In this paper, we study a Riemann problem at a junction of two reaches. We consider the 1D Saint-Veant equations in each reach combined with particular conditions at the junction of the two reaches. The flow is the canal system is assumed to be subcritical. We have shown under a certain condition that the Riemann problem has a unique solution. However such a condition is not always satisfied, meaning that there are states where no solution to the Riemann problem can be found.