For a century, USGS measured steam discharge using current meters.

Measuring stream discharge using the "current meter" method

The most common method used by the USGS for measuring discharge is the mechanical current-meter method. In this method, the stream channel cross section is divided into numerous vertical subsections (diagram to the left). In each subsection, the area is obtained by measuring the width and depth of the subsection, and the water velocity is determined using a current meter (left-side picture below). The discharge in each subsection is computed by multiplying the subsection area by the measured velocity. The total discharge is then computed by summing the discharge of each subsection.

► Find out how streamflow is measured.