowtail Dam. From the outflow of the dam clear, cold water flows from the man-made structure. This cold water provides ample habitat for hatches and trout. Only four public access points are available to anglers on the first thirty miles of river. In the first thirty miles of river is where most of the fly fishing occurs. Additionally, the river upstream of Bighorn Fishing Access Site provides the highest concentration of fish, declining as the river travels downstream to Mallards Landing Fishing Access Site.  

Floating is the primary method to fish the Bighorn River, as only four access sites exist. Stream flows in May historically average around 2,000 to 3,000 cubic feet per second (cfs), rising to around 5,000 by month’s end. Even with flows around 5,000 cfs, walk-and-wade anglers can utilize the access sites and then walk upstream or downstream. The Bighorn River’s small sized cobble bottom is easily waded. Floating anglers also primarily use the boat to float from one area to the next, anchoring the boat in shallow water and getting out to walk-and-wade a productive riffle, run, or flat.  

May fishing: what to expect

May on the Bighorn River, in most years, is pretty straight-forward. With average stream flows between 2,000 and 3,000 cubic feet per second (cfs), anglers can expect consistent conditions. The main hatch in May are Blue Winged Olive (BWOs) mayflies. Emergences can be very strong if overcast and rainy weather occur. 

With brown trout being the predominant species—outnumbering rainbow trout about 3 to 1—anglers committing to fishing streamers will relish in the opportunities. Because stream flows and clarity are consistent in May, most streamer fishing is done with floating lines, but with weighted flies. Slowly stripping streamers is the most common method for catching Bighorn River brown trout on large flies, but lately more and more anglers are dead-drifting or dragging streamers from a boat. Sleek, but weighted flies are the best choices in solid colors. Black and olive seem to catch the most fish, but white, yellow, and brown produce well. During May on a cloudy day choose a black, brown, or olive colored streamer and on a sunny day choose a white or yellow colored streamer. 

If BWOs do not hatch or if the streamer bite is minimal, fish a two-fly weighted nymph rig with a size 16 to 20 midge or mayfly nymphs, then around midday the BWOs begin to hatch and anglers with moderate casting ability switch to targeting trout eating BWOs on the surface. A good reach cast and quality floatant are important. 

For tandem nymph rigs most Bighorn River anglers use 9- to 12-foot leaders with 3X and 4X leaders with fluorocarbon tippets. Dry fly anglers fish nothing shorter than 12-foot leaders and often use 5X and 6X, supple, monofilament tippets. Successful patterns include Wondernymphs, JuJu Baetis, Hairwing Duns, and CDC Sparkle Duns. 

The Bighorn River is also home to massive populations of scuds and sowbugs. These aquatic insects do not hatch like a midge or a BWO, but Bighorn River trout gobble up these subsurface insects on a regular basis. Dead-drifting Ray Charles, Firebead Sowbugs, Carpet Bugs, and other scuds and sowbugs in May is a great way to pad the numbers if a hatch of BWOs is non-existent or if brown trout are not chasing streamers. 

Although the Bighorn River is synonymous with consistency, there are times when stream flows above 5,000 cfs or Soap Creek, a small tributary about ten miles downstream of After Bay, alters the otherwise consistent fishing. During heavy spring rains, Soap Creek can muddy the river downstream of its confluence with the Bighorn River near Bighorn Access. 

If flows are above 5,000 cfs, walk-and-wade fishing becomes a little more limited. Once flows rise above 7,000 cfs walk-and-wade fishing is very difficult and not safe, additionally, floating at stream flows above 7,000 cfs should only be done by experienced floaters. 

At these high flows tandem nymph rigs are the go-to options. Choose weighted worm patterns and large scuds and sowbugs fished on 10-foot leaders with nothing lighter than 3X fluorocarbon tippets. Fish will still chase streamers at flows above 7,000 cfs. Dragging them on a tight-line behind the boat, angled from the shore, or slowly stripping them off the bank are the best tactics for fishing streamers during high flows on the Bighorn. 

Where to find May trout on the Bighorn River

On the Bighorn River in May, if a Blue Winged Olive (BWOs) hatch occurs, before noon and after 12 pm will fish a little different from each other. 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. During a thick hatch of BWOs target current seams, slow seams next to fast seams, any shallow water adjacent to any deep water, eddy lines, and seams behind rocks or seams of mixing fast and slow water.  

Just before a strong hatch—in the morning or mid-day hours—many of the Bighorn River’s trout move to large seams and foam lines created by submerged structure, or they may lie near the drop-off of a long riffle just before the water enters a deep run. Well-presented two fly weighted nymph rigs can rack up trout numbers in these current seams. However, dry fly anglers must be patient because Bighorn River trout often require a thick hatch to rise and feed from the surface. 

If hatches of BWOs are sparse or if flows rise above 5,000 cubic feet per second (cfs), trout will be in deeper, slower water because that is near the available food. In the deep and slow water, fish are probably eating midges, scuds, or sowbugs and possibly active BWO nymphs. The Bighorn River is home to thousands of fish per mile and any small change in depth or break in current can create a holding lie. 

If fish are not rising or actively feeding near the surface, it is highly likely they are still feeding. Target any change in current speed or change in depth, such as a bucket or small depression where the depth may be a foot or so deeper than the nearby structure. 

Important May hatches

During May, scuds, sowbugs, aquatic worms, and crane flies are abundant in the Bighorn River, with sowbugs accounting for over half or a Bighorn River trout’s diet. Strictly subsurface, these insects are active throughout May and available to hungry trout. Scuds and sowbugs are not exactly what an angler may think of when it comes to hatches, yet they are a key food source for trout on the Bighorn River. 

Oftentimes thought of as the same insects, a scud resembles a freshwater shrimp and a sowbug looks like a “rollie-pollie” bug. Even if they are different, a trout on the Bighorn River in May often gobbles-up a well-presented scud or sowbug. 

In most years water temperatures hover in the high 40 degrees F, creating strong hatches of BWOs. On sunny days these mayfly species will trickle off and may not bring trout to the surface, but Bighorn River trout are most likely chowing on them subsurface. On cloudy days expect more intense hatches and look for rising trout in slower water, in runs, or along seams. 

Midges are active year-round on the Bighorn River and make-up a massive chunk of a Bighorn River trout’s diet. Fishing midge patterns is best done subsurface as part of a weighted two fly rig. 

May hatches on the Bighorn River are entirely dependent on the temperature of the water being released from the dam. Although regularly consistent from year to year, there are occasional variations. For example, in 2018 releases from the dam were well above 5,000 cubic feet per second (cfs), this resulted in very weak hatches of Blue Winged Olives (BWOs) throughout May. Ideal flows for BWO hatches in May are between 2,500 and 4,500 cfs. 

Bighorn River fly box for May

BWO dry flies size 14 to 18

BWO emergers size 16 or 18

BWO nymphs size 14 to 18

Scuds, especially in pink, in sizes 10 through 20

Sowbugs, especially in pink, in sizes 10 through 20

Midges in sizes 16 through 22

Sculpin patterns in sizes 2 to 6

Streamers in olive, black or brown in sizes 2 to 6