I have a question relate with the convergence of two reaches upstream and two reaches downstream. I know that it is not possible to create a junction in this situation. As a solution I propose to create a small river reach between the two reaches upstream and the two reaches downstream, with a length negligible to the total length of the river.
What do you think is the best geometric solution to this situation?
Are you modeling this as a steady state or a unsteady state model?
If unsteady a trick I use often is to use a storage area as the junction. You can then drag and drop the ends of the reaches into the area and use it to split the flows.