On cold nights katabatic winds flow off the mountains into the Derwent Valley and, if the air is moist enough, this cold air forms a fog which then drains down the Derwent valley like a second river. This is the "Bridgewater Jerry"
It flows into Hobart over Bridgewater, hence the first part of its name but the "Jerry" is a bit of a mystery. One theory says that "jerry" is London thieves slang for fog, which sounds plausible, but no one really knows...
(been offline for a bit - will catch up soon)