August Fishing on the Missouri River

August weather, streamflow's, and summary

August on the Missouri River is unique. As many rivers in Montana may be experiencing low flows and warm water temperatures, trout on the Missouri River tend to enjoy consistent streamflows and water temperatures in the mid 60 degrees F. With hatches of tricos, caddis, a few Pale Morning Dun mayflies, and plenty of terrestrials, this central Montana tailwater remains a viable option when other rivers may be in their late summer doldrums. 

Streamflows in August are rarely above 5,000 cubic feet per second (cfs) and in most years average around 3,000 to 4,000 cfs. Water temperatures are often at their highest early in the month and gradually lower as the month progresses. These low flows and water temperatures in the high 60 degrees F make the Missouri River perfect for wading anglers. 

The later part of August feels much different than early August. Because cool nights in late August generally average twenty minutes longer than early August, weather later in the month is actually more favorable than earlier in the month. During August on the Missouri River there are less than five days with measurable precipitation, averaging slightly less precipitation than July. Daily high temperatures during the first two weeks of August average well above 85 degrees F dropping to 80 degrees F by month’s end. Whether it is nymphing with two-fly weighted flies or targeting trout sipping on trico mayflies or prospecting with hoppers in the midday sunshine, August on the Missouri River is surprisingly diverse. 

Unlike many Montana freestones, the Missouri River is home to an abundance of weed beds. These weed beds provide habitat for millions of aquatic insects. As one of the main causes of the Missouri River’s prolific hatches, weed beds can also work against anglers as they grow in size in late summer. As the compounding nature of prolonged direct sunlight builds, weed beds increase in size on the Missouri River in August. This can create more habitat for fish—cover and food—but as weeds dislodge and float in the current, they can also cling to floating flies hindering a quality drift. Weeds are not a deal-breaker for an August fly trip on the Missouri River, they are just a fact of life for fly fishing the river in August…much like the potential for high water for a trip in late May or early June. 

Fishing the Missouri River in August is different than June or early July. But with hatches of tricos and caddis and the emergence of terrestrials and the tailwater nature of the river, trout are still active. Because June and July get most of the notoriety of the Missouri River’s summer calendar, fly fishing the Missouri River in August can mean some solitude on a river that is usually busy. 

August fishing: what to expect

A typical day in August usually begins early, often before daybreak. Pale Morning Dun mayfly and caddis nymphs are active year-round, but during August on the Missouri River are only active when water temperatures are around 65 degrees. Trico mayflies—tiny black-bodied mayflies—can hatch on the Missouri River around sunrise. Missouri River trout will group together in “pods” and feed on hatching, adult, or spent—mated and dead—tricos. These mayflies are small, with most being size 18 to 22. For dry fly anglers looking for a challenge, these Missouri River trico hatches can serve up some unique sight-fishing opportunities. 

Even though the Missouri River’s stream flows are regulated by several reservoirs, the amount of water in the reservoirs is dependent on the previous winter’s mountain snowpack. In years with normal mountain snowpack, August on the Missouri River can see fish-friendly streamflows and abundant hatches of PMDs, caddis, and trico mayflies. In years with below average snowpack, lower than average streamflows may hinder hatches and fishing becomes entirely subsurface or dependent on available terrestrials. 

Mayfly and caddis nymphs, midge larvae, scuds, and sowbugs are active year-round and these are the go-to choices for prospecting with a two-fly weighted nymph rig in August. Because grasshoppers, crickets, ants, and beetles become active later in the day, beginning the day with a two-fly weighted nymph rig makes the most sense. As the sun gets higher on the horizon and air temperatures rise, terrestrials become more active. If the wind begins to blow, which is common on the Missouri River, more terrestrials land in the river enticing even more trout to be opportunistic feeders. 

These terrestrials are the reason many anglers fish the Missouri River in August. Flanked by acres of farms and grasslands, ample habitat exists for grasshoppers, crickets, ants, and beetles. Blowing into the river, these terrestrials provide plenty of food for hungry trout and a wide range of opportunities for anglers committed to fishing dry flies. A tactic gaining popularity in recent years is to fish two dry flies simultaneously. Choose a grasshopper in size 8 to 12 and an ant or beetle in size 14 to 18. Similar to fishing a two-fly weighted nymph rig, fishing two dry flies increases the chances for success. 

Where to find August trout on the Missouri River

Trout on the Missouri River in August move around to seek out any available food. Most trout on the Missouri River remain active if water temperatures are below 68 degrees F. Once the water temperature rises beyond 68 degrees F trout will slow their feeding and seek out refuge in deeper, cooler water. Because hatches are less consistent in August than in July, finding trout on the Missouri River in August is about finding the food. 

Caddis and mayfly nymphs, midges, scuds, and sowbugs are active year-round on the Missouri River. In August, as the sun rises and penetrates deeper into the water, trout may stay in holding lies, occasionally moving to feed on a nymph, midge larvae, scud, or sowbug floating by in the current. These trout can be found in classic subsurface lies: deeper water near shallow water, behind or in front of structure, or any place that provides cover from predators or bright sunlight. 

The prolonged exposure to the bright summer sun also affects the behaviors of Missouri River trout. In early morning or late evening hours, trout may feed in shallow water. As the sun rises trout will move to deeper water. In this deeper water they may feed on nymphs, midge larvae, scuds, or sowbugs. Throughout the day as subsurface insect actively becomes less active and more terrestrials land on the water, a hungry trout may be willing to rise from the depths to eat a large hopper. 

Because the Missouri River flows for nearly 50 miles and is home to numerous island channels, braids, canyon walls, and plenty of bank-side structure, there are miles of streambank that provide habitat for trout to wait for a terrestrial that has blown into the water. Banks with deeper water, structure that provides cover, and many of the Missouri River’s long flats and riffle corners are ideal places for opportunistic trout. 

Important August hatches

August on the Missouri River, compared to other months, experiences minimal hatches. The main hatches are trico mayflies, caddis, terrestrial insects, and a few sporadic emergences of Pale Morning Dun (PMDs) mayflies. On any given day during August anglers can experience a strong emergence of any or all of the four. 

Trico mayflies hatch in the early morning hours. Trout rarely feed on trico nymphs but will often rise to adults floating on the surface or “spent” adults—insects that have mated and are now dead. Tricos usually hatch along bankside structure or from many of the Missouri River’s riffle corners. During a strong trico emergence, Missouri River trout group together in “pods” to rise to hatching tricos. 

Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Missouri River trout’s diet in August. Grasshoppers, crickets, ants, beetles, spiders, and any other land-dwelling insect that may inadvertently find its way onto the surface will be targeted by trout as well. 

During August caddis hatch primarily in the evenings on the Missouri River. Home to dozens of species of caddis, hatching caddis in August tend to be smaller than earlier in the season ranging in sizes 16 to 22. 

PMDs in August hatch exclusively in the morning hours. Often concurrently with tricos, it is important to know the difference. Tricos are black-bodied and have white-wings while PMDs are light-bodied and have pale or grayish wings. Additionally, trout may feed on PMDs spinners—adults that have mated and are “spent”—floating lifeless on the water. 

Missouri River fly box for August

Trico nymphs in sizes 18 to 22

Trico duns/adults in sizes 18 to 22

Trico spinners in sizes 18 to 22

Caddis pupae size 14 to 18

Caddis CDC emergers size 14 to 18

Caddis dry flies with dark grey, black or brown bodies in size 14 to 18;

PMD nymphs in sizes 16 and 18

PMD emergers in sizes 16 and 18

PMD dry flies in sizes 16 and 18

PMD spinners in sizes 16 and 18

Grasshoppers, in tan or peacock, in sizes 4 to 14

Ants and beetles in sizes 12 to 18

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

Sculpin patterns in sizes 2 to 6

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