Non-decreasing flow

[Bearman]

I have a river model I am working on. Flow is based upon a hydrograph at the upstream cross-section. The hydrograph peaks and returns to Zero flow. I can see the peak and return to zero flow in the first cross-section, but every cross-section after that will not return to zero flow. All downstream hydrographs reach a certain flow and will maintain that non-zero flow throughout the duration of the analysis, no matter the length of time used. Any suggestions?