Loch Lomond is a large lake about twenty miles northwest of Glasgow. Geoffrey of Monmouth tells of an offensive by Arthur against the Picts and Scots at Loch Lomand. One can note that one of the candidates for the battles at the River Douglas that Nennius mentions is a mountain river that flows into Loch Lomond at the town of Inverbeg.
Nennius also mentions Loch Lomond (which he calls Loch Leven) in his list of "The Wonders of Britain" . Geoffrey's description of the loch is almost identical to that Nennius. In both accounts, the loch is fed by sixty rivers and has sixty islands, each with sixty eagle's nests. Geoffrey may of course been copying from Nennius.
"[Arthur] now led his army to Moray, where the Scots and the Picts were
under siege. They had fought three times against
the King and his nephew, suffering defeat at Arthur's hands and then seeking refuge in this particular district. When they
reached Loch Lomond, they took possession of the islands in the lake, hoping to find a safe refuge on them . . . Arthur collected together a fleet of boats and sailed round the rivers. By besieging his enemies for fifteen days, he reduced them to such as state of famine that they died in their thousands."
History of the Kings of Britain, 218-219.
Scottish Arthurian Sites