River Difficulty Rating Calculator

The gradient score curve flattens deliberately at high values — each additional foot-per-mile above 60 adds only 0.05 points, versus up to 1.0 point per unit below 5 ft/mile. That mirrors the real Class scale: a river at 150 ft/mile isn't meaningfully harder than one at 100 ft/mile once both are already firmly in Class V territory, so extra gradient beyond that point doesn't move the needle much.

The calculator doesn't score raw discharge (cfs) directly — it divides flow rate by channel width to get flow-per-foot, a proxy for how concentrated the hydraulic power is. The same 2,000 cfs spread across a 200 ft wide river is much gentler than that same volume squeezed through a 20 ft slot, so a narrow, high-flow channel can score much higher difficulty than a wide river carrying far more total water.

Cold water can add up to 20 points to the total difficulty score, which is enough on its own to push a run up a full class or more — adding 15-20 points to a river that would otherwise sit at the low end of Class III can tip it into Class IV territory. The scoring gets steeper as temperature drops below 50°F, reflecting that hypothermia and swim time become the dominant safety factor rather than the whitewater itself.

Treat it as a rough floor, not a personalized recommendation — it's a fixed lookup by class (0 years for Class I up to 10 for Class V) and doesn't account for your specific skills, the boat you're paddling, or how that particular river behaves at the flow you entered. A paddler with five years of general whitewater experience but no big-water time could still be underprepared for a Class IV river running unusually high.