May weather, streamflow's, and summary
May on Montana’s Missouri River—and the potentially excellent fishing that can occur—is the perfect antidote after a long winter of cold and snow on this central Montana tailwater. But like with any wild trout fishery in spring, there are variables that can swing the angling pendulum. On the Missouri River below Holter Dam, near Wolf Creek and Craig, and downstream to Cascade, during May hatches of Blue Winged Olives, caddis, and a few Drakes, mean anglers looking for feeding trout will be happy.
However, because May in Montana is often the start of snowmelt runoff, any fly fishing during this month requires attention be paid to streamflows and snowmelt runoff. Unlike a freestone river, the Missouri River’s flows are regulated by several dams. In most years, early May on the Missouri River experiences relatively consistent streamflows—levels hovering between 3,000 cubic feet per second (cfs) and 6,000 cfs. In the second half of the month the likelihood of warm and wet weather increases, resulting in the possibility of high streamflows.
During May, weather on the Missouri River is often pleasant, yet with the possibility of substantial rain. Average daily highs in early May are in the low 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.
Despite the Missouri River’s flows being regulated by dams, a few tributary streams can create muddy conditions on the river. If a large rain event occurs in the Little Prickly Pear Creek or Dearborn River drainages, the river downstream of their confluences with the Missouri River can create muddy conditions. However, even during mud or rising streamflows, the Missouri River’s trout tend to still actively feed but desirable aesthetics—clean water and rising trout—are non-existent.
In years with consistent flows—even as high as 10,000 cfs—and clear water, the fly fishing on the Missouri River in May can be some of the best of the year. Blue Winged Olive mayflies can hatch in early May and by the middle of the month daily emergence of caddis creates thousands of hungry, active trout.
The section of the Missouri River below Hauser Dam—known as the Land of the Giants—has the same defining weather and streamflow attributes as the river below Holter Dam.
May fishing: what to expect
It is fair to have high expectations for fishing the Missouri River in May. Because water temperatures are on the rise throughout the month, rarely dipping below 50 degrees F and rarely rising above 55 degrees F, trout on the Missouri River are active. Along with these trout-friendly water temperatures, are strong emergence of Blue Winged Olive (BWOs) mayflies and caddis. Because of the perfect pairing of water temperatures and hatches, anglers on the Missouri River almost always have a hatch to fish.
But, given all of that, it is still best to manage expectations a little. And this is best done by differentiating between river conditions in early May versus late May. Most years streamflows increase throughout the month, creating different fishing opportunities as the month grows. Hatches can continue during increasing streamflows, but the potential for dry fly fishing diminishes as streamflows rise.
In the first half of May anglers should be prepared to fish two-fly weighted nymph rigs subsurface. Blue Winged Olive mayflies or caddis can be the main hatch. Choose beadhead nymphs in sizes 12 to 18 for either insect. When fishing subsurface with either a caddis or a mayfly it is a good idea to have one fly be a scud or sowbug pattern in size 10 to 16. Scuds and sowbugs make up a huge portion of the Missouri Rivers underwater biomass.
For dry fly anglers wanting to experience some of the Missouri River’s world-famous “pods” of rising trout, early May can be the golden ticket. As long as water temperatures remain above 50 degrees F look for hatches of Blue Winged Olives or caddis. For BWOs look for fish to rise to a well-presented dry fly in sizes 14 to 16. For caddis choose flies in sizes 14 to 16. For either species, flies tied with CDC—Cul du Canard—are favorites of many of the river’s dry fly experts.
Because the Missouri River is home to over 4,000 trout per mile, anglers desiring to chase fish with streamers can also expect good action. With water temperatures during the month at fish-friendly ranges, many of the river’s large brown trout actively seek out prey. Plus, with the Missouri River’s abundance of weedbeeds and fine-silted flats, the river is home to thousands of crayfish. As these crayfish molt, they are easily picked-off by large trout. Slowly drifting or dragging a crayfish pattern can entice some large Missouri River trout.
Where to find May trout on the Missouri River
Finding trout on the Missouri River in May depends on the condition of the river. For the first half of May flows traditionally remain consistent, rarely rising above 6,000 cubic feet per second (cfs). When flows are below 6,000 cfs trout will follow the available food source.
For hatches of Blue Winged Olives look for trout to start in deeper, slower water, then move closer to the surface to feed. During a hatch of Blue Winged Olives target slower currents, eddy lines, and seams behind rocks. For the few Drakes that may hatch, focus on slower runs and soft water downstream of structure.
If caddis hatch, look for trout in faster, shallow areas of the river. Target water with a stream bottom made of small rocks or pebbles. The Missouri River is also home to a variety of large cliffs and rock shelves that protrude into the water. During May these rocks warm in the bright sunshine and become havens for hatching caddis.
Shallow flats are also very common on the Missouri River. Many of these flats are dotted with weed humps and potholes, both creating structure and cover for trout. During a caddis hatch these flats can be filled with rising trout.
Large protruding rocks are prominent on the Missouri River and these large rocks create large seams—places where several currents join. By mid-May trout begin to use these slower, mixing currents, as both holding and feeding lies. Two-fly subsurface nymph rigs can rack up trout numbers in these current seams.
Because Missouri River trout feed in high and off-color water, rarely does a day exist in which trout cannot be found. But when flows on the Missouri River are rising, hatches are less strong. If a hatch doesn’t occur and the river has rising flows, target slow eddys, foam lines, and any soft water near structure with subsurface nymphs. Trout will be in lies where they can enjoy access to food floating by without spending too much energy swimming in the rising streamflows.
If flows rise above 10,000 cfs many of the river’s trout will migrate to feed in the softer water—inside bends of islands, slower deeper runs below long shelfs, or slow currents of large eddys. During high flows or after a rain event, worms--both aquatic and land-based--can be flushed into the river. Many longtime Missouri River locals know this and when this occurs out come the worm patterns.
Important May hatches
Hatches of BWOs, caddis, and Drakes can occur in May. Blue Winged Olive mayflies and caddis are the primary hatches of the month. Drakes are very sporadic and it is rare to see more than a few dozen adults floating the surface in any given section.
As water temperatures climb to 50 degrees F, these insects can hatch on sunny or cloudy days. BWOs will hatch in abundance on cloudy, rainy days but on sunny days might only hatch in small numbers. Caddis tend to hatch on the Missouri River when the water temperatures rise above 55 degrees F. This occurs later in the May, but when caddis hatch on the Missouri River expect strong emergences.
Worms—both aquatic and terrestrial based—are abundant on the Missouri River. These worms do not hatch like mayflies or caddis, but after a large rain event or rise in streamflows, these worms are dislodged and become easy pickings for trout.
Missouri River fly box for May
Caddis pupae size 14 to 16
Caddis CDC emergers size 14 to 16
Caddis dry flies with dark grey, black or brown bodies in size 14 and 16;
BWO dry flies size 14 to 18
BWO emergers size 16
BWO nymphs size 16
Drakes, brown and green, dry flies in sizes 12 or 14
Scuds in sizes 10 through 20
Sowbugs in sizes 10 through 20
Midges, especially Zebra midges in black and olive, in sizes 16 through 22
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