Some difficulties arise in predicting the stream morphology and sediment runoff, in and from a sediment deposited area typical in mountain streams, because a stream channel often forms in the restricted region and changes its course. This paper describes the mechanism of a stream formation and the relation between the sediment runoff and the process of a stream change. A mathematical model, which is useful for a regime criterion of channel formation as well as for calculating the cross-sectional shape of a stream, is presented, and applies to the flume tests. Numerical predictions are made with modified one-dimensional model for the sediment runoff and channel variation, and compared with data obtained from a flume experiment.