From WordNet (r) 3.0 (2006) [wn]:
billabong
n 1: a stagnant pool of water in the bed of a stream that flows
intermittently
2: a branch of a river made by water flowing from the main
stream only when the water level is high
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Billabong \Bil"la*bong`\, n. [Native name.]
In Australia, a blind channel leading out from a river; --
sometimes called an anabranch. This is the sense of the
word as used in the Public Works Department; but the term has
also been locally applied to mere back-waters forming
stagnant pools and to certain water channels arising from a
source.
[Webster 1913 Suppl.]