Have just been reading a web-site about the Thames, which tells you pretty much everything you need to know about the Thames. Thirty eight tributaries feed into the Thames on its way to the sea; its source is as thesweetcheat said, is disputed between Trewsbury Mead and Seven Springs a little further north – which is the source of the Churn.
http://www.the-river-thames.co.uk/thames.htm

Scrolling down the link page there is a good map of the Thames catchment area which shows the Kennet; it also illustrates well that there could be many contenders for the title of ‘source’. To the people who built Silbury from the chalklands they lived on and who somehow transported the massive sarsens from Fyfield Down to build the stone circle and the Avenues, the Kennet was their source … their link to great sea beyond.

Up around Lechlade the land is a flood plain, the soil is not chalky and there are no remaining burial grounds to my knowledge. Except perhaps at the at Inglesham the site of a lost village where now only a farm and the redundant church of John the Baptist stands. It is a very special little church and stands on a mound surrounded by water meadows in a meander of the Upper Thames. Anyone interested in the Christianisation of ancient sites will know that John the Baptist represents midsummer eve.