Pittsburgh is located in southwest Pennsylvania at the confluence of the Allegheny River and the Monongahela River, which combine to form the Ohio River. Pittsburgh is known both as "the Steel City" for its dominant role in the history of the U.S. steel industry, and as the "City of Bridges" for its 446 bridges. Aside from steel, Pittsburgh has led in the manufacturing of other important materials—aluminum and glass—and in the petroleum industry. Deindustrialization in the 1970s and 1980s laid off both blue- and white-collar workers as steel and other heavy industries declined.