Abstract: A measurement system for collecting data relating to the total, partial or distributed flow of a river or like course is disclosed. A generally planar array of flow velocity sensors is erected transversely with respect to the general direction of flow. The array comprises a number of vertically extending strings of sensors which are anchored at one end of the bed of the course and the free end of the string is supported by a buoyant float. The sensors in a string are attached at fixed intervals along the string so that each always resides at the corresponding fixed proportion of total depth up to the maximum which can be accommodated by the length of the string. Alternative sensor designs are also discussed.