Henry is a city in Marshall County, Illinois, United States. The population was 2,320 at the 2020 census. It is part of the Peoria Metropolitan Statistical Area. History Henry is named after General James D. Henry and was initially surveyed in 1834. The topography of the land on the west side of the Illinois River, with relatively steep banks rising well above river level, assured early settlers that their homes would not flood. Its slogan, "Best Town in Illinois by a Dam Site", is derived from the city's distinction of having the first lock and dam built on the Illinois River. It was completed in 1870 at a cost of $400,000. The retreat house of the Roman Catholic Diocese of Peoria (Nazareth Retreat House, formerly King's House) is located here. Geography Henry is located in northern Marshall County at (41.113152, -89.360218). It is bordered to the east by the Illinois River, which forms the boundary with Putnam County. Illinois Route 18 passes through the center of Henry as Third Street and School Street, leading southwest across the Illinois River on the Henry Bridge, then east to Streator. Illinois Route 29 passes through the west side of the city, leading north and east to Spring Valley and southwest to Peoria. According to the U.S. Census Bureau, Henry has a total area of , of which are land and , or 3.85%, are water, consisting of the Illinois River and its inlets. Geomorphology and geology Henry lies on a segment of a river terrace that is about long and just over wide. This fluvial terrace is underlain by stratified; yellowish brown to grayish brown; calcareous; and usually clean and moderately well sorted sand and gravel with cobbles and boulders. These sands and gravels contain occasional beds of silt and clay and unconformably overlie older sand and gravel deposits, glacial till]], or bedrock. The sands and gravels underlying the terrace on which Henry lies were deposited by the Kankakee Torrent about 19,000 BP calibrated years ago. Within the Illinois River valley, these sediments typically range from thick. These and other Wisconsinan coarse-grained, fluvial sands and gravel within Illinois have been named after the city of Henry and are currently known as the Henry Formation, a geological formation. The parts of the Henry Formation which are of fluvial or glaciofluvial origin are designated as the Mackinaw facies of the Henry Formation. The original exposure