River Foss

The River Foss is an improved river in North Yorkshire, England, and a tributary of the River Ouse. It rises at Oulston reservoir at Yearsley and runs south through the Vale of York to York itself, which stands at the confluence of the Ouse and the Foss.William the Conquerer dammed the River Foss in 1069 just south of York Castle close to its confluence with the Ouse in order to create a moat around the castle complex. The damming also caused the river to flood further upstream in what is now the Hungate and Layerthorpe areas, forming a large lake that would become known as the "Kings Pool". The Kings Pool became an integral part of the city's inner defences during the Middle Ages (which explains the absence of defensive wall in the area today) and became well-known for its abundance of fish. By the 17th Century, however, the Kings Pool and the Foss as a whole were in a state of decline. Silt from upriver had been collecting in the lake and there was not enough water coming down to move it on. This continued over the centuries to a point that the lake had become too shallow to remain viable as a defence of the city. Indeed by 1644 the lake was shallow enough for Parliamentarian forces under Sir Thomas Fairfax to consider crossing it on foot as a way of breaking the Siege of York during the English Civil War. A century later the waters were so low that marshy islands had begun to appear, giving the area its modern name of Foss Islands. During the summer months the river smelled foul and was mostly dry and stagnant. The authorities knew that something had to be done.
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