This paper is concerned with the study of the unsteady problem associated with numerical simulation of bore propagation through junctions, and the statement of some of its main features from the point of view of a one dimensional approach. In order to ensure the correct treatment of the involved internal boundary problem when trying to include it in a modelling system, compatibility conditions must be applied with care. The usual technique is to assume the equality among the water levels at the junction. It is shown that this is a valid approximation only for a few simplified cases. A theoretical approach is used to state the unsteady compatibility conditions to be used in the development of a one-dimensional simulation model including supercritical flows and shock propagation through the junction. Some numerical results are presented to illustrate the performance of the suggested techniques. The study does not pretend to be a programming guide but a basic discussion and, as a contribution to the evidence for and against the current technics, it becomes useful to improve modelling systems involving tributaries.