One of the Kennet's sources is Swallowhead Spring near Silbury Hill in Wiltshire, the other being a collection of tributaries north of Avebury near the rural settlements of Uffcott and Broad Hinton which flow south past Avebury and join up with the waters from Swallowhead Spring.
The upper reaches of the River Kennet have two tributaries: the River Og which flows into the Kennet at Marlborough and the River Dun which enters at Hungerford.