The river originates in the hills above Dallas, in Moray, and has its source 400 metres (1,300 ft) above sea-level.

By the time it moves through Elgin its rate of flow, in normal conditions, is best described as very slow.

The gradient between Elgin and Lossiemouth is almost imperceptible with a total fall of less than 5 metres (16 ft).

The hydronym Lossie was recorded either as Loscyn or Lostyn in 1189, and may be of Pictish origin.

[1] The Lossie has been identified with the Loksa recorded by Ptolemy,[1] although River Findhorn is a more likely candidate for such identification.