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 — solidly 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.
General gradient classes (American Whitewater scale): Class I: under 10 ft/mile — moving water, gentle riffles, easy Class II: 10–20 ft/mile — clear channels, some maneuvering needed Class III: 20–40 ft/mile — moderate rapids, irregular waves, scouting advisable Class IV: 40–60 ft/mile — powerful rapids, precise technique required Class V: 60–100 ft/mile — extremely difficult, violent water, expert only Class VI: over 100 ft/mile — 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.
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%.
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.