During the late 19th century, city leaders debated the responsibility and expense of creating a waterworks system and disposal of sewage. Downstream users complained about Pittsburgh's dumping of sewage into the Ohio River. Allegheny County cities did not stop discharging raw sewage into rivers until 1939. Pittsburgh's smoke pollution, seen in the 1890s as a sign of prosperity, was recognized as a problem in the Progressive Era and was cleared up in the 1930s–1940s. Steel plants deposited mountains of slag until 1972, especially in Nine Mile Run Valley.