River Gradient Calculator
Calculate river gradient in feet per mile from elevation and distance.
Matches gradient to whitewater difficulty class from Class I to Class VI.
Gradient is how much a river drops per mile. It is the single most useful number for judging how challenging a stretch will be before you put in.
Gradient (ft/mile) = (put-in elevation - take-out elevation) / river length in miles
A river dropping 300 ft over 15 miles has a gradient of 20 ft/mile, which usually means Class III territory on average. But averages hide reality. Most rivers pool and drop. A stretch averaging 20 ft/mile might be 90% flatwater with a few concentrated drops that rate Class IV or V. Always verify with current trip reports.
These bands are a rule of thumb, not the official scale.
That distinction matters more than it sounds. The International Scale of River Difficulty, the one American Whitewater maintains, rates a rapid on what is actually in it: the size and power of the waves, whether the route is obvious, how consequential a swim would be, whether rescue is possible. It says nothing about feet per mile. No governing body has ever published a gradient-to-class conversion, and any table claiming to be one, this page’s included, is paddlers’ shorthand.
What the shorthand is worth: it is a decent first filter for a river you have never seen, and it is wrong often enough that you should never put on the water with only this number.
| Gradient | Usually rates around | What that means |
|---|---|---|
| Under 10 ft/mile | Class I | Moving water, gentle riffles |
| 10-20 ft/mile | Class II | Clear channels, some maneuvering |
| 20-40 ft/mile | Class III | Moderate rapids, irregular waves, scouting advisable |
| 40-60 ft/mile | Class IV | Powerful rapids, precise technique required |
| 60-100 ft/mile | Class V | Extremely difficult, violent water, expert only |
| Over 100 ft/mile | Class VI | Limits of navigability, extremely dangerous |
Rock character matters as much as gradient. A 40 ft/mile granite gorge with siphons rates differently than a 40 ft/mile sandstone river with sandy banks and no strainers. Volume matters too, and the shorthand ignores it completely: a big-water river moving 20,000 cubic feet a second at 15 ft/mile can be far more serious than a creek at 60.
To find elevations, use USGS topo maps or the National Map Viewer. River length should be measured along the actual river path, not straight-line distance. Meandering rivers are always longer than they appear on a map, sometimes by 20-40%, and measuring the straight line inflates your gradient by the same proportion.
Water level changes everything. A Class III rapid at medium flow can become Class V at flood stage. Gradient tells you the baseline; gauge readings tell you the current conditions.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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