About

How the gauge decides

Gauge Rafting is a made-up outfitter, built to show a rafting site whose numbers are computed, not asserted.

The formula

Every rapid has a base class at its river's reference flow, and a sensitivity: how many classes it gains each time the flow doubles. At any level, class = base + sensitivity × log₂(flow ÷ reference), kept between I and V+ and shown to the nearest half class.

Every rapid, at its reference flow and at double and half of it
River	Rapid	Base	Sensitivity	At half	At reference	At double
Elk Fork	Riffle Row	1.5	0.4	I	I+	II
Elk Fork	Boulder Garden	1.8	0.5	I+	II	II+
Elk Fork	The Staircase	2	0.5	I+	II	II+
Granite Run	Warm-up	2	0.5	I+	II	II+
Granite Run	Pinball	3	0.8	II	III	IV
Granite Run	Dentist's Chair	3.5	1	II+	III+	IV+
Granite Run	Washboard	2	0.5	I+	II	II+
Black Narrows	Gatekeeper	3.5	0.9	II+	III+	IV+
Black Narrows	The Maw	4	1.2	III	IV	V
Black Narrows	Coffin Corner	4.5	1.4	III	IV+	V+ (cap)
Black Narrows	Exit Exam	3	0.8	II	III	IV

What else is computed

A river is good between its minimum and maximum flow, pushy in the top fifth of that range or whenever its hardest rapid reaches class IV+, too low below the minimum and closed above the maximum.
A group may be asked to run no rapid harder than its limit: class II+ with no experience, III+ with a few easy trips, V+ with class III experience; and II+ under age 10, III+ under age 14. A trip that would pass a rapid above the limit is refused, with the rapid named.
The class is capped at V+: where the formula would give more, tables print "(cap)", because the model has no number beyond V+.
The group discount is 5% from four people and 10% from eight; the deposit is 25% of the total, rounded up to the dollar.
The page's colour is the river level on a heat scale, on a log scale: shallow acid yellow at the lowest flow on the site, cobalt blue at a normal flow, flood crimson at the highest.

cfs is cubic feet per second: the volume of water passing a point each second. A bathtub holds about 8 cubic feet.

A test that runs on every change recomputes each figure on this site from its source data and fails if a page prints a different number.