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Note: Until the 1920s, the proportion of the tri-county population living in incorporated areas generally increased as municipalities annexed suburbanizing areas. As annexation waned and more growth occurred in unincorporated areas, especially in Multnomah County, special districts proliferated. The proportion of the population living in municipalities has increased and reliance on special districts has decreased with new municipal incorporations and annexations since the 1970s.
Sources: Population from decennial census, various years; number of special districts for 1920–40 from Oregon, Problems of the Urban Fringe, 2: 51–5, and for later periods from Census of Governments, 1952–2002.
Taylor, Zack, "Figure 7.5 Incorporated area population and special districts, tri-county area, 1860-2000.pdf" (2019). Shaping the Metropolis: Institutions and Urbanization in the United States and Canada by Zack Taylor. 26.