When the name Thames was originally considered for the river flowing through London, Ontario, the river was known mostly by reputation. The river was reputed to be big and long. It was said that it might well be the biggest river system in southwestern Ontario. It's reputation brought to mind its English namesake. It is long but big? I don't think so. The biggest river in the area is the well named Grand River.
The picture shows the Thames River flowing out of London heading toward Chatham before it empties into Lake St. Clair. Head up river from here and after very few kilometres one reaches the Forks of the Thames in the core of the city.
The North Branch of the Thames meets the Thames River proper at the forks but one would not know it from the local stories, mostly wrong. The North Branch is bigger than the Thames River itself. The North Branch tributary carries more water than the Thames River. This leads folk to call the Thames River the South Branch. Many believe the Thames River is the result of the merging of the two branches. Nope.
The Thames River flows east of the city to Woodstock and then turns north to its headwaters in some marsh land near Tavistock. In truth, the Thames River in southwestern Ontario is a slow, meandering, shallow river that picks up a little extra water at The Forks of Thames and then continues its lazy way west.