Data vintage Flows last recomputed 2026-06-14 17:54 UTC. Fishable-flow bands last reviewed 2026-06-13.
Where the data comes from
- USGS NWIS — instantaneous values — live river discharge (parameter 00060, cfs) and gage height (00065, ft) from the U.S. Geological Survey's stream gages. This is the live "is it fishable now" reading. (waterservices.usgs.gov/nwis/iv)
- Open-Meteo — free hourly cloud-cover forecasts plus sunrise/sunset for each river's coordinates (the "sky" context and the dawn prime-window time). (api.open-meteo.com)
- Moon / solunar model — the moon's phase computed deterministically from the date (a known new moon + the synodic month); bite strength peaks near the new and full moon. No external feed.
We are not affiliated with the USGS or Open-Meteo; we reproduce and combine their public data and cite it here.
How the score is computed
For each river and each day we combine three factors into a 0–100 score:
- Flow vs. the river's fishable band (the dealmaker). Each river has its own published prime-flow range (in cfs). Live flow inside the band scores full credit (Prime); the shoulders score partial credit (Fishable); well below the band reads Low (skinny), and well above reads Blown (high and off-color). The band is per-river because a prime flow on a small tailwater is a trickle on a big one.
- Season. A curated month-by-month rating per river. Dam-fed tailwaters fish year-round; freestone rivers in spring runoff are treated as out of season. Season scales the odds: a river in prime shape during its peak month scores higher than the same flow in a shoulder month.
- Moon / solunar (the modifier). The day's bite strength from the moon phase, kept between 70% and 100% of the flow×season odds — a nudge, never a dealbreaker.
The score maps to a plain verdict: Blown, Low, Fishable, Prime. We do not penalize cloud cover — an overcast day can actually fish better.
The fishable-flow bands
The prime cfs range for each river, with the gage we read it from. These come from outfitter and guide-service flow guidance cross-checked against each river's USGS record, last reviewed 2026-06-13:
| River | Prime (cfs) | Fishable | USGS gage |
|---|---|---|---|
| Madison River Montana | 800–1,500 | 600–2,500 | 06038500 |
| Bighorn River Montana | 2,500–4,500 | 1,500–9,000 | 06287000 |
| Missouri River Montana | 3,000–5,000 | 2,500–9,000 | 06066500 |
| Gallatin River Montana | 400–1,000 | 250–2,200 | 06052500 |
| Green River Utah | 800–2,600 | 700–4,600 | 09234500 |
| San Juan River New Mexico | 500–1,000 | 350–5,000 | 09355500 |
| Colorado River at Lees Ferry Arizona | 8,000–14,000 | 6,000–25,000 | 09380000 |
| Fryingpan River Colorado | 100–250 | 60–900 | 09080400 |
How fresh it is
An automated job re-pulls the USGS flow and the weather every hour, recomputes every river's score, and rebuilds these pages. We track 8 rivers. If a gage is stale or failing, we keep the prior reading and flag it rather than show a flow we can't stand behind.