A new shape function for self-formed straight threshold channels in uniform bed material is proposed, based on the static equilibrium of forces acting on individual particles lying at threshold on the channel boundary. The combination of this function with the Shiono and Knight depth-averaged momentum equation, the threshold condition of Ackers and White and the resistance law of White, Paris and Bettess gives a physically-based geometric model for the design of threshold channels. Geometric dimensions predicted by the model are shown to be consistent with experimental data.