This difficulty is caused by the fact that FEFLOW does not allow selecting faces for BCs, but chooses the faces from selected nodes. In such a case we typically refine the mesh a little bit and and/or move some nodes (Moving nodes will have to be done in the Classic interface at the moment), until we get two elements on top connected to the node where the rivers meet. Both of these elements will have one node without the BC, so the top face will not be covered between the rivers.