I’ve long wondered about half the Danube being emptied into a field in Serbia, I’m glad to see we have @Duja to manually survey! 
The problem is n6731666597. Water flows in from w716661430 (which is tagged waterway=river), and only flows out through w716661432, which is only waterway=canal. After 2 (a & b) other =canal ways, it rejoins the Danube at n10098027262.
I can’t see any mapped pumping station in OSM 
. Are you sure =canal is correct for w716661432? Does the water flow along that way? I thought canals were “always” still water. If you change the tags to something else which is more accurate, it would make the calculations work. waterway=canal,flowing_water=yes sounds too much like (mis)tagging for the renderer waterwaymap, so I don’t want to encourage it.
ⓐⓜⓐⓝⓓⓐ 