The solution technique used for the unsteady flow equations is completely different from that used for the steady flow equations; so the paradigm of “upstream” or “downstream” computations does not apply. The unsteady flow equations are mixed partial differential equations that are transformed into finite difference equations. The result of all this is a set of non-linear equations that are solved simultaneously by a matrix solver for each time step. This is a major difference between steady and unsteady flow modeling and leads to the cross sections being described by tables, etc. A more detailed description of the mathematics involved can be found in the HEC-UNET manual (CPD-66). (UNET was the precursor to RAS for HEC unsteady flow modeling.)