The mechanism of sandbars initiation and formation is unresolved. The occurrence of sandbars has been investigated using stability analysis, which assumes that sandbars occur due to the inherent instability of a riverbed. However, there are no data, either from riverine observations or model experiments, to support this assumption. Here, we conducted flume experiments in which sandbars were formed from a flatbed by simultaneously measuring the water surface and bottom surface. The results showed that the process of sandbars initiation and formation first involves the generation of small periodic bedforms; then, the bedforms transition to small three-dimensionally shaped rhomboid bars, and finally the rhomboid bars transition to sandbars. The measurements also suggested that wave trains occurred on the water surface. We then conducted fixed-bed experiments under the same conditions as a moving bed to ascertain the behavior of the water surface. The results of these fixed-bed experiments showed that standing waves were observed on the water surface even when the experimental conditions were steady and the flatbed channel was straight. A two-dimensional wavenumber analysis showed that the dominant wavenumbers of the standing waves and initial small bedforms were in good agreement. The whole set of results indicated that standing waves were already present on the water surface before bedforms occurred and that one of the factors in sandbars initiation was the presence of standing waves on the water surface.