Water flow dynamics of a river has significant effect in the ecological functions played by the river. There exists an intricate relationship between discharge in a river and phenomenon such as sedimentation, prevalence of vegetation and river morphology. In order to better understand these phenomena, it is important to determine the velocity of the river. While there exist conventional in-river velocity measurement techniques, UAS based method offers the capability to make large number of measurements at a large number of locations without the need to place measuring instrument at each of these locations. In this paper, we present the results of application of UAS in determining surface velocity of river. In this study, we released floats along the river and captured videos by mounting RGB sensor as well as thermal sensor in the UAS. The videos were processed using OpenCV library as well as Matlab’s Image Processing Toolbox. This paper discusses the learnings of our study which indicates that videos captured from UAS can be used to determine surface velocity of a river.