When the banks of a meandering river channel are reveated all along and the amplitude of meandering is kept within certain limits, the alternating sand bars keep creeping along the river channel. But when the amplitude becomes larger and reaches a certain critical point, the creeping stops and the form of the river bed becomes stabilized. It has been a very important question in view of bank protection to discover the conditions accompanying this critical point in order to secure a stabilized water route. The author, since 1963, has advocated the use of photogrammetry on flood flows, and as a result of his research he has clarified the relation between the distribution of the surface velocity vector and the movement of the sand bars. He has also studied the yearly transitions of the sand bar forms in many river channels, using aerial photos, and, along with his study in the experimental channels using photogrammetry, he has discovered a general law governing the conditions accompanying the critical point. The author applied photogrammetry to one of the main rivers in Japan, the Yodo River, at the time of flood when its waters reached the design flood discharge. He then contributed to the planning and designing of the large scale improvement project on this river. Several examples of photogrammetry showing various phenomena that can be observed in flood flows are also shown as illustrations.