Idaho Rivers


What to Expect in May

Note: This What-To-Expect is from Westfly's Legacy pages and may not accurately reflect the current fishing at this venue

Warm weather or extended rainy periods will cause snow to melt quickly, and rivers will respond with high, muddy, unfishable flows. Cool weather, on the other hand, will melt snow more slowly; rivers might become higher and colored, but remain fishable. Look for spells when the temperature is near or below freezing for several nights in a row; sometimes that will give you a narrow window of a day or two when you can fish. If the latter scenario comes to pass, nymphs will usually be the way for anglers to stay productive.

If a river is fishable and open, you'll find trout taking drifting salmonfly and golden stonefly nymphs if it hosts populations of those mega-sized insects. The key to effective nymph fishing is to get your fly to the bottom. Spilt shot, beadhead, extra weight under the body dubbing--whatever it takes and is still legal. If you aren't losing a few nymphs, you're not doing it right (it must be true--all the people who sell nymphs say this!).

Salmonfly nymphs are most active at dawn and dusk, so that's when they are most likely to get knocked loose and drift in the current--and when trout will be waiting for them. But that doesn't mean you can't catch fish all day, either. It just tells you when you'll probably do best. Fish below riffles, among boulder fields, and through drop-offs. Kaufmanns Stoneflies, Rocky Nymphs, Rubber Legs, Bitch Creeks--they all work.

By mid-month, adult salmonflies will start to hatch. The nymphs crawl to shore, and once it's out of the water, the adult emerges, dries its wings, and flies to an alder tree where it utters the insect equivalent of "Hey, Baby, Baby." In their relentless pursuit of the opposite sex, adult stoneflies often fall or are blown out of the trees, land in the water, and are devoured by trout. (Take whatever moral lesson you wish from this sequence of events.)

So if you cast a MacSalmon, Clarks Stonefly, Low Ball Stonefly, or Stimulator near shore and just downstream or downwind from overhanging vegetation--especially in the afternoon when the bugs and the wind are at their most active --you may catch a fish.

Note that just because the adult salmonflies are out, it doesn't mean trout are taking them yet. Trout are creatures of habit and they can be slow to make the switch from nymphs to adults.

One hint: if you buy your salmonfly flies, buy them as soon you can because the fly shops only stock-up once and won't re-order until next year. If you wait too long all you'll find are empty bins or flies that are the wrong size, pattern, etc.

By late May the green drake nymphs will be active. Hatches of this large insect usually occur in early afternoon and probably won't begin until June. But the nymphs are usually active by late May, and trout will be on them. A Poxyback Green Drake can catch fish when drifted through a run with a slow to moderate current. This fly has a shiny back, based on the fact that top of the thorax of most mayflies gets shiny just as it is ready to emerge.

March browns and pale morning duns are other mayflies you may encounter this month.

Grannoms (genus Brachycentrus), the "Mother's Day Caddis," are important on many Idaho rivers. Hatches usually begin around mid-May, plus or minus a week depending on the weather. They should be rolling by opening day. If the rivers are fishable at that time, use a Sparkle Pupa or Deep Sparkle Pupa with a green body and a tan shroud before and during the hatch. Dead drift it near the bottom, then let it swing up to the surface. If you see trout feeding consistently just subsurface or making splashy rises, cast a Sparkle Pupa upstream-and-across and let it drift drag-free just under the surface.

Other active caddis species at this time of year are green caddis (genus Rhyacophila), spotted caddis (genus Hydropsyche), and saddle-case caddis (genus Glossosoma).

With caddis fly patterns, "close" is usually good enough, so you only need a couple of fly patterns; just vary the color and size to match the natural insects. For a dry, use an Elk Hair Caddis, Deer Hair Caddis, Casanova Caddis, X Caddis, or similar pattern.

Many caddis species lay eggs by swimming or crawling underwater, and they are often taken by trout. Use a wet fly such as a Soft Hackle or a Diving Caddis in the sizes listed above for adults; caddis get darker when ready to lay eggs, so use darker versions of the listed colors.

Trout and steelhead have been spawning in many Idaho rivers, so if you're wading over gravelly areas or see small pebbly spots that are "cleaner" than their surroundings, you're probably on a redd. Get off it, and don't fish in that area. Spawners need their rest.

Also, this is the time of year that salmon and steelhead smolts migrate to sea. They congregate in backeddies, below riffles, and near shore. They're suckers for a dry fly or anything near the surface. If you're catching a bunch of5-8 inch "trout" that are shiny and silvery and maybe have the adipose fin clipped off, you're into smolts. Move on and fish another area. They are likely to be damaged by your fly, whether it's barbless or not.


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