The River Foss is an improved river in North Yorkshire, England, and a tributary of the River Ouse. It rises in the Foss Crooks woods near Oulston reservoir close to the village of Yearsley and runs south through the Vale of York to the Ouse. The name most likely comes from the Latin word Fossa, meaning ditch and is mentioned in the Domesday Book. The York district was settled by Norwegian and Danish people, so parts of the place names could be old Norse. Referring to the etymological dictionary "Etymologisk ordbog", ISBN 82-905-2016-6 dealing with the common Danish and Norwegian languages - roots of words and the original meaning. The old Norse word Fos (waterfall) meaning impetuous. The River Foss was dammed, and even though the elevation to the river Ouse is small, a waterfall was formed. This may have lead to the name Fos, and later changed to Foss.