We shall consider the one-dimensional Saint-Venant equations for the numerical simulation of shallow water flows on a canals network. We assume that no source term is present and we deal with the delicate problem of defining and simulating the dynamic at the canals junction. After revisiting the basics of Runge-Kutta (RK) discontinuous Galerkin (DG) on a single canal, we describe the numerical coupling condition at the junction according to the two main used approaches, which impose the equality of water height and the continuity of energy (besides the conservation of flow) between incoming and outgoing canals. We conclude giving some numerical tests on one-to-two and two-to-one junctions.