Stage vs. flow: the two numbers every gauge reports

Every stream gauge reports two numbers, and they answer different questions.

Stage(or "gage height") is the height of the water surface in feet, measured not from the creek bottom or from sea level, but from an arbitrary per-site reference elevation called the gauge datum, a local zero chosen so the readings stay positive. That means "8 feet" is meaningless without knowing the site: at one gauge it's a trickle, at another it's a flood.

Streamflow (or discharge) is the volume of water passing the gauge, in cubic feet per second (cfs). One cfs is about 449 gallons a minute. Flow is comparable across creeks and over time, which is why swimmability thresholds are written in cfs.

Here's the interesting part: gauges don't measure flow. They physically measure stage, and flow is computed from stage using a site-specific rating curve built from years of manual field measurements. The relationship is nonlinear: at low flow, one extra foot of stage might mean 200 more cfs; at flood stage, that same foot can mean 20,000 more cfs as the channel spills onto the floodplain.

The rule we follow on this site: use cfs to describe swimmabilityand to compare creeks; use stage only against that same gauge's own bankfull and flood thresholds.