August Fishing on the Jefferson River

August weather, stream flows, and summary

August on the Jefferson River is a reminder to most anglers that trout really do need plenty of cold water to thrive. Because stream flows on the Jefferson River during August rarely are above 600 cubic feet per second (cfs) and the average daytime high temperature rarely dips below 80 degrees, water temperatures on the Jefferson River in August are often too warm for actively feeding trout. 

During August on the Jefferson River there are less than three 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. 

Early August features plenty of long, sun-filled days. While wonderful for making the packing list short and sweet, 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 68 degrees, the majority of the Jefferson River’s hatches subside. With the lack of a prolific hatch, trout on the Jefferson 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 Jefferson River altogether, however these slight changes can make a big difference resulting in actively feeding trout, making late August a much better prospect than early August. 

August fishing: what to expect

August on the Jefferson River sees the highest average water temperatures paired with the lowest average stream flows. Because of this, it is best to lower expectations for fly fishing the Jefferson River in August. In years with average or above average snowpack, early August on the Jefferson River can see fish-friendly stream flows and hatches of Pale Morning Dun (PMDs) mayflies, caddis, and trico mayflies. In years with below average snowpack, lower than average stream flows will 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 Jefferson River are only active when water temperatures are below 65 degrees. Trico mayflies can hatch around the coolest time of the day, typically around sunrise. These mayflies are small, with most being size 18 to 22. 

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.

These terrestrials are the reason many anglers fish the Jefferson 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. However, the challenge on the Jefferson River is always the low stream flows combined with warm air temperatures creating water temperatures that tick close-to or rise-above 70 degrees F. 

If water temperatures stay below 68 degrees F and fishing is a viable option, 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 Jefferson

Trout on the Jefferson River in August will migrate around to find food. Trout on the Jefferson River will remain active if water temperatures remain below 68 degrees F. By mid-July, daily high-water temperatures often rise above 70 degrees causing trout to slow their feeding habits. 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 Jefferson River in August is about finding the food and finding cool water that is conducive to feeding trout. 

Caddis, stonefly, and mayfly nymphs are active year-round on the Jefferson 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 Jefferson 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. Banks with deeper water or structure that provide cover are ideal places for opportunistic trout. 

Important August hatches

Hatches on the Jefferson River in August are very small compared to other months. Trico mayflies can emerge in the early morning hours, followed by sporadic hatches of Pale Morning Dun (PMD) mayflies. A few small caddis species can hatch throughout the day, with most strong emergence occurring in the evening hours. Tricos and small caddis can hatch in several of the Jefferson River’s long riffles, but tricos will be early in the morning and caddis will be late in the evening. 

In August, emergence of many species of terrestrials are the focus of anglers and trout on the Jefferson River. But for trout to feed actively on these land-based insects, water temperatures need to remain below 68 degrees F. Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Jefferson 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, fall under the terrestrial moniker. 

Jefferson 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 white, yellow, black or brown in sizes 2 to 6