The Flood Pulse Concept

Australia: The Land Where Time Began

The Flood Pulse Concept (Junk et al., 1989)

Central Australia's floodplain rivers of the semi-arid and arid parts of the continent are so different from the typical rivers in other places, particularly in areas with more, and more regular, rain that their requirements need to be understood in order to maintain them in a healthy state. One of the features of these unusual rivers, that is different from 'normal' rivers, is that parts of the river-floodplain are only connected in times of flood. This occasional connection between the parts of the floodplain system is essential to their healthy functioning as an ecosystem.

In the floodplain ecosystem there are 3 main aspects to the connectivity to different parts of the system. The connection between different parts of the channel, at times of flood the water connects all parts of the channel, bringing sediments and nutrients.

Laterally, as the floodwater spreads out across the plain, distributing materials and resetting ecological systems.

There is also a vertical component, the water passes from the channel and plain to the watertable. This sediment-water interface is important as a refuge for many organisms in between the infrequent, erratic floods.

In the dynamics of the river-floodplain system the floods are a driving variable. Exchange of biota, nutrients and other materials between the river channel, floodplain and wetlands. Flood pulses vary from place to place. In some places the floods occur annually, while in others they may be less frequent and erratic.

Drivers of river functions are variations of rates of rise and fall, frequency and predictability. One example of the Barwon-Darling River. Before the regulation of the rivers there were highly variable flows that resulted in a complex channel morphology. These channels were characterised in cross-section by a series of 'benches' - flat surfaces. These benches were important to habitat heterogeneity, performing some functions usually associated with floodplains. They enabled the accumulation of organic matter and other nutrients. The number of benches varied with the variability of the flow. The number of benches increased with an increase in variability. Increased physical channel complexity provided an increased surface area, where organic matter was stored and provided a food source, providing a more diverse habitat for aquatic organisms.

In places where the flood timing is variable species with a flexible lifecycle are favoured. Attached algae or aquatic macrophytes would be disadvantaged in the environments that undergo rapid rise and fall of water level.

Floodplain Inundation

An immediate surge of nutrients is released when the floodplain soils are wet, usually by an ephemeral flood. This nutrient release has 3 main components.

partly from nutrients deposited in the previous flood
partly from biological and physical effects of wetting dry soil
partly from the breakdown of organic matter accumulated since the previous flood

In the soil are many small freshwater species that have been dormant since the drying after the previous flood, when the flood arrives the dormancy of these creatures is broken. The soil bacteria have been respiring aerobically in the dry soil, water cuts off the oxygen supply, so they switch to anaerobic respiration, then release nutrients into the water in forms usable by other organisms. In the floodwaters the biological productivity peaks quickly but falls off slowly until it reaches a new plateau. Standing water cannot support the same rapid biological growth as fresh floodwater. The species found in the floodplains are adapted to the sudden nutrient surges at flood time.

The tiny aquatic species that are the basis of food webs on the floodplains proliferate rapidly within hours of the soil being wet by the flood. At this time many opportunistic species arrive, often from great distances, to take advantage of the ephemeral bounty. When the water overflows the main channels and spreads out over the floodplain all the isolated waterholes and channels are connected, allowing the spread of aquatic species throughout the system. The plants and animals begin their breeding cycles, needing to complete them before the water is gone. The fact that floodplains act as sources or sinks for different materials have important effects on the riverine ecosystems. The health of the aquatic organisms such as fish, and whether or not the blooms of cyanobacteria produce toxic chemicals, depends on how the materials are distributed.

When the rivers are not artificially controlled they contribute water, nutrients and sediment to floodplains. The floodplains supply carbon, living organisms and water treatment of the floodwater, and breeding grounds for plants and animals. Rivers that are separated from their flood plains are impoverished and degraded because the floodplains supply food and life to the rivers and acts like a filter for the river, removing pollutants and excess nutrients that can lead to toxic algal blooms.

Floodplains are biologically rich, having 100-1000 times more species than the rivers that supply the water to them. Habitat diversity is supported by billabongs, intermittent lakes, backwaters, anabranches, swamps and wetlands. Billabongs that are spaced out across a floodplain contain a high number of endemic species, in spite of being connected in flood times. Some have been likened to islands where speciation occurs. The fact that endemic species persist means that removing a billabong would lead to extinctions.

Sources & Further reading