605 ILCS 5/10-301
Each county of this State, is authorized to acquire by purchase or otherwise, to construct, repair, maintain and operate a bridge or bridges over and across any navigable or non-navigable stream within such county, or forming a boundary line between such county and any other county in the State of Illinois, or forming a boundary line between the State of Illinois and any adjoining state, and shall have authority to exercise all such powers within its boundaries and in adjacent territory within this State, and in any adjoining state, after first having obtained authority, if any be necessary, from the Department and the United States. Such powers may be exercised either directly through the county board of any such county, or through a bridge commission created as in this Division of this Article provided.
For the purposes of this Division of this Article "Bridge" means any bridge over or across any stream, navigable or non-navigable, including any bridges and the approaches thereto, and all necessary elevated structures to eliminate intersection at grade with any streets, tunnels, public roads, thoroughfares, highways, railroads or street railroads.
605 ILCS 5/10-301