The Jefferson River begins when the Beaverhead and Big Hole Rivers join near Twin Bridges, Montana. It then flows for over 60 miles to Three Forks, Montana where it joins the Madison and Gallatin Rivers to form the Missouri River. Less known and less crowded than many its neighboring rivers, the Jefferson River as a dedicated following of locals anglers who value experience solitude over catching plenty of trout.
Because water in the Jefferson River is in high-demand from ranchers, stream flows drop rapidly after snowmelt runoff. In most years, there is a short window where between runoff and lower summer flows when warmer water temperatures occur. Despite consistent low water during high-summer, the Jefferson River contains relatively healthy populations of aquatic insects and terrestrials.
Blue Winged Olives (BWOs)
These mayflies hatch in spring and fall on the Jefferson River. Beginning in late March and lasting through May and again in mid-September lasting well into October, these mayflies can hatch on any day during those two seasonal windows. Overcast days may see the strongest emergence, especially on the river around Silver Star and the Highway 287 bridge. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Jefferson River is overcast, with scattered showers, and air temps in the low 50s. Spring BWOs range from size 14 to 16 and fall BWOs range in size from 16 to 22.
Skwala stoneflies
On the Jefferson River skwala stoneflies are the first large insect to hatch yet are very sporadic and nearly impossible to plan for or expect. On the Jefferson River skwalas emerge in random fits and spurts as flying adults are rarely seen. Unlike salmonfly ad Golden stonefly hatches on well-known freestones where hundreds of adults can be seen fluttering in the air, a skwala emergence is subtle at best with only a few adults spotted airborne. But because these large insects hatch in mid- or late-March, opportunistic trout may rise to a well-presented dry fly. Most adults are size 8 or 10.
Western March Browns
March Browns can hatch on the Jefferson River, but their emergence is very unpredictable. They can hatch on the entire length of the river in late-March and can be seen throughout the month, but consistency of when or where a hatch might occur is difficult to pin point. Conditions must be ideal—overcast, water temperatures between 44 and 48 degrees F, and very light winds. March Browns on the Jefferson River range in size from 14 to 16.
Caddis
The Jefferson River is home to a large amount of caddis species. Like many other freestone rivers the potential for a Mother’s Day caddis hatch exists, however because that timeframe can also line-up with snowmelt runoff fishable conditions are always touch-and-go. If snowmelt runoff us delayed or air temps and water temperatures align—daytime highs not above 70 degrees F and water temps in the mid-50 degrees F—the Jefferson River can experience a strong Mother’s Day caddis hatch.
If a Mother’s Day caddis hatch does occur, after it, runoff usually begins in earnest. Snow melt runoff can last a few weeks and most years the Jefferson River is fishable post-snowmelt sometime between June 10 and June 15. Caddis hatches are thick immediately after runoff and last well into July.
Salmonflies
Dependent on the end of snowmelt runoff, the salmonfly hatch on the Jefferson River can occur anytime during the first two weeks of June. However, because the river doesn’t have the abundance of aquatic insects compared to the Big Hole River, salmonflies on the Jefferson River are sporadic at best.
About a week or so before any insects have actually hatched from the river, nymphs crawl to bankside structure and remove their outer shell—also known as their shuck. When this occurs an adult insect, complete with legs and wings, emerges. This typically occurs when water temperatures range between 54 and 58 degrees F. These adults cling to structure until it is time to fly and find a mate. Once airborne males and females will mate, females deposit eggs on the surface of the water, and males will soon die. From migration to egg laying can take as long as three to four days. Weather, sunlight, and streamflow are all factors in the quality of fishing during a salmonfly hatch. Nymphs and adults (dry flies) range in size from 4 to 8.
Being able to fish the salmonfly hatch on the Jefferson River is a matter of timing—as snowmelt runoff wanes the river must drop enough so trout can hold tight to bankside structure to feed. Unlike many western rivers, Jefferson River trout feed voraciously even when water clarity may only be a few inches.
Golden stoneflies
Immediately after—and sometimes during—the salmonfly hatch on the Jefferson River, Golden stoneflies hatch. Although not a strong hatch on the Jefferson River, trout can feed with gusto on stonefly nymphs. These insects are size 8 to 12 and usually emerge slightly later than salmonflies. Beginning in mid-June and lasting into early July, a Golden stonefly hatch occurs in the same sequence as a salmonfly hatch, however golden stoneflies are smaller. Golden stoneflies are equally important to trout as salmonflies, they just don’t have the same celebrity status because they are the second species of stoneflies to hatch after runoff.
Yellow Sally stoneflies
On the Jefferson River, the smaller Yellow Sally stoneflies make-up a large chunk of a trout’s diet during their emergence. These stoneflies may hatch concurrently with salmonflies and Golden stoneflies. Yellow Sally stonefly nymphs become active in early-June, with adult insects hatching from mid-June and into July. Yellow Sally nymphs range in size from 8 to 16, with size 10 and 12 being the most common.
Pale Morning Duns (PMDs)
These summer-time mayflies can hatch in abundance on the Jefferson River as long as stream flows and water temperatures allow. Hatches of PMDs will certainly occur in late June and early July but in most years by mid-July stream flows are low and water temperatures rise above 65 degrees, causing PMD hatches to wane. Primarily found in the river’s riffles and longer runs, PMDs emerge when water temperatures hit 58 degrees F. PMDs hatch on sunny or overcast days and on the Jefferson River they can hatch in either condition. Nymphs are available to trout year-round, but during a hatch, the hatching insects require time to allow for their wings to dry before taking flight, therefore being easily available to a sipping trout. Most PMDs on the Jefferson River are size 14 and 16, with size 16 being the most common.
Trico mayflies
These small mayflies are primarily important for dry fly anglers who enjoy casting very tiny flies—sizes 18 to 22—to rising trout. Although not a major hatch on the Jefferson River tricos offer something for anglers desiring small fly, technical dry fly fishing. Tricos usually hatch at sunrise and an emergence rarely lasts longer than a few hours. Adult tricos swarm in mating swirls above the water, after mating they fall to the water and die, creating “spent” mayflies, also called “spinners” by many anglers.
Terrestrials: Grasshoppers, ants, and beetles
With the Jefferson River flowing through abundant ranchlands and hay fields, summer season fishing with terrestrials is some of the most reliable in the southwest Montana. As the heat of summer grows and hatches become more sporadic and less predictable, trout look to land-based insects for food. Grasshoppers, ants, and beetles dominate a trout’s diet by late July. Having been blown into the river these land-dwelling insects are a common occurrence on the Jefferson River. Once on the water they become easy prey for hungry trout. Jefferson River trout are not particular to color or size of grasshoppers or ants and beetles on the river, but most grasshoppers are sizes 8 to 12 and most ants and beetles are size 14 to 18. Colorations vary but natural tones like gray and tan tend to fish better than bright colors.
October caddis
Although talked about on the Jefferson River, October caddis are rarely seen. October caddis can hatch as early as late September and will typically peak in late October. Compared to other hatches, an emergence of October caddis is limited to a few flying insects in a given section of the river. But an opportunistic trout may feast on one of these large caddis species if it happens float by at that same time the fish feels hungry. Most October caddis are size 8 to 10.