Overbourn Marshes
From Tolkien Gateway
The Overbourn Marshes were located in the Southfarthing, south of the Shirebourn river, where it flowed into the Brandywine river.[1]
Etymology
The name probably means 'over (across) the bourn (stream)'. Old English ofer means "border, margin, edge" and the element -bourn, common in English river- and place-names, refers to the Shirebourn.[2]