what is source of Thames river Ontario?

Answer 1

The Thames rises at three distinct points near Mitchell (North Thames), Hickson (Middle Thames) and Tavistock (South Thames). The Middle and South Thames join east of London and the North and South Branches meet at the Forks in London, the city’s most important historical landmark. From there, the river flows southwest passing through several communities including Chatham and four First Nations Reserves before it empties into Lake St. Clair at Lighthouse Cove.

The upper branches of the river flow through ancient glacial spillways that formed following the last glacial retreat. The river beds are rocky and the valley slopes are steep, with bluffs or terraces on at least one side. In contrast, the lower Thames carved its own shallow channel into the flat plains of clay and sand. Here, the river bed is soft and the water flow is gentle.
The majority (about 98%) of the Thames watershed is in private ownership with several parcels of publicly owned land scattered throughout.
http://www.chrs.ca/Rivers/Thames/Thames-F_e.htm
Here is a map showing it:
http://www.chrs.ca/Rivers/Thames/Thames-M_e.htm

Answer 2

Technically, there are three sources:

Mitchell (North Thames), Hickson (Middle Thames), and Tavistock (South Thames).

The Middle and South Thames join east of London (to become the South Branch). The North Thames and South Branch meet at the Forks in London, the city’s most important historical landmark. From there, the river flows southwest passing through several communities including Chatham and four First Nations Reserves before it empties into Lake St. Clair at Lighthouse Cove.