Salinity is the amount of salt (sodium chloride) in a solution. In this case it is the Avon River. The Avon provides a natural drainage system for the river basin. It affects the whole Avon catchment, as it starts from the soil in the first place. Salinity is a major environmental concern in Western Australia, especially the southwest. In fact out of the whole Australia region; Western Australias southwest is the most affected area. Salinity has always been around, since Australia was settled. If you were to blame it on someone the first settlers would be the ones to blame as they are the origin of the problem. The Avons water usage, flora and fauna are all in danger with the threat of salinity.
There are many causes for salinity in the Avon catchment. Overtime many things/people have implemented into the bad problem of salinity. The first settlers were the first to start the problem. When they came, they dug up the native plants, which had deep roots, and replaced them with other foreign plants, which had shallow roots. This caused the underground water to rise (due to evaporation because there was no deep roots for the water to feed), bringing salt crystals or seeps with it. After reaching the surface it was washed down into the river by rainfall. So when farmers clear the land, for agricultural uses, the soil (containing salt) gets washed away into the Avon River from mostly rainfall.
Gibb, D. 1998. The Avon Region. (Online) http://www.wrc.wa.gov.au/ribbons/avon_region.html, March 30, 2005.