Whitewater Flow Calculator
Evaluate river flow conditions for paddling suitability using flow rate, width, and depth to calculate velocity, Froude number, and feature suitability.
About this calculator
A gauge reading in cubic feet per second doesn't tell you whether a rapid will be a fun wave train or a violent hydraulic — the Froude number does. This calculator first turns your flow rate, river width, and average depth into a cross-sectional area, then divides flow rate by that area to get velocity in feet per second. From velocity it computes the Froude number — velocity divided by the square root of gravitational acceleration times depth — a dimensionless ratio that describes the flow regime: below 1 the flow is subcritical (pools and gentle current), around 1 it's critical (this is where standing waves and surf spots typically form), and above 1 it's supercritical (fast, steep, hydraulic-prone water).
The calculator then scores how suitable the current Froude number is for the feature you're chasing — surfing waves peak near a Froude of 1.0, play holes near 1.2, eddy lines near 0.6, and general runs are scored more permissively across a wider range — and estimates a rough whitewater class from the same number. Treat the class estimate as a starting-point heuristic, not a substitute for a scouted read of the actual rapid: real whitewater difficulty depends heavily on gradient, obstacles, and channel shape that this simplified model (a rectangular cross-section assumption) doesn't capture. Flow-per-foot-of-width is included separately because it's a handy way to compare two different rivers' relative power independent of width — useful when translating your home run's flow to an unfamiliar river's gauge reading.
Inputs
Results
Flow velocity (ft/s)
2.78
How to Use This Calculator
- Look up the current flow rate in cfs from a USGS stream gauge for your river section.
- Measure or estimate the average river channel width in feet at the feature you are evaluating.
- Estimate the average water depth in feet across the channel.
- Select the type of feature you are assessing: surfing wave, play hole, eddy lines, or general class run.
- Read Flow Velocity (ft/s) and Froude Number — a Froude number above 1 indicates supercritical (fast, steep) flow.
- Check Feature Suitability Score (0–100) and Estimated Whitewater Class to decide if current conditions match your skill level.
How the result changes with Flow rate (cfs)
| Flow rate (cfs) | Flow velocity (ft/s) |
|---|---|
| 250 | 1.39 |
| 375 | 2.08 |
| 750 | 4.17 |
| 1,250 | 6.94 |
What each input means
- Flow rate (cfs)
- River discharge in cubic feet per second. Check USGS gauges for real-time data.
- River width (ft)
- Average width of the river channel at the feature.
- Average depth (ft)
- Average water depth across the channel.
- Feature Type
- Type of whitewater feature to evaluate.
What each result means
- Flow velocity (ft/s)
- Water speed calculated from flow rate and cross-section.
- Froude number
- Dimensionless ratio indicating flow regime. <1 subcritical, >1 supercritical.
- Flow per ft width (cfs/ft)
- Flow concentration — useful for comparing different rivers.
- Feature suitability (0-100)
- How suitable current flow is for the selected feature type.
- Estimated whitewater class
- Rough whitewater difficulty class (I-V) based on flow dynamics.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFlow rate (cfs) = 500, River width (ft) = 60, Average depth (ft) = 3, Feature Type = 1 = 4 input(s) provided
- Calculate Flow velocityFlow velocity = flowRateCfs / crossSection2.78 = 2.78
- Calculate Froude numberFroude number = velocity / sqrt(g * avgDepthFt)0.283 = 0.283
- Calculate Flow per ft widthFlow per ft width = flowRateCfs / riverWidthFt8.3 = 8.3
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator treat the river channel as a rectangle when real riverbeds aren't flat-bottomed?
Cross-sectional area is computed as width times average depth, which is a rectangular approximation — real channels have varied bottom contours, so this can understate area (and thus overstate velocity and Froude number) for rounded or V-shaped channel beds. Treat the velocity and Froude figures as reasonable approximations for planning, not a precise hydraulic survey of the actual rapid.
Why does the ideal Froude number differ between surfing waves, play holes, and eddy lines?
Each feature type has its own built-in target range and peak: surfing waves peak near a Froude number of 1.0, play holes near 1.2 since holes want more aggressive recirculating flow, and eddy lines peak near a gentler 0.6, since eddies form best in calmer transitional flow rather than critical or supercritical water. The suitability score measures how close your calculated Froude number is to the selected feature's specific peak, not a single universal ideal.
How reliable is the estimated whitewater class this calculator produces?
It's a rough heuristic that maps Froude number ranges directly to class I through V+, without accounting for gradient, obstacles, channel constriction, or drops — the factors that genuinely determine difficulty on the water. Use it to compare relative flow states on a run you know, not as a substitute for scouting or a guidebook's rated class.
What's the point of "flow per foot of width" if I already have flow rate and velocity?
It strips out channel width so you can compare the raw power of two different rivers independent of how wide each one is — useful when translating experience on your home river's gauge reading to an unfamiliar river with a very different channel width. Flow rate alone can be misleading for that comparison since a wide, low river and a narrow, powerful one can share the same cfs reading.
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