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River Foss
The River Foss at York
Origin	Near Oulston Reservoir, Yearsley
Mouth	River Ouse, York
Basin countries	England
Source elevation	160 m (525 ft)

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 "King's Pool" (or "King's Fishpool"). The King's 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 King's 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, and the townspeople had taken to throwing animal waste and other rubbish into the water. During the summer months the river smelled foul and was mostly dry and stagnant. The authorities knew that something had to be done.

Improvement works were promoted by Acts of Parliament in 1793 and 1801. In the works that followed what remained of the King's Pool finally disappeared and the newly-formed Foss Navigation Company canalised the river and sought to make it navigable as far as Sheriff Hutton. Today, it is only navigable for just over a mile from Monk Bridge to Blue Bridge, where it joins the Ouse.

The Foss Barrier is built across the river near its mouth. When closed, it prevents floodwater from the much larger Ouse flowing up the Foss and flooding those parts of York in its vicinity. Water flowing down the Foss can be pumped over the top of the barrier and into the Ouse.

• Evans, Antonia (ed) (2002). The York Book. York: Blue Bridge. ISBN 0-9542749-0-3. 

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