The present invention concerns a device (1) for measuring the position of the bed and surface of a river, in which the device comprises a support (2) along which a plurality of measurement areas (5) and means (3, 5) suitable for determining the local heat dissipation in each measurement area (5) are defined; the present invention also concerns a method for determining the position of such interfaces, in which a device (1) according to the present invention is arranged so that it intersects the interface, ensuring that one or more measurement areas (5) are immersed in each of the means constituting the interface and finding the value of the power dissipated in the measurement areas (5) from the temperature measured at said means.