August weather, stream flows, and summary
Fly fishing the Yellowstone River in August is defined by long, sunny days fishing one of the West’s most scenic rivers. The Yellowstone River flows out of Yellowstone National Park for 150 miles—unimpeded by any dams—making it the longest free-flowing undammed river in the lower 48. The river is home to four species of trout, including the native Yellowstone cutthroat trout.
Gone are the salmonflies and stoneflies of July, but appearing in full force are grasshoppers, ants, beetles, and crickets. Hatches of caddis are abundant on the Yellowstone River in August and a few sporadic hatches of PMDs happen in many of the Yellowstone’s riffles as well.
During August on the Yellowstone 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 79 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 Yellowstone River is surprisingly diverse.
Because the Yellowstone River is a freestone river dependent entirely on snowmelt runoff, stream flows gradually drop throughout the month. In many years the streamflow on August 1 can be double the streamflow on August 31. Fishing the Yellowstone River in August requires an understanding of how water temperatures affect hatches and the habits of feeding fish.
Early August features plenty of long, sun-filled days. While wonderful for suntans and flip-flops, prolonged high air temperatures and bright sun does affect the habits of trout and insect hatches.
In early August as the peak daily water temperature climbs past 65 degrees, the majority of the Yellowstone River’s hatches subside. With the lack of a prolific hatch, trout on the Yellowstone River become more opportunistic, focusing on terrestrials—grasshoppers, ants, beetles, and crickets—for the bulk of their diet.
Overlooked by many anglers are the last two weeks of August. During these two weeks, the nightly low temperatures are almost five degrees F cooler than earlier in the month. By the end of the third week of August, usually around August 25th, there is one less hour of daylight compared to earlier in the month. These longer, cooler nights result in a lower daily high temperature. Many anglers think late-August is a time to forego fishing the Yellowstone River altogether, however these slight changes can make a big difference resulting in actively feeding trout.
August fishing: what to expect
August fly fishing on the Yellowstone River is best done with a flexible attitude. Unlike a tailwater river with consistent flows, the Yellowstone River’s stream flows are entirely dependent on the amount of remaining snowpack in the headwater’s basins. In years with average or above average snowpack, early August on the Yellowstone River can see fish-friendly stream flows and abundant hatches of PMDs, caddis, and trico mayflies. In years with below average snowpack, lower than average stream flows may hinder hatches and fishing becomes entirely subsurface or dependent on available terrestrials.
A typical day in August usually begins early, often before daybreak. PMD and caddis nymphs are active year-round, but during August on the Yellowstone River are only active when water temperatures are below 65 degrees. Trico mayflies—tiny black-bodied mayflies—can hatch on the Yellowstone River around sunrise. These mayflies are small, with most being size 18 to 22. For dry fly anglers looking for a challenge, these Yellowstone River trico hatches can serve up some unique sight-fishing opportunities.
PMD and caddis nymphs are active year-round as well, so a stonefly nymph and a mayfly or caddis nymph 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 Yellowstone River, more terrestrials land in the river enticing even more trout to be opportunistic feeders.
These terrestrials are the reason many anglers fish the Yellowstone 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 Yellowstone
Trout on the Yellowstone River in August move around to seek out any available food. Most trout on the Yellowstone 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 Yellowstone River in August is about finding the food.
Caddis, stonefly, and mayfly nymphs are active year-round on the Yellowstone 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 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 Yellowstone 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. Throughout the day as nymphs become 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 Yellowstone River flows for nearly 150 miles from Gardiner to Columbus, 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 or structure that provides cover are ideal places for opportunistic trout.
Important August hatches
Hatches of salmonflies, Golden stoneflies, and Yellow Sally stoneflies are finished. Trico mayflies will emerge in the early morning hours, followed by sporadic hatches of PMD mayflies. Caddis can hatch throughout the day, with most strong emergence occurring in the evening hours. Tricos and caddis can hatch in abundance in many of the Yellowstone River’s long riffles.
But in August, emergence of the many species of terrestrials are the focus of anglers and trout on the Yellowstone River. Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Yellowstone 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.
Yellowstone River fly box for August
Grasshoppers in sizes 4 to 16
Ants; black, brown, or cinnamon in sizes 12 to 18
Beetles in sizes 10 to 18
Crickets in sizes 4 to 16
PMD nymphs sizes 12 to 18
PMD emergers sizes 12 to 18
PMD dry flies sizes 12 to 16
Caddis pupae sizes 12 to 16
Caddis CDC emergers sizes 12 to 16
Caddis dry flies with dark grey, black or brown bodies in sizes 12 to 18
Trico dry flies in sizes 18 to 22
Crayfish patterns in sizes 2 to 8
Sculpin patterns in sizes 2 to 6
Streamers in olive, black or brown in sizes 2 to 6