"jogging on towards the Tweed, by the side of a small river called Ellan"
The Mouth of the Allan Water as it runs into the River Tweed
Creative Commons Attribution Share AlikeThe Mouth of the Allan Water as it runs into the River Tweed - Credit: Iain Lees

This refers to Allan Water, a river which joins the Tweed just east of Selkirk. The Tweed itself is a 97-mile long river which begins in the Southern Uplands of Scotland, forms the Scottish-English border for 17 miles and empties in the North Sea in Berwick, north-east England.

 

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The point where Allan Water joins the River Tweed.