Complex sentence: Urban, industrial, and intensive agricultural use have made it one of the most polluted rivers in Quebec, especially from agricultural waste and pesticides; nevertheless, many municipalities use it as their source for drinking water.
Split into simpler sentences: Urban, industrial, and intensive agricultural use; have seriously polluted it, making it one of the most polluted in Quebec, especially from agricultural waste and pesticides. Nevertheless, many municipalities use the river as their source for drinking water.