The environmental characteristics of the catchment are presented largely on the basis of land systems described by C.S.I.R.O. Division of Land Research. The main features of the catchment are:-
(1) a large area of shallow, dispersible duplex soils;
(2) irregular and intense summer rainfall;
(3) nil discharge for prolonged periods and major discharges on rare occasions;
(4) grazing of native pastures
The C.S.I.R.O. land systems are also used as a basis for assessment of erosion incidence because they provide useful data concerning soil characteristics and susceptibility to erosion. Existing erosion was mapped by aerial photograph interpretation, supported by ground studies and aerial reconnaissance. Most of the erosion occurs in the north-western part of the catchment, on land characterized by the Drummond Basin geological formations. There is a lesser, but still very serious incidence in large portions of the remainder of the catchment. The extent of existing deterioration is assessed as follows:-
very severe gully erosion – 213 square miles (3.3% of catchment);
severe sheet erosion/moderate gully erosion 487 square miles (7.6%);
slight sheet and rill erosion – 4,200 square miles (65.2%);
plus at least 150 miles of actively eroding stream banks.
Alternatively, the gully erosion alone in the north-western part of the catchment can be expressed as 6,000 miles of channel averaging more than 1 chain in width.
Assessments of rate of erosion were made on the basis of estimates of depth of soil lost since settlement, comparisons of eroded areas on air photographs of different ages, sedimentation in small dams, and known loss rates on similar soils elsewhere. It is estimated that the average rate of soil loss is of the order of 9,000 acre feet per annum (14 million tons per annum).
It is considered necessary that immediate action be taken to plan and implement a comprehensive reclamation and conservation programme. Major adjustments to land use, including exclusion of stock from some areas, restriction of stocking on others, and the overall control of burning and clearing are needed. A research/demonstration programme should begin as soon as possible to compare costs and effectiveness of the many possible techniques of reclamation. A more detailed assessment of conservation needs, starting with a new air photo coverage, is needed for accurage detailed planning.
In the absence of reliable long-term sediment discharge records for the Nogoa River, the proportion of eroded soil reaching the river is assessed largely by argument. The Nogoa is notoriously turbid, and the combination of the intense rainfall and dispersible soils indicates that the sediment will be more mobile than experience in catchments lacking this combination would suggest.
Methods of reducing the amount of sediment reaching the Farbairn Reservoir are discussed, and it is concluded that stabilization of eroding soils is the most effective.