Some fly fishers dream of summer in Idaho, when streams flow tranquil and clear through the mountains. But there’s another underrated season you don’t have to wait as long for: the pre-runoff months of spring.
On many Idaho rivers, the period from late spring into early summer—when the weather couldn’t be more perfect for fly fishing—have some of the year’s most challenging conditions. Warm sun might activate hatches and get trout moving, but it also melts the snow in the high peaks, transforming normally calm rivers into angry, muddy torrents.
The advantages of getting on the water before the thaw are significant. During the pre-runoff season, rivers are uncrowded and many trout haven’t seen a fly since the previous fall.
Timing Idaho’s Pre-Runoff Fly Fishing Season
Timing is crucial for successful spring fly fishing in Idaho, but predicting when the rivers will balloon with runoff is virtually impossible. Every river is different, and every year is different.
Across much of Idaho, snowmelt causes rivers to get blown out sometime between May and June during a typical year. When and how badly depends on how much snow is up in the mountains, and how quickly the sun melts it. This usually happens quite a bit earlier in warm, low-elevation areas like southwestern Idaho, and an unusually warm spring can cause high, muddy conditions to develop earlier statewide.
But generally speaking, the months of March and April offer a great window of late winter and early spring pre-runoff fly fishing that can at times rival summer. To check on water levels, the USGS River Flows website is a great resource.
Key Spring Hatches on Idaho Rivers
Just like the post-runoff period, the pre-runoff period often offers some of the year’s most prolific insect hatches on Idaho streams. Some of the most important insects early in the season are the diminutive midges, which in many cases are the only things hatching on Idaho streams through the winter months.
Skwala Stoneflies are the first “big” bugs of the year. They hatch as early as February and often last into April. The Skwala Hatch in Idaho happens on countless rivers and streams, especially in the northern part of the state.
March Browns are the major spring mayfly hatch. The March Brown hatch in Idaho is usually just behind the Skwala Hatch, often in April, though on many streams it may not begin until May.
Other hatches you may encounter during the pre-runoff period include Blue Winged Olive, Western Green Drake, Grey Drake and Little Black Caddis. Little Winter Stoneflies may still be hatching in early spring, and Golden Stoneflies might just be getting started when runoff starts picking up.
Essential Spring Fly Patterns for Idaho Waters
Start with Nymphs
High-sticking or Euro-nymphing account for a lot of trout strikes in spring, especially for cutthroat trout, and using a strike indicator can also be a good call. Patterns like Copper John, Hare’s Ear and Prince Nymph excel in sizes #14 to #16, and Stonefly Nymphs are also effective at times. Many anglers tie a second, smaller nymph (around size #20) behind the lead fly to imitate midge larvae.
Dry Dropper Options
Fishing a dry dropper setup is ideal when water is a bit high and stained, pushing trout toward the banks. Water Walker, Gypsy King, Fat Albert and Skwala Stonefly patterns (size #6 to #10) are great dries from which to suspend a nymph dropper.
Streamer Fishing
Streamers offer bigger, higher-visibility meals to brown trout and big cutthroats in springtime. Most rivers have leeches and sculpins, so be ready with a Balanced Leech or Muddler Minnow. Another tried-and-true pattern is the venerable Wooly Bugger, which can resemble anything from a leech to a stonefly larva, or even a baitfish in lighter colors..
Worm Patterns
As streams rise, a lot of worms become dislodged from the banks and swept into the current. That makes spring a perfect time to tie on a San Juan Worm.
Top Idaho Rivers for Spring Fly Fishing
Coeur d’Alene River
This Panhandle river is a gem for pre-runoff cutthroat trout fishing, and the same goes for its North Fork. Conditions are likely to be chilly, but the Coeur d’Alene River is great for nymph and streamer fishing this time of year. Skwalla, BWO and March Brown hatches also offer dry fly potential from March through May.
Big Wood River
One of the best bets in the Sun Valley area for spring fly fishing, the Big Wood River supports prolific midge hatches throughout the colder months, along with BWOs in March. The Big Wood is usually blown out in May, but it doesn’t get much pressure pre-runoff.
Henry’s Fork
Fed by springs, Henry’s Fork isn’t quite as heavily affected by runoff as some southeast Idaho streams. There’s almost always fishable water here in April, though May can be more hit-and-miss. Big hatches include BWOs, March Browns and Mother’s Day Caddis.
Boise River
The twin tailwaters of the Anderson Ranch Dam and the Lucky Peak Dam make this southwest Idaho river a great bet for big brown trout pre-runoff. The outflow from the dams will be quite cold, so find a spot downriver where the water has had a chance to warm up a bit.
Expert Tips for Spring Fly Fishing in Idaho
Dress Right
You might get 50 or 60-degree days on Idaho streams in April, or you might find yourself fishing in a blizzard. In any case, it’s always better to plan for cold conditions and assume that the water will be cold even if the air isn’t. Wear fleece under your waders, and dress in layers to adapt to changing conditions.
Be Ready for Stained Water
You might get lucky and find streams running clear in spring, but it’s more likely that there will be at least some sediment clouding the water. Larger, higher-visibility flies will help catch trout’s attention in high, muddy conditions.
Gear Up
In most situations, a 5 or 6-weight rod with a floating line is the ideal choice for spring fishing in Idaho. If the water is flowing more heavily, you might want a sink tip line. At the end of the day, choose an outfit that you’re comfortable with and has a fair degree of versatility.
Focus on Edges and Breaks
Trout seek shelter from the current when flows are high. Key areas are eddies and any structure that provides a current break. Trout may also stay close to the edge of the stream, where the current is less strong, or head into side channels away from the main flow.