Discharge measurements in rivers rely on direct measurements of the velocity field, or on water-level measurements and stage-discharge relations. With the development of digital imagery and photogrammetry, it possible to obtain surface-velocity measurements in streams from aerial images acquired for example using a drone flying over the considered river-reach. The key challenge is then to transform these velocity measurements into discharge measurements. Research on these topics is performed both in the lab and on the field to solve the related issues.