Ottawa is a city located at the confluence of the navigable Illinois River and Fox River in LaSalle County, Illinois, United States. The Illinois River is a conduit for river barges and connects Lake Michigan at Chicago, to the Mississippi River, and North America's 25,000 mile river system. The population estimate was 18,063, as of 2019. It is the county seat of LaSalle County and it is the principal city of the Ottawa, IL Micropolitan Statistical Area.