The evolution of natural river channels is strongly affected by the interplay between the altimetric pattern and the planimetric configuration acting contemporary in branches of braided rivers. Very few quantitative observations are presently available to characterize such interactions. In the present contribution we discuss the results of experimental runs performed with both uniform and graded sediments. The experiments were aimed at describing quantitatively the evolution of a single laterally unconstrained channel until the occurrence of the first bifurcation. An objective criterion for the occurrence of the bifurcation has been established using the data provided by the Fourier analysis of the evolving bank profiles; the procedure enabled us to characterize the morphodynamic sequence leading to flow and channel bifurcation. The sought outcome of the investigation is to derive a suitable description of the bifurcation process to be implemented in predictive models for braiding evolution, for which physically based nodal point conditions would be highly desirable.