James Leech filmed this clip of the marine mammal in the River Wharfe - more than 50 miles from the Humber Estuary, where it is likely to have come from. It appears to be a grey seal.
The Wharfe is a tributary of the River Ouse, which in turn enters the Humber.
Back in April, a common seal was spotted in the River Swale at Topcliffe Weir, near Thirsk. The adult is thought to have swum up from the Humber during winter flooding and become lost.
A charity, British Divers Marine Life Rescue, worked with several environmental organisations to monitor and capture the seal with the intention of returning it to the North Sea. It was finally caught on August 10 using a specially constructed net, but died during the rescue attempt. A post-mortem established that the animal was underweight.