See area 7 on Aerial Photo Map. The purpose, history and current management of the Back Paddock and its fence is described on the Bandicoot page. In particular that page describes the problems with management of Kangaroos in the back paddock.
Also of concern are the Swamp Wallabies. A small number of these were introduced into the Back Paddock soon after it's construction (National Parks Service 1988), despite the fact that there appears to have been no suitable habitat except the scrub on Gellibrand Hill. Gellibrand Hill is no longer in the Back Paddock but the Wallabies still are, and they are now considerably overpopulated. Of their preferred foods, the only ones present in the Back Paddock are a few shrubs of Tree Violet and perhaps some fungus in the cooler months. The Tree Violets have almost no foliage left, and the Wallabies have been eating the bark instead. They are also eating Red Gum leaves, a food supply which is not yet exhausted but which they may only be able to tolerate a limited amount of given its toxicity. Another food available in winter is the weed Bridal Creeper, which covers several hectares in the north-west corner of the back paddock. The Bridal Creeper grows to about two metres high on Gellibrand Hill just outside the back paddock, but inside the fence is only a few centimetres high.
In summer the Wallabies eat the bark from Sweet Bursaria. Some Bursarias I inspected had obviously had their bark eaten off in at least two previous years. On Gellibrand Hill, previous episodes have killed some Bursarias. I saw at least one Sweet Bursaria in the Back Paddock where all the leaves are dead after recent ring-barking by wallabies.
The Wallabies pose a threat to themselves and to the vegetation, but unlike the Kangaroos they are unlikely to be a threat to the Bandicoots.
Creeks and Gullies
There are three gullies or drainage lines running from north to south through the back paddock. The eastern one has Cumberland Dam on it but is the driest of the three.
The central gully is glorified by the name Greenvale Creek, although like the other gullies it rarely flows. It drains a large area of the former Greenvale Sanatorium land before entering the back paddock. Since European invasion it has undergone several sudden changes in the amount of water that flows down it, as well as more gradual changes due to alterations in vegetation and soil. The first sudden change was the construction of a dam which we call the Sanatorium Dam even though it was constructed before 1869. It was enlarged in 1923 as described on the page about the Sanatorium water supply.
Another sudden change occurred in 1929 when the Sanatorium was connected to mains water. From then on there was a more or less continuous flow of drainage and sewage effluent down the creek, increasing when the complex was enlarged in the 1950s and again in 1974. An Environment Protection Authority licence issued in 1981 allowed the discharge of up to 9000 l/h of treated effluent. This would have come to an abrupt halt in 1998 when the hospital was closed.
Enlargement of the complex would also have resulted in increased rainfall run-off due to all the paved roads and rooftops, but this would have flowed through the Back Paddock only when the dam overflowed.
Erosion of the creek probably started long before the Sanatorium was built. The continuous flow of effluent down it may have supported vegetation growth which would have decreased erosion in the Back Paddock. Now that the hospital is closed there is no grass in the creek to slow erosion. Erosion starts at the culvert under the Sanatorium access road, but the worst erosion occurs just below the track crossing constructed by Parks Victoria in the centre of the back paddock.
The eastern gully starts just inside the north-east corner of the back paddock. About half way down is a dam which is the only one in the park that has never been known to dry out. Most of the water flowing into it seems to come from the erosion zone south of Gellibrand Hill, which accounts for the muddy colour from suspended clay.
Dams contribute to erosion because water flows less often downstream of them. The Eastern gully is indeed eroded downstream of the dam, and the erosion is worse downstream of a second dam which rarely fills.
Another thing to observe about the eastern gully is that except at the lowest end, the Red Gums along it are all dead or dying. There are only a couple of patches of live Red Gum saplings, one of which is dependant on the lower dam.