I think, that the problem can be in the Flow Regime or in the Manning´s values.
1. You could check a Freude number, if these values are around 1, so you have to use a mixed flow regime.
2. The error can be also in the Maninng´s values- roughness parameters. If you defined high values or you have big difference between channel n-value and floodplains n-values in the CS.
I noticed that there is a pumping station in my river reach (where a mine is pumping water for its processes), which I had overlooked earlier. The pump is situated at the upper part of the modeling reach. At this case I will not model any water reservoirs concerning the mine, so how can I add a pump that has no “ending point” where the water flows from the river. Is it even possible?