In the sentence, "effluent" refers to waste or discharge, especially liquid waste or pollutants released into a body of water. It often includes substances from industrial processes or sewage systems. The word "sewage" is the closest in meaning to "effluent" in this context. Sewage refers to the waste matter, including human and household waste, carried away in sewers or drains. It commonly includes both liquid and solid waste. So, in the given sentence, the water in the stream has become unsafe for drinking due to the pouring of effluent, which is essentially sewage or pollutants.