There is a short stretch every spring, after the shelf ice lets go and before the snowpack starts to move, when Western freestone rivers fish better than they have in months and better than they will again until July. The window can be five weeks or five days depending on elevation and how fast the weather turns. This page is about that window: how to recognize it, what hatches the charts put in it, and how to know when it is closing. What happens after the river blows out is on the runoff page.
Ice-out to melt: how the window opens and closes
The window opens when the river sheds its ice and the water temperature starts climbing through the high 30s and into the 40s. Low-elevation rivers get there first, sometimes in late February in the desert Southwest and Utah's lower valleys; Montana and Wyoming freestones follow through March and April. The window closes when warm days start melting the snowpack faster than the ground can absorb it. On a USGS gauge that shows up as a daily rhythm: flow rises through the afternoon and evening, drops overnight, and each day's low is a little higher than the last. When you see that pattern, you have days, not weeks.
The tension is that the same warm afternoons that bring the best hatches are the ones that start the melt. A cold, cloudy week extends the window. A sunny warm week ends it.
The March and April Blue-Winged Olive peak
Baetis mayflies are the defining hatch of pre-runoff spring, and the charts on this site say so plainly: Blue-Winged Olives are marked at peak in March on 86 of the 92 rivers, and in April on 88 of them. On the freestones the row typically reads on in January and February, peak in March and April, and on in May, which lines up almost exactly with the pre-runoff window.
Montana freestones with that pattern include the Madison, Gallatin, Yellowstone, Big Hole, Bitterroot, and Rock Creek. In Colorado the Roaring Fork, Arkansas, and Eagle show BWO at peak from February through April, and the Cache la Poudre from March. In Utah the Logan, Blacksmith Fork, and Duchesne are all peak in March and April.
Spring Baetis are small, size 18 to 22, and the best emergences come on cool, overcast afternoons, often with a little drizzle or snow. Fish a Pheasant Tail or a Baetis nymph in the morning, switch to an RS2 or a CDC emerger when you see the first noses, and go to a Parachute Adams or Comparadun size 18 to 20 once the duns are on the water. The Blue-Winged Olive guide covers the insect in detail.
March Browns, Red Quills, and the other spring mayflies
The hatch charts on this site do not carry a separate March Brown row, so treat this as general knowledge rather than data. March Browns (Rhithrogena) are a larger spring mayfly, size 12 to 14, best known on Pacific Northwest rivers, and they emerge sporadically on warm afternoons in the same window as the Baetis. The March Brown guide covers the insect. On the Northwest rivers charted here, the McKenzie, Yakima, and Deschutes, the rows that do move in spring are BWO (peak through April) and caddis (on in March, peak in April on the McKenzie and Yakima).
The one spring mayfly besides Baetis that does appear on a chart is the Red Quill on the Frying Pan, marked on in March and peak in April and May. It is a tailwater, so it has no runoff window to worry about.
Rising temperatures and the stonefly nymph migration
As the water warms, fish leave their wintering pools and spread back into the runs and riffle tails where the food is. The other thing that happens in the last weeks before runoff is that the big stonefly nymphs, Golden Stones and Salmonflies, begin crawling toward the banks ahead of their early-summer emergence. The Salmonfly row is marked on in April on the Logan and the Deschutes, the two earliest on the site, and on for most Montana and Idaho freestones in May.
That migration is why a big rubber-leg stonefly nymph, size 6 to 10, dead-drifted tight to the bank, is the most reliable pre-runoff fly on a freestone when nothing is hatching. Fish it as the point fly with a Baetis nymph dropped off the bend.
Freestone, tailwater, spring creek: which one to spend the window on
Spend it on freestones. That is the whole point. A tailwater like the Bighorn or a spring creek like Silver Creek (BWO at peak in March and April on its chart) will still be there in June. The Madison and the Yellowstone will not; they will be brown and unfishable for weeks. If you have a choice on a good April afternoon, fish the river that is about to blow out.
The exception is a cold, high-water day after a rain, when the freestone is already off color. Then the tailwater is the fallback, and the Baetis will be hatching there too.
The Mother's Day Caddis as the hinge
Several Utah and Idaho rivers here carry a separate Mother's Day Caddis row (Brachycentrus): the Provo shows it on in March and peak in April and May, and the Weber, Ogden, Middle Provo, and Boise show it on in April and peak in May. That caddis is famous for arriving at the same moment the freestones start to rise, which is why it is best known as a tailwater hatch: the blanket emergences happen on the rivers that do not blow out. When you see the first Mother's Day Caddis on a freestone, the pre-runoff window is closing.
Reading a gauge for the start of melt
- Pull up the river's USGS gauge from its page and look at seven days of flow, not the current number.
- A flat or gently rising line means you are still in the window.
- A sawtooth pattern, rising every afternoon and falling overnight with each day's low a little higher, means melt has begun on the lower snow. Fish now.
- Water turning from clear to tea-colored to brown in the afternoon confirms it.
- Check the state fish and wildlife agency for spring closures on the water you are planning to fish; several rivers have them to protect spawning rainbows and cutthroat, and they vary by state and year.