The goal of this paper is to present in situ data for the reflectance and transparency of river plumes and to interpret the observed variability in terms of hydrodynamic processes. The data was obtained for river plumes in the Black Sea and in Arctic Ocean regions. Spectral reflectance was measured in the 400- 700nm band by a ship/helicopter-mounted spectroradiometer and was supplemented by shipboard measurements of vertical profiles of the light attenuation coefficient. Remotely-sensed data shows the transition from freshwater to sea water with both sharp boundary fronts and wide mixing zones. Internal fronts and quasi-periodic spatial variability of reflectance were also observed within the river plume. The observations are related to the relevant hydrodynamic processes (fronts, advection and settling of particulate matter, internal waves). A radiance model was developed to match the vertical profile of optical properties to the observed variation of remotely-sensed reflectance.