With increasing pressure on Australia’s water resources, many have looked to northern Australia to provide water for agriculture, urban development and other human needs.

Much of northern Australia is in the wet-dry tropics, which stretch from north Queensland through to the Kimberley in Western Australia. This area has one of the largest concentrations of free-flowing rivers in the world. The climate is characterised by a long, hot dry season with little or no rain, and intense flooding rains for a few months in the wet season.

These extremes of climate result in a unique suite of animals and plants that have adapted to survive and flourish in this environment.

About 60% of Australia’s surface water runoff occurs in northern Australia. Recent media stories have highlighted the potential economic and social benefits of developing irrigated agriculture in the wet-dry tropics. Indeed, the federal Regional Development Minister Simon Crean is quoted as saying that there is “plenty of water there that is all now going to waste” (The Australian, Inquirer, October 6-7 page 21). This statement, typical of those voiced by politicians keen to develop northern Australia, is a fallacy and reflects a lack of understanding of how rivers and estuaries in northern Australia work.

Many rivers in the wet-dry tropics reduce to a series of disconnected waterholes in the dry season, with estuaries receiving little or no freshwater. These waterholes become important refuges for many species such as fish (iconic species such as barramundi and sawfish), crustaceans and birds. Water extraction from these rivers for irrigation and other agricultural needs could, if not done sustainably, affect the numbers and kinds of species in these waterholes.

The wet season transforms the landscape. Heavy rainfall causes large-scale flooding, particularly in areas that are low lying and flat. The rivers swell and overflow their banks, creating huge floodplains across vast inland areas and saltflats near the sea.

These floodplains provide a critical food source for fish and crustaceans as they move out onto the floodplain. Fish fatten and breed here, and migrate their length to get suitable food and breeding sites. Extracting water for irrigation, or building on-river dams, will affect fish migration, food availability and breeding. This will affect recreational and commercial fishing downstream.

Dams also fragment river-estuarine systems which rely on the transport of sediment and nutrients downstream, ultimately reducing the productivity of estuaries and coastal areas. So the use of off-channel, rather than on-channel dams is an important component of ensuring that agricultural development minimises impacts.

As the floodwaters drain after the wet season, large volumes of water, and associated nutrients, flow into the sea via estuaries. Salinity drops.

This is a cue for species such as the commercially important banana prawn, which cannot tolerate low salinity, to move out of the estuaries and into deeper waters with higher salinity. Commercial fishers take advantage of this movement, harvesting banana prawns offshore.

The Northern prawn fishery is substantial – in 2002, the Australian Bureau of Agricultural and Resource Economics valued it at $165 million. In some areas, such as the Gulf of Carpentaria, the scale of the freshwater flow is used as a predictor of the scale of the banana prawn catch in the offshore fishery. So, the bigger the flood, the bigger the catch.

Substantially reducing the volumes of flow downstream by extracting water for irrigation, or regulating the flow by damming rivers is very likely to reduce catches of banana prawns.

The benefits of extracting water for irrigated agriculture need to be weighed up against the benefits of recreational and commercial fisheries, and maintaining the inherent value of these ecosystems.

Our northern wet-dry tropical rivers and estuaries are unique habitats that have remained relatively unchanged. The animals and plants are highly adapted to the extremes of climate between the dry and wet seasons, and vulnerable to any future changes due to human impact. Human activities such as water extraction, and dam building for irrigation, can have substantial impacts on both the ecosystems and humans, reducing the environmental, social and economic values of these systems.

Maintaining the natural flow, and ensuring thresholds of water availability, is critical for both the health and sustainability of rivers and estuaries. Managing these systems in the future needs to examine the trade-offs between environmental, social and economic needs.