Effects of sediment supply and grain sorting on the formation process of low-watercourse in a straight channel with alternate bars are investigated by means of flume tests using uniform and non-uniform sediment. Results are summarized as follows:(1) The low-watercourse is shallow and unstable when the bed level does not vary by sediment supply, whereas the deep and stable low-watercourse is formed when the bed degradation progresses by no sediment supply.(2) The emerged bars in the non-uniform sediment bed are formed with grain sorting, and their upper and lower part mainly consists of coarse grains and fine grains, respectively. These emerged bars are more stable than those in the uniform sediment bed, and contribute to the stability of the low-watercourse.(3) The meandering wavelength of the low-watercourse increases considerably with progression of the bed degradation, and is about 2-3 times the mean wavelength of the alternate bars.