The River Windrush was another factor in Witney's success: it provided large volumes of clean water for many cloth-making processes and a good millstream to power fulling mills.

Straight from the loom a piece of woollen cloth is a coarse material with little substance: after it has been fulled, dried, stretched and raised it is transformed into a softer thicker and fluffier cloth. In the fulling process the cloth was repeatedly pounded in water with fuller's earth to scour and shrink the wool; the beating action matted the fibres until the cloth looked like felt. Urine was another essential ingredient to the process and Witney inns had large tanks for its convenient collection.

In the very early days the fullers used to trample the cloth by foot in vats but fulling was the first process to be "industrialised'.

By 1277 there were three fulling mills in the manor of Witney. Water power was used to alternately raise and drop pairs of heavy wooden hammers in a trough called a stock.