The best method for measuring the velocity of flow in rivers is

Explanation:

current meter:

• For measuring discharge or velocity of a streamflow current meter is used. 
• In this, the stream channel cross-section is divided into numerous vertical subsections. 
• 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. 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.