The Darling catchment contains at least 10 different soil types. However, the principal soil on the river floodplain is best described as cracking clays on alluvial plains (grey and black vertosols). They often contain Gilgai’s (saucer-shaped depressions) which are interspersed on higher areas with brown and red soils, sandy soils, dunes and islands of red earth.
The various soils of the Darling catchment have influenced agricultural development and land use and are believed to have influenced the quantity and quality of water reaching the river.
Land use and soil type are interrelated. Land use since white settlement has had a significant impact on soil health and condition. Sheep and cattle grazing are the principle land use industries.
Soil erosion is the most widespread form of land degradation and remains the biggest environmental problem in much of the catchment. It is a problem which can, to a large extent, be controlled by land use, management of grazing, cultivation and structural works. Soil erosion has increased because of the unreliable climate and as cropping has expanded into marginal areas.
Erosion throughout the Darling catchment has been caused by water erosion, rill erosion (caused by concentrated run-off), gully erosion and wind erosion. Stream bank erosion is also a problem and is a major contributor to turbidity and sediment loads in waterways. Bank slumping has been a problem following flood events in the Darling River, particularly upstream of Wilcannia where banks are relatively high and steep.
Wind erosion is a particular problem for sand and sandy loam soils and soils with poor vegetation cover. Serious wind erosion occurs in western NSW when vegetation cover becomes depleted by grazing, drought and cultivation. In recent years, wind erosion has been greatly reduced by native woody shrubs which have proliferated on most red calcareous and sandy soils.
The frequency of wind storms is a crude indication of wind erosion severity, although dust storms have declined significantly since the 1950s. The greatest wind erosion hazard is cultivation for cropping in the lower Darling catchment where the soil is susceptible to erosion, rainfall is low and summer fallowing often corresponds with strong winds. Conservation farming practices have reduced this hazard but in drought years soil loss and drift can still occur.
While it is difficult to map, the decline of soil structure is also believed to be a major cause of erosion. This results from continuous cultivation, heavy trampling by stock and overstocking, particularly in dry times.
Flood irrigation can also have a detrimental impact on soil structure.