A mechanical or electromagnetic instrument used to measure the velocity of flowing water at a specific point. Current meters were the traditional standard for stream discharge measurement before being largely replaced by ADCPs.
A current meter is an instrument designed to measure the velocity of water flow at a specific point in a stream, river, or open channel. Mechanical current meters consist of a rotating element—either a horizontal-axis cup-type (Price AA meter, the USGS standard for decades) or a vertical-axis propeller—connected to a counting mechanism that registers the number of revolutions per unit time, which is converted to velocity using a calibration equation. Electromagnetic current meters use Faraday's law of electromagnetic induction to measure velocity, offering advantages including no moving parts, better performance in low velocities and debris-laden water, and ability to measure in shallow depths. The velocity-area method of discharge measurement using current meters involves dividing the stream cross-section into vertical subsections, measuring velocity at each vertical (typically at 0.6 depth for shallow sections, or at 0.2 and 0.8 depth for deeper sections to capture the logarithmic velocity profile), and summing the products of velocity, depth, and width for each subsection. This method, standardized by the USGS and ISO, was the foundation of streamflow measurement for over a century. While ADCPs have replaced current meters for many applications, mechanical meters remain important for small streams, shallow flows, wading measurements, and as backup instruments. The USGS maintains a current meter calibration facility to ensure measurement accuracy.