The Yarra Yarra Catchment in the Northern Agricultural Region (NAR) is centred about 350 km NNE of Perth and straddles the wheatbelt – rangelands boundary. We are concerned only with the agricultural portion — an area of around one million hectares, including the towns of Carnamah, Kalannie, Morawa, Perenjori and Three Springs. The climate is Mediterranean, with hot, dry summers and cool, wet winters. Drainage is internal. Irregular surface flow is directed to a chain of several thousand ephemeral saltlakes and samphire-covered claypans, approximately 300 km long and 250,000 ha in area. Some of these lakes rarely contain free water and there is no recorded instance of continuous flow from one end of the chain to the other. In most years (and this is the nub of the problem), surface water does not flow to the lake system at all. Instead, it ponds in waterlogged depressions or poorly defined drainage lines, and eventually drains away to the local groundwater table.
These drainage lines, as well as the broad valley floors that host them, are becoming progressively saltier, as groundwater builds up – often to within 1-2 m of the surface. Valley floors that once supported crops or diverse woodland communities, are now bare salt-scalds or samphire-covered flats. Some small depressions high on the valley sides have also become afflicted by this combination of saline groundwater and waterlogging.
Gradients are extremely low throughout the region, and the distinction between valley floor and valley sides is not always obvious. There is only about 50 m fall ‘downstream’ along the saltlake chain from saline wetlands near Burakin to Yarra Yarra Lake near Carnamah.
Location map, showing the Yarra Yarra catchment, the clearing line and the jurisdiction of the Yarra Yarra Catchment Management Group.