Stream Flow Calculator

Discharge is calculated as Velocity x Width x Depth, a simple product rather than a formula where one term is weighted more heavily than the others. Nudge any one of the three measurements by some fraction and Discharge moves by that same fraction, so no single measurement dominates the result the way it might in an additive or squared formula -- getting all three measurements right matters equally.

Supercritical flow (Froude Number above 1) means the water is moving faster than a surface disturbance could travel upstream against the current -- typically shallow, fast water like a steep mountain stream, a spillway chute, or a constricted channel section. Most natural streams and rivers at typical flow are Subcritical (Froude Number below 1), so a Supercritical reading usually signals either a steep gradient or an unusually constricted, shallow section.

Froude Number is velocity divided by the square root of gravitational acceleration times depth, so depth sits in the denominator under a square root -- as depth increases, that denominator grows, which lowers the ratio for the same velocity. Physically, this reflects that a surface wave travels faster in deeper water, so the flow has to move proportionally faster relative to a deeper channel to reach the same Froude Number it would at a shallower depth.

It's a simplification -- treating Cross-Section Area as Channel Width times Average Depth works well for a wide, shallow, roughly rectangular channel, but it overstates the true area of a channel with a rounded, V-shaped, or trapezoidal natural streambed, since a true rectangle assumes full depth extends edge to edge. For a precise gauging-quality discharge measurement, a surveyed cross-section with multiple depth verticals is more accurate than this single-average-depth approximation.