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Indian Ocean drainage division

From BIRD: linking the biodiversity community

Categories: Drainage divisions

Cattle grazing is the primary land use. Mangroves, white shell beaches, and red sand dunes: Shark Bay. Even when not flowing, the Murchison River supports a large number of waterbirds. The lower Murchison River cuts through the limestones of Kalbarri National Park. Salinity in degraded agricultural land is a major issue: the Irwin River.

The Indian Ocean drainage division extends over 520,000 square kilometres of arid Western Australia. Although it covers a vast area — more than double the size of Victoria — rainfall is low and conditions harsh. Dominant vegetation types are hummock grasslands, saltbush, and mulga scrub. The entire basin supoprts only 12 species of freshwater fish, including 5 endemics, two of them rare species that are found only deep underground in the caves of the North West Cape district, the Blind Cave Eel and the Blind Gudgeon.

The Pilbara

The division falls naturally into two main areas. To the north, the Pilbara is composed of deeply weathered plateaus with many rock and river pools, and extensive coastal plains. Daytime temperatures regularly exceed 45° in summer; the opal mining town of Marble Bar once experienced a world record 161 consecutive days over 100° Fahrenheit (38°C).

Yearly rainfall averages up to 400 mm and is more common in the summer months, when tropical cyclones can travel well to the south. As with most parts of outback Australia, however, yearly average figures are misleading: dry periods with little rain can last for months or years, yet a summer cyclone can deliver 200 mm of rain in a few hours. The Pilbara has fine soils sand erosion during rain events can be severe.

Major river systems in the Pilbara include the De Grey, Yule, Fortescue, and Ashburton. All flow only intermittently. Although little of the native vegetation in the area has been cleared, overgrazing has degraded much of the land area. Another concern is the over-extraction of groundwater, which threatens the unique subterranean faunas of Bundera Sink Hole and Camerons Cave.

Nevertheless, the region harbours significant biodiversity. It has 12 declared wetlands of national importance, many different waterbirds, several frogs, and a vast area of sparsely populated arid ground which, grazing, rabbits and foxes aside, is relatively intact.

The Gascoyne-Murchison

The southern part of the division is known as the Gascoyne-Murchison, after its two largest river systems. All rivers in the region flow intermittently, contracting during dry periods to a series of semi-permanent waterholes. The Gascoyne River, for example, usually flows once or twice a year, but does not flow at all about once every five or six years.

Rainfall is erratic but tends to be more common in the winter months. There are 11 listed wetlands of national importance in the Gascoyne-Murchison, and several significant coastal sites (notably Hamlin Pool), many of them protected by national parks. The rivers of the district contain a wide variety of small aquatic fauna, but at very low population levels, and just four native fish species.

Most of the Gascoyne-Murchison is too arid to support agriculture, but some areas, mostly in the south, have been cleared for grazing and, in some cases, cropping.

The highly seasonal nature of streamflows in the district means that waterholes and billabongs have a vital function as refuges for aquatic life until the next major flow event can reconnect isolated populations and rejuvenate both channel and floodplain. For this reason, management of livestock to prevent water pollution and damage to streamside vegetation is particularly important.

Overgrazing is a major issue and has led to loss of native vegetation, depletion of in-stream fauna, and erosion. Large duststorms were recorded as early as 1887. Along with the Lyndon-Minilya system further north in the Pilbara, the Murchison is regarded as the most degraded pastoral catchment in Western Australia (Halse, Scanlon & Cocking, 2001). Another threat is an introduced African pest species, the Mozambique Mouth-brooder, which is feral in all major streams of the catchment except the Wooramel and Murchison Rivers.

References and further reading:

Allen, Midgley, & Allen (2002). Freshwater Fishes of Australia. Western Australian Museum, Perth.
Halse, Scanlon & Cocking (2001) Australia-Wide Assessment of River Health: Western Australian Bioassessment Report (WA Final Report) (http://www.deh.gov.au/water/rivers/nrhp/wa/chapter4d.html), Commonwealth Department of Environment and Heritage.
Rangelands NRM Co-ordinating Group (2005) Rangelands Draft Natural Resource Management Strategy, (several PDF files, linked here (http://strategy.rangelandswa.info/)).
Western Australian Water and Rivers Commission (no date) Statewide Assessment of River Water Quality (http://www.wrc.wa.gov.au/under/statewqassess/mapchoice.html)

— The Australian drainage divisions —
Indian Ocean	Timor Sea	Gulf of Carpentaria	Northeast coast
Western Plateau	Lake Eyre Basin	Murray-Darling Basin	Bulloo-Bancannia
Southwest coast	South Australian gulf	Tasmania	Southeast coast

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