The mystery of the floating islands near Colac (Vic)

Back in the early 1990s, I remember seeing a news story on some floating islands – which the locals were towing to the other side of a pond after the wind had blown them to one side. It was one of those quirky stories which run at the end of the news edition, but it’s a story that has intrigued me ever since, and more so since I’ve been working around wetlands.

For background, the Floating Islands Reserve, which is where this story came from, is located on the Princes Highway, on the Stoney Rises near Pirron Yallock, about 20km west of Colac in western Victoria. I’ve dropped in there for a look several times but have never seen any water there, let alone floating islands. When I first moved to Warrnambool in 1992, there was also the Koala Motel, a petrol station and animal sanctuary in the vicinity. The petrol station burnt down in the early 2000s, the animal sanctuary is long gone and the wetland hasn’t held water for a very long time. But, I always think about that news story and ponder what happened to the floating islands every time I drive past.

The floating islands are thought to have formed after a bushfire burnt out the peat in what was then shallow wetlands. The resulting deeper basin then filled with water and the remaining, unburnt peat areas became detached and started to float. A combination of outlet blocking by road construction and high rainfall in 1952 raised the level of the wetland even more, and the islands reportedly floated uninterrupted for 30 years. After a drought in 1982, combined with abstraction of water by the nearby motel, the water level declined and the islands became grounded. However, the news story I refer to above suggests that they did fill again. This certainly corresponds with a period of higher rainfall through the early 1990s as shown in the chart below, supported by aerial images showing the wetland pretty full in summer 1989.

When the management plan for the reserve was drafted in 1983, these floating islands were considered the only example in Australia and one of only a few in the world. With this recognised importance, a strong emphasis was placed on water management and a hydrological study was undertaken. Priority recommendations were to cease local abstraction, and to siphon water into the wetland from a dam which caught runoff from the adjacent highway. Obviously local abstraction has ceased now that the motel has disappeared, but this wetland has not filled for a long time.

If we look at the rainfall patterns (above) we can see that rainfall has remained close to or below average since the later 1990s. Groundwater trends (based on the nearest observation well) also show that levels have been dropping since the millennium drought. So, both their formation and disappearance have occurred over a very short time-frame, geologically speaking. None the less, for the relatively short time they existed, they were a pretty special occurrence. And, now I can drive past without wondering whatever happened to those mysterious floating islands!