Using the principle of maximum entropy and field data, this paper derives the mechanism and conditions for change in the channel pattern of alluvial rivers. Theoretical analyses indicate (i) the mechanism of formation of different channel patterns in a river system is one of maximizing entropy of the system under different local flow and boundary conditions, and (ii) the river pattern with sinuosity S = π/2 ≈ 1.57 conforms to the principle of maximum entropy. The equations governing the change in a channel pattern are derived and thresholds, as the conditions of channel pattern changes, determined, using theory and field data from 70 alluvial rivers having different channel patterns. The result of this study suggests that river management and training works should help rivers achieve and maintain a sinuosity S close to 1.57 and the stability criterion Scr> 0.2.