Australia: The Land Where Time Began
Australia - The land of the Floodplain (White, 2003) See Hydrology
On the Australian continent the only drainage systems flowing to the sea are around the coastal fringe. The rest of the land surface is inward draining. Some of the biggest flows go to Lake Eyre, though the run-off often evaporates and sinks into the bone-dry sand along the desert water courses. As a result the only river systems to be affected by the rise and fall of the oceans are the coastal systems. The marine incursions over large areas of the the land surface has resulted from the subsidence of the basins of the inland areas several times during the continent's long history.
The fluvial landscapes of Australia are mostly in the driest, inland parts of the continent. The rivers of this arid region differ from those of the coastal strip in a number of ways, one of the main ways they differ from the coastal rivers is that they are inward-draining. They have much longer courses, are of much lower gradient, as they cross vast areas of very flat ground, they spread over huge distances from the main channels during the infrequent but very A feature of these inland rivers is that they are very very ancient. The Finke River, eg., is probably the oldest river in the world.
Most river systems of central Australia arise in the central Australian ranges. In these ranges the river patterns are constrained by the ridge and valley topography of the ranges that are constructed of resistant, ancient rocks.
There are a wide range of wetland habitats scattered throughout the floodplain of Cooper Creek. Peripherally, there are usually dry, saltpans (salinas) and claypans of heavy clay soils. For much of the time these pans are dry, with a network of cracks separating the blocks of curling mud. After rain they are covered with ephemeral herbs.
There are Coolibah fringed billabongs, secondary channels, that receive water when the main stream of Cooper Creek rises in times of flood. Billabongs also drain water from the floodplain into the Cooper.
There are also semi-permanent waterholes in the main channel of the Cooper, and the sand dune-lined Coongie Lakes. There are stands of River Redgums around the Coongie Lakes Complex. If the Cooper stops flowing for 9 months or more these more permanent water holes, and even the lakes, dry out.
This vast epicontinental sea reached its peak between 115 and 110 million years ago, when all the interconnected continental basins were covered by it, dry land being restricted to4 islands.
At this time the drainage pattern on the pre-Cretaceous were obliterated and the only patterns now visible on satellite photos are those on the 4 remaining blocks of land. The pattern on these blocks tends to be radial, the water finding its way to the nearest part of the surrounding sea.
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 water 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 to 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. Befoe the regulation of the rivers there were highly variable flows that resulted in a complex channel morphology. These channels were charaqcterised in cross-section by a series of 'benches' - flat surfaces. These benches were important to habitat heterogeneity, performing some functions ususlly 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.
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.
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 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 al 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 on 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 degradation because the floodplain 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.
A warm shallow sea that covered much of Australia during the Ordovician. In the sea wee coral reefs, a wide diversity of many invertebrate groups, particularly the molluscs. In places like the Georgina Basin carbonate deposits formed, and in the Daly River Basin there are halite crystals in the sediments. The country of the northern block had low relief, but that of the southern block was rugged and volcanic.
These occur where the water in the aquifers of the Great Artesian Basin finds it way to the surface. In some it results from the aquifer beds deep underground narrow and some of the water is forced to the surface where it seeps, and in the past, wetter times, probably ran out with more force. The mounds at the far end of the basin, where the beds carrying the aquifers slopes up to the surface, the water remaining in the aquifer comes to the surface in a number of mound springs that form an arc to the west of Lake Eyre. The mounds form from the minerals and associated debris and mud that comes up with the water,
The largest concentration of springs occurs in part of the arc to the southwest and northwest of Lake Eyre. Dalhousie Springs lies 250 km northwest of Lake Eyre. In an area of 70 square km there are 80 active springs.
Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
|firstname.lastname@example.org Sources & Further reading|