The many plants and animals, together with a diverse range of physical and climatic environments, combine to make thousands of different types of ecosystems in the Murray–Darling Basin.
An ecosystem is a community of living things (i.e. animals, plants and microorganisms) interacting with non-living things (i.e. soil, rocks and water) and the physical environment in which they exist (e.g. desert, river or mountains).
Ecosystems perform a range of functions that ensure the environment of the river, and its wetlands and floodplains, are healthy for the organisms that live within the system and healthy for the people and industries who rely on water in the system.
The functions of an ecosystem are complex and finely balanced. The removal of just 1 species from an ecosystem or a change in the physical conditions of an ecosystem, such as when certain flows occur (seasonality), can change the way the ecosystem works; sometimes such a change can have adverse effects on the natural environment and the people and industries that depend upon it.
Rivers
The Basin contains a vast network of rivers, creeks and other watercourses. The largely flat nature of the Basin means that its rivers are long, meandering and slow flowing. The actual course of the Darling River is about 3 times as long as the direct distance that it travels. Evaporation of water from the river system is high, due to the slow flowing nature of the rivers and semi-arid climate of most of the lower catchments.
Many of the rivers and creeks carry water only during times of flood. For the rest of the time, the beds of the waterways are dry or a string of pools along the watercourse. These rivers and creeks are called 'ephemeral' watercourses.
Rivers provide habitats for fish and other aquatic species. Each part of the river system, be it fast flowing, slow flowing or strings of pools, is important for particular species of fish at particular stages of their life cycle. The ebbs and flows of the river provide an environment in which other aquatic species and insects thrive. Birds and mammals depend on the fresh water of the river for basic water needs as well as for their food source.
Water flow plays an important role in the river system, helping to flush the river of salt, sediment and nutrients. High water flows are the process by which floodplains and wetlands receive sediment and nutrients, and that groundwater systems are recharged. Many types of vegetation and ecosystems depend on regular or periodic flooding.
Floodplains
A floodplain is the land alongside a river that is slightly higher in elevation, and, as the word suggests, becomes flooded when there are high volumes of water in the river system. The plain itself is mainly made up of sediment that has been deposited by the existing river in times of flood, or sometimes by ancient rivers that flowed through the general area of the floodplain. Some floodplains have a more complex geology, influenced by previous high sea levels or earth movements at a local scale that lifted or created specific soil and rock formations.
The areas of the floodplain that remain wet after flood waters recede are called wetlands.
Floodplains are an important feature of the Basin. They contain unique ecosystems, such as the Barmah–Millewa Forest in the central Murray catchment, and they provide fertile ground for irrigated agriculture, such as the area alongside the Darling River from Bourke to Menindee. Below the surface of the floodplain there is usually a complex system of underground streams and aquifers that are connected to the river. The quality of the groundwater beneath floodplains is highly variable, and can also be influenced by the groundwater that flows from upper catchment areas.
Floodplain vegetation is important as it uses groundwater in the soil profile. This prevents water and salts rising to the surface or being transported to the river. Irrigation can increase the height of the watertable, dissolving salt in the soil and bringing salt into the root zone of crops or into aquifers that flow into the river.
Flooding is the natural process by which excess salts are periodically removed from the floodplain and the river system.
Wetlands
Wetlands are areas on the floodplain that are at times inundated with shallow, still (or slow moving) water. They are among the Basin's most productive and biologically diverse ecosystems, providing essential breeding and feeding habitat for many kinds of organisms ─ waterbirds, fish, invertebrates, and plants. Wetlands are also important in absorbing, recycling and releasing nutrients and trapping sediment, acting as natural filters which improve water quality. At the same time, excessive pollutants will degrade or destroy wetlands. Wetlands also increase the productivity of associated aquatic and terrestrial ecosystems
Wetlands add to the diversity of the landscape and are the focus for a number of recreational activities. Given the Basin's generally dry but highly variable climate, the wetlands act as environmental buffer areas. During wet periods, they spread flood peaks and store floodwaters, releasing them gradually and reducing the effects of flooding. During drought, they provide refuges for wildlife and grazing for stock.
The Basin has over 30,000 wetlands, some of them listed internationally (Australia's Ramsar wetlands)for their importance to migratory birds that visit the site. To be healthy and promote life, the wetlands of the Basin need to be alternately wet and dry.
