The following extract from the Dumfries & Galloway Standard in 1949 answers this question.
The presence of mind and forsight displayed by Paul Roxburgh, Lochmaben, have resulted in an excellent specimen of an early period dugout canoe being added to the collection in Dumfries Museum. At the weekend, young Roxburgh saw the canoe at the water's edge in the Castle Loch and realised it might have some historic value. He mustered his friends--Asher Banks, National Bank House; Robin Richardson, Westcroft and Ernest Ferguson, Princes Street--and they salvaged the boat and left it in a place of safety.
On returning to Dumfries Academy, he reported his find to his History master who got in touch with Mr R C Reid, Cleuchbrae, a former president of the Dumfries & Galloway Natural History and Antiquarian Society. Mr Reid communicated with Mr A E Truckell. the curator of the local museum, who in a statement about the find, said: "We went out to the Castle Loch earlier this week and examined the canoe and then arranged for its transport to the museum. It is an unusually good specimen of a period we do not have represented in the museum--the end of the Roman occupation until about 1200. The canoe is of oak, is 14 feet long and has a beam of three and a half feet. It is almost flat, most of the wooden upper structure having disappeared".
Following the spell of dry weather, the water level in the Castle Loch is unusually low, and it is believed that is why the canoe has not been spotted below.
Postscript: One of the Castle Loch sculptures--a canoe--is a reminder of this find.
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