A view of Thames Head in The Cotswold near Kemble, England.  It consists of a set of seasonal springs in fields that come from limestone aquifers.  In actuality, the source of the River Thames does not have a fixed location – rather, it changes according to the level of the groundwater in the limestone. In dry periods, the groundwater level falls, causing the Thames Head springs to dry up and the river to begin lower down in its course. In wet conditions, the groundwaters rise and the river start here at Thames Head springs.  This spring, known as Lyd Well, is the hollow that is lined with stones.  The springs continue immediately south of the A433.  The Thames Head Stone reads the following:

THE CONSERVATORS OF THE RIVER THAMES

1857–1974

THIS STONE WAS PLACED HERE TO MARK THE

SOURCE OF THE RIVER THAMES.

From: https://en.wikipedia.org/wiki/Thames_Head