Methodology and data sources
How every number on this site is made
Float the Yellowstone covers 29 Montana Fish, Wildlife & Parks access sites from Gardiner to Laurel. Every figure it shows traces to a public source or to a measurement described on this page, and each estimate is published with its uncertainty.
Sources
How often it updates
Live flow and water temperature are re-read from USGS at most every 5 minutes; USGS itself publishes on a 15–30 minute cadence. Wind forecasts refresh every 15 minutes. Historical statistics, river miles, channel slope and shuttle times change on a timescale of years and are computed ahead of time rather than on every visit.
How the float time is estimated
The estimate is water speed multiplied into river distance. Water speed is not taken from a textbook: it was measured on this river, from how long changes in flow take to travel the 49.6 river miles between the Corwin Springs and Livingston gauges. The fit uses binned-median regression over 134 celerity windows, 2021-2026, and it is validated from 1,096 to 16,698 cfs.
Speed then scales with the day’s flow, and each stretch is adjusted for its own channel gradient from NHDPlus, because a steep stretch runs faster than a flat one of the same length.
Every estimate carries a range of ±25%. Outside the validated flows the range widens to ±40% and the page says the figure is an extrapolation.
River miles
Distances are measured along the NHDPlus river centerline, not in a straight line. Each access point is projected onto that centerline; the method reproduces Montana’s published river-mile markers to within half a mile. Sites that sit on a tributary rather than the Yellowstone are identified by their distance from the channel and left out.
Stretches with no gauge
No real-time USGS gauge measures the Yellowstone between Livingston and Billings. Below the Boulder River confluence (river mile 103.6) the flow shown is estimated by adding the Livingston gauge to every tributary gauge upstream of the stretch, and every such page labels it as an estimate.
That sum was checked against the Billings gauge, which sits below every tributary, over 153 days (2025-05-01 to 2025-09-30): mean error +2.6%, standard deviation 9.6%, worst case -23% to +22%. Because water speed rises much more slowly than flow, a 20% flow error moves a float time by roughly 7%.
Typical flow by month
“Typical” means the median day for the month across the full USGS record for that gauge — a statement about history, not a forecast. Monthly figures were last rebuilt 2026-09-28. Flow by month shows them in full.
Water temperature thresholds
50°F is the widely used cold-water danger line, below which an unplanned swim becomes serious quickly. 73°F is the temperature at which Montana FWP considers hoot-owl angling restrictions. Hoot-owl rules restrict fishing, not floating.
Reported float times
After a float, anyone can report how long it actually took. Each report is stored with the flow, wind and air quality that were live at launch and with the estimate the site gave, so the two can be compared. No name, email or account is collected. Unusual reports are held for review rather than discarded.
Once a stretch has 12 reports, a model fitted to real trips can begin to replace the measured-speed estimate there. Today there are 1 reports across 1 stretches. Individual reports are not published.
What the estimate leaves out
- It assumes continuous drifting with no stops. Anglers working the banks routinely take two to three times as long.
- It uses reach-average water speed. A boat riding the main current runs somewhat faster; that difference is what reported trips will calibrate.
- It does not model wind, which on summer afternoons can materially slow a boat, or differences between craft.
- It is not a judgement that conditions are safe for any person or craft, and it does not replace current Montana FWP regulations.
Coverage
36 float stretches have their own page. A stretch gets one when both ends have a launch Montana FWP lists and it runs between 2.5 and 20 river miles. See them all. The 22 sites with a launch each have a page listing the floats and shuttles from there: access sites.