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 there may be too many fish and other species in the water because the upstream cities use it all up so the river runs completely dry

Explanation:

Treated sewage may still contain contaminants, posing risks to downstream drinking - water sources. The presence of fish is not directly related to drinking - water issues from upstream, and while water over - extraction is a concern, it's less common for rivers to run completely dry due to upstream use for drinking - water problems.

Answer:

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