Discharge is derived from the stage data through the use of a relation between stage and discharge. The stage- discharge relation for a specific stream location is defined from periodic discharge measurements made at known stages. The most practical method of measuring the discharge of a stream is through the velocity- area method. This method requires the physical measurement of the cross-sectional area and the velocity of the flowing water. Discharge is determined as the product of the area times the velocity.
Velocity is measured by using a current meter.
The meter consists of a propeller that is rotated by the action of flowing water. The rotation depends on the velocity of the water passing by the propeller. With each complete rotation, an electrical circuit is completed and recorded in some fashion. Given the number of revolutions in a given time interval, velocity can be determined for the location of the current meter. Measuring the average velocity of an entire cross section is impractical, so the method uses an incremental method.