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GASCOYNE RIVER

SIZE AND FLOW OF THE RIVER

The Gascoyne River is 865 kilometres long, making it the longest river in Western Australia. The Swan River, in comparison, is about 72 kilometres long, and the Canning River is about 110 kilometres long. During floods it can discharge around 6000 m³/second, which is enough to fill two Olympic sized swimming pools in one second.

The Gascoyne River flows most years, and flows are the main source of recharge to the underground water stores which are directly connected to the river. There are however extended periods of time when there is ‘no-flow’ but if aquifers are managed carefully water supplies can be maintained for up to four years without flow.

The type of sediment around and in the river is closely linked to how easily water is stored. High clay areas are not good at storing water. Layers of sand and gravel can trap and hold much larger volumes of water.

HOW INDUSTRY ACCESSES THE WATER

The Carnarvon Horticulture District water supply comes from aquifers (underground water systems) beneath and next to the Gascoyne River.

Horticulturalists on the river banks have ground water licences allowing them to take water to irrigate their properties by using private bores in the river bed. Scheme water is also available to growers which is supplied from areas upstream of Carnarvon. Having two supplies makes water supplies very reliable for growers. On average about 12 GL* of water is abstracted per year which services approximately 1200 ha of farmed area.

Many horticulturalists in the region use water efficient irrigation techniques such as micro-irrigation technology, subsurface drip tape and ground covering mulch to use water effectively and efficiently, to grow large quantities of fruits and vegetables. Carnarvon produces about $6 to $7 million of crop per GL compared to the national average of $1 to $2 million per GL.

WHICH INDUSTRIES RELY ON THE RIVER

Thanks to fertile soils around the Gascoyne River there are many types of fruit and vegetables grown in the region. These include bananas, table grapes, tomatoes, capsicums, cucurbits, avocados and mangos. Local fisheries are also impacted by the river, especially when a flood causes fresh water to flow into coastal waters. Inland pastoralists rely on the river and subsurface water for maintaining livestock.

MANAGING SALINITY DURING FLOOD AND DROUGHT

Salinity is a major factor in managing water availability in the Gascoyne River. There are highly saline waters around and below the main river channel. This saline water can move into the river sediments, due to manmade causes (e.g. people taking too much water from the river) or natural features such as limestone (which water can move through easily) and occasional brackish flows after very large flood events.

These changes to water quality can happen over short distances and affect some areas of the river system and not others. Once salinity of bores reach 1000mg/L Total Dissolved Solids the bores are switched off to protect the aquifer and topsoils of Carnarvon.

Good quality and reliable water supplies are important to help industry. The Department of Water works with state and local government, industry and local growers to maximise water use and availability, whilst minimising the risk to water quality.