Altered water flows and quality

Aquatic ecosystems in the Wet Tropics

The river systems of the Wet Tropics World Heritage Area are often overlooked because people think mainly about the rainforest. However, the Wet Tropics has a variety of aquatic ecosystems - fast moving streams flowing down the ranges, rapid rivers cascading through gorges and over waterfalls, meandering coastal rivers and associated wetlands, mangrove communities and estuaries. The majority of these are freshwater ecosystems, but coastal mangrove communities and estuarine systems support both freshwater and saltwater ecosystems.

The Wet Tropics bioregion has 13 major river systems, most of which drain eastward into the Great Barrier Reef World Heritage Area. There are also 30 wetlands of national significance. A map of the major wetlands and water resources of the Wet Tropics was produced for the Conservation Strategy. Wet Tropics freshwater systems contain extremely rich biodiversity. For instance, 80 of the 190 species of Australian freshwater fish can be found in the Wet Tropics. Freshwater systems are a preferred habitat for 30 frog species, 16 reptile species and 73 bird species. Rivers and waterfalls also have scenic and recreational value such as waterfalls, swimming holes and rapids used for rafting. Small areas of the Wet Tropics World Heritage Area may also include marine species in the mangrove and estuarine systems. Some freshwater species such as barramundi and eels spend parts of their life cycle in the marine environment. These aquatic values are shared with the adjoining Great Barrier Reef World Heritage Area.

Aquatic ecosystems also have special qualities such as their linear connectivity across the landscape and the inherent variability of water flows. The linear nature of aquatic ecosystems means that they also have the capacity to move and accumulate threatening processes such as siltation and pollution across large distances. For example, run off from upland agriculture affects the Great Barrier Reef.

Environmental water flows and water quality

Environmental water flows include both the volume of water flow and the natural variations in water flow that are needed for aquatic plants and animals to survive. Not enough water, or water at the wrong time, can result in a loss of habitat, breeding failure and even death for some species. Water quality can be lowered by increases in nutrients, eroded sediments and pollutants, sometimes combined with reductions in water flows.

Lowered stream flows and groundwater levels, decreased water quality, and competition for rights to use water are nationally important issues. The needs of natural ecosystems must compete with demands for increased domestic, agricultural, industrial and recreational water supplies. Increased demand for water leads to calls for additional impoundments, extraction facilities and other infrastructure.

There are a number of impoundments in and adjacent to the World Heritage Area for community water supply and power generation. Koombooloomba, Copperlode and Paluma Dams are within the Area and Tinaroo Dam directly affects flows on the Barron River. There are also 22 water intakes within the Area.

Threats to water flows and water quality

There are many threats to environmental water flows and water quality in the Wet Tropics. Some of the main ones are listed below:

• Dams can have local environmental impacts and drastically reduce water flows and quality downstream. They may interfere with movement and successful reproduction of aquatic species. Streams below dams often become eroded and river deltas shrink as sediments are trapped behind the dams and water flows decrease.
• Agricultural and urban expansion may also affect water flows and water quality through soil erosion, the clearing of extensive tracts of native vegetation, irrigation, drainage, effluent discharge, weed invasions and increased chemicals and nutrients.
• Lowland wetlands and their surrounding dunes and catchments have suffered enormous impacts from levees and drains which alter water table levels and flow regimes. In some areas along the coast this has allowed the invasion of saltwater into freshwater systems, killing off large areas of vegetation
• Groundwater extraction from the Tablelands’ basalt aquifer is also of concern as it is thought to be vital for recharging many higher order stream tributaries which originate in the World Heritage Area
• Potential acid sulphate soils underlie many flood prone coastal ecosystems and may be exposed by lowered groundwater levels or soil disturbance
• Invasive weeds such as hymenachne have the capacity to dramatically alter aquatic systems and affect water quality and flows
• Land use activities in and adjacent to the Wet Tropics World Heritage Area may also affect marine ecosystems in the Great Barrier Reef World Heritage Area. Excessive amounts of sediments, nutrients and chemical pollutants in the water can damage coral and seagrass communities.

What is being done to help?

On a local scale, new designs for roads and culverts are being devised to help aquatic wildlife move upstream past roads. Catchment groups and private landholders are working to rehabilitate river banks and minimise erosion. Shire Councils are working hard to eradicate weeds such as hymenachne from their wetlands.

On a regional scale, the Australian and Queensland Governments have developed a Reef Water Quality Protection Plan to minimise pollution from land use that is harming the reef. A ‘Catchment to Reef’ study will provide new tools to monitor of water quality and ecosystem health.