Sediment yields and turbidity records from small upland subcatchments in the Warragamba Dam Catchment, southern New South Wales

Abstract

Accelerated rates of soil erosion, following European settlement in Australia, pose a major threat to river ecosystems and water supply storages with agricultural catchments, such as Warragamba Dam (the main water supply dam for Sydney).

Sediment yields from 1988 to 1999 have been determined by stream monitoring for 6 small grazing catchments located near Goulburn in southern New South Wales, in the outer Warragamba catchment. Average specific sediment yields were 0.07 t/ha.year from 3 small catchments of around 10 ha, where the drainage lines were not continuously gullied, and the drainage lines were undisturbed. Sediment yields from larger gullied catchments of 29, 52, and 510 ha were at least an order of magnitude higher than from the ungullied catchments at around 1 t/ha.year. Gullies continue to be the dominant source of sediment from these subcatchments despite indications that yields from gullies in the Southern Tablelands are much less than in the first few decades following settlement and are gradually declining as gullies move towards a stable vegetated state. Measured sediment yields from this study are comparable with yields for southern New South Wales determined using other methods, such as radionuclide and farm dam survey techniques. The impact of soil conservation works, such as sediment trapping and grade stabilisation works, on sediment yields can be mixed depending on the activity of the gully being treated and the mobility of the bed sediments.

Turbidity levels from runoff events from the gullied catchments were an order of magnitude higher than from the ungullied catchments. For the Warragamba catchment turbidity levels are scale dependent; at the farm paddock scale turbidities are generally low where the land use is grazing, they increase dramatically at the subcatchment scale where active gullies dominate the drainage network, and then drop back again at the large river catchment scale. Turbidity is positively correlated with sediment concentration, so works that reduce sediment loads will also reduce the turbidity of water leaving the subcatchment. However, the link between high turbidity levels in small upland catchments and downstream water quality in large complex river basins is tenuous.