The song celebrates the profession of log driving, a practice in the lumber industry which involved transporting felled timber by having workers walk or run on the logs as they floated down rivers. This occupation required a great deal of strength and physical agility, and Hemsworth was struck by how much the sight of log drivers at work resembled dancing. The song's chorus is:
For he goes birling down a-down the white water
That's where the log driver learns to step lightly
It's birling down, a-down white water
A log driver's waltz pleases girls completely.
From Wikipedia
