A mathematical model for erodible channels is formulated, developed and tested with field data. A significant feature of the model is its ability to account for changes in both channel width and channel-bed profile of aggrading and degrading streams. Since changes in channel width and channel-bed elevation are closely interrelated, this model represents an improvement in this direction. This model employs the techniQues for water and sediment routing; an additional condition based upon the concept of minimum stream power is used as the width predictor. Analysis using this model shows that the total stream power of a reach is reduced if the widths at all cross-sections are so adjusted as to reduce the variation in energy gradients at these sections. When this model is applied, a varied flow will eventually develop into a uniform flow for which the stream power is an eventual minimum.