The Thames River begins in a swampy area of southwestern Ontario and meanders quietly for 273 km past the cities of Woodstock, London and Chatham-Kent to empty into Lake St. Clair. At the Forks, in London, the North Thames River joins it. The Thames was one of the first rivers in Ontario formed following the retreat of the last continental ice sheet 15 000 years ago. The river's upper reaches still flow through the ancient spillways. The lower reaches gently flow through flat plains of clay and sand that were laid down by glacial lakes.

Along much of its length, the Thames is flanked by rich Carolinian forest with tree species that include tulip, paw paw, Kentucky coffee and sassafras. Wildlife and fish found in its muddy waters or along its banks include many that are rarely found elsewhere in Canada, such as the eastern spiny softshell turtle, queen snake, black redhorse (see Sucker) and Virginia opossum. The diversity of species is reflected in the rich cultural heritage of the Thames. Its fertile valley has been home to people for over 11 000 years. Wars have been fought here, and commercial farming in Canada had its roots here. Much of the Thames River valley still appears as it did 200 years ago, and many early buildings are still standing.

Called Askunesippi, "Antlered River," by the Neutrals, it was renamed La Tranchée (later La Tranche), "The Trench," by early French explorers and settlers. It was given its present-day name in 1792 by the lieutenant-governor of Upper Canada, John Graves Simcoe after the Thames River in England. In 2000 it became part of the Canadian Heritage Rivers System.