The source of this river is a spring situated adjacent to, and flowing into, Oulston Reservoir near Newburgh Prior, 4 miles north of Easingwold. From there to the Blue Bridge in York, where it joins the River Ouse, it is 19.5 miles in length. In 1806 part of the river was canalised as far as Sheriff Hutton Bridge but today is only navigable for 1.5 miles upstream of Castle Mills Lock. 

For walkers the Foss Walk follows much of the river course from Blue Bridge to Oulston Reservoir and then on to Easingwold, a distance of 28 miles. From Oulston Reservoir there is an alternative route via Coxwold and Husthwaite adding a further 11 miles to the walk.

Historically within the city centre the Foss formed a massive pond (the Kings Fish Pond) which provided fish for the markets and a marsh which served as part of the city's defences. There is no city wall between Layerthorpe Postern and the Red Tower as the marsh is virtually impassable.

Castle Mills Lock

Castle Mills Lock is 112 feet long and 20 feet wide. There are mooring points in the lock basin (River Ouse side of Castle Mills Lock) but NO overnight mooring on the River Foss itself. There are very limited opportunities for turning beyond Rowntree Wharf.

To access the upper river vessels have to pass through the lock and this can be arranged through either City of York Council's Engineering Consultancy for which there is a charge, or alternatively, during the autumn/winter cruising season of 2008 Castle Mills Lock can be operated by local members of the Inland Waterways Association (IWA) Ouse/Ure corridor section. Please contact them on 07588236597 (24 hour answer service) or by emailing amartin_assoc@yahoo.co.uk at least 4 days before wishing to enter the River Foss. There is no charge for this service. Please make sure you have valid insurance documents with you.