what county is idaho falls id in
Courthouse1024.jpg. http://www.co.bonneville.id.us/images/SiteImages/Courthouse1024.jpg. Bonneville County, in southeast Idaho, is part of the Upper Snake River Valley. Its eastern border is also the state border of Idaho and Wyoming. Idaho Falls, originally called Eagle Rock, is the county seat. Incorporated cities in Bonneville County include Ammon, Idaho Falls, Iona, Irwin, Swan Valley, and Ucon. Bonneville is the fourth largest county in the state with a population of more than 104,000.