Question

why is using river water for drinking water a possible problem for cities downstream from other cities? because treated sewage is pumped back into the river by upstream cities because the upstream cities use it all up so the river runs completely dry because there may be too many fish and other species in the water

Explanation:

Treated sewage from upstream cities still contains contaminants. Even after treatment, it may have residual pollutants like chemicals, pathogens, or trace substances. These can affect the water quality for downstream cities using river water for drinking. Upstream cities don't typically use all water to dry the river (as rivers have sources like rainfall, groundwater inflow). And fish and species presence isn't a direct problem for drinking - water (unless in extreme pollution - related die - off scenarios, but that's secondary to sewage input).

Answer:

because treated sewage is pumped back into the river by upstream cities