The name is a tautology, since "burn" is a Lowland Scots/Northern English word referring to a small river or large brook.
The name is a tautology, consisting of the word "loch" (of Gaelic origin) and vatn, a Norse word meaning the very same, found in such names as "Þingvallavatn" and Myvatn in Iceland, and "Røssvatnet" and "Møsvatn" in Norway.