If you mean waste water treament plants (eg: municipal sanitary waste for example), they are built "on" creeks because they discharge treated water ... if the plant was not discharging to an existing creek or river, the discharge would make a creek or river of its own! Many millions of gallons of waste water per day, from homes of a town or city, for example, has to go somewhere (once the contaminants are removed).
Good luck.
