The source of the Thames lies at Trewsbury Mead, Thames Head in a remote Gloucestershire meadow beneath the boughs of an elderly ash tree. For most of the year the spring is dry and you may find the riverbed remains without water for some distance. The infant river follows through fields and near to several small Cotswold villages, before passing through the middle of the Cotswold Water Park. Eventually the river reaches the town of Cricklade where the right of upstream navigation ends. As the river continues its path to Inglesham the flow of the river is increased by the flow of the River Coln which joins it. Inglesham is marked by the Inglesham Round House.
The OS map reference for the river's source is ST 980 994