Non-decreasing flow

[Bearman]

I am working on a stream analysis in unsteady state flow. The flow input is a hydrograph at the upstream most cross-section. The hydrograph starts at 0, peaks, and then returns to 0. The problem is, I can see the flow in the first cross-section peak and return to 0. The flow in the remaining cross-sections will never return to 0, but level out at some flow other than 0. Any suggestions