Water is an abundant resource in the Shire and is present as groundwater, freshwater, tidal and estuarine water and coastal water. Surface waters play an important role in maintaining urban water supply, irrigation, industrial needs, in-stream needs and river health. The main threats to waterways are contamination by nutrients and suspended solids and pesticides.

Quantity of Surface Water and Demand

Estimated mean annual discharge from the Johnstone Basin is some 4.7 million megalitres per annum. This amounts to an average runoff of about 2020 millimetres per year. Total demand for water supplies in the Basin in 1991/92 was 27 900 megalitres. The catchments of the Johnstone and South Johnstone Rivers contribute some 2.7 million megalitres (approximately 70%) to the total discharge.

Streamflow records and water quality records are available from five gauging stations in the Johnstone Shire. The major industrial water extraction sites in the Johnstone Shire are the South Johnstone and Mourilyan Sugar Mills located on the South Johnstone River. Some 5160 hectares of crops have been licensed to be irrigated from surface water resources. Water extraction for residential use is about 3200 megalitres per year.

Instream use of water resources encompasses use by wetlands, river systems and estuaries. Importantly, it must be recognised that often there will be conflict between different types of instream use and that these should essentially be able to be sustained over time without degradation of the resource, loss of riverine values or loss of essential ecological processes.

River Health

Two biological studies of river health have been undertaken in the Johnstone Basin. These studies assess the ecological condition of streams using macroinvertebrates as indicators. Macroinvertebrates are present in a whole range of water systems and they can be affected by a range of impacts including pollution, lack of suitable habitats and changes in flow conditions. The diversity, composition and abundance of macroinvertebrates can be used to indicate the health of the whole ecosystem.

Overall, the in-stream health of the Johnstone Basin seems to be in generally good to moderate condition, though some sites in the smaller tributaries were assessed to be in poor condition. Those assessed to be in good condition are generally in rainforest areas, which have good riparian vegetation and diverse in-stream habitats. Sites assessed to be in poor condition are generally the smaller tributaries which run through intensive grazing or agricultural land and have poor riparian vegetation, high infestation of exotic bank vegetation, poor in-stream habitat and poor water quality.

Industrial and urban development on the floodplain has been minimal. Except for the Mourilyan and South Johnstone sugar mills, the Consolidated Meat Group, Innisfail and small townships with associated infrastructure, there are few point sources of pollution in the Basin. Nutrient concentration is contributed from the Innisfail Sewage Treatment Plant and from runoff from the cane, banana, and dairy industries.

A water quality study in the Johnstone Catchment found that there is community concern on nutrients and sediment discharges to coastal rivers and the effect on the Great Barrier Reef. To date lack of reliable data and quantitative information has limited a balanced assessment of the nature, extent and causes of the problem. The community has also identified the need for a water management plan for the catchment to establish on-going monitoring programs to detect future trends in nutrients, sediment and pesticides.

Source: Johnstone River Catchment Management Association