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Northwestern Illinois is generally considered to consist of the following area: Jo Daviess County, Carroll County, Whiteside County, Stephenson County, Winnebago County, Ogle County, and Lee County. Northwestern Illinois borders the states of Iowa to the west and Wisconsin to the north.
The largest city in this region, located near the eastern edge, is Rockford. Rockford's Metropolitan Area includes Winnebago County and Boone County (to the east), with an estimated 2009 population of 353,722. 
The geography of northwestern Illinois varies from that of the rest of the state. The area is primarily in the Coulee Region, which is characterized by bluffs, valleys, and rolling hills. Two counties most characteristic of the Coulee region are Jo Daviess County and Carroll.
- "Table 1. Annual Estimates of the Population of Metropolitan and Micropolitan Statistical Areas: April 1, 2000 to July 1, 2009 (CBSA-EST2009-01)". 2009 Population Estimates. United States Census Bureau, Population Division. 2010-03-23. Archived from the original ( CSV) on June 15, 2010. Retrieved 2010-03-26.