Australia: The Land Where Time Began
The Channel Country
- Northern South Australia and south-west Queensland
Considered the most unusual of Australia's deserts, the Channel Country is a riverine desert. It is a very flat floodplain and consists of a vast network of interconnecting channel-like rivers and creeks, accounting for about 5 % of the surface area of the continent. Some of the larger and better known streams are the Barcoo River, which arises on the northern slopes of the Warrego Range, the Diamantina River and Cooper's Creek, which rises near Windorah. The water that flows in the many channels is delivered by the summer monsoon in the north and flows towards Lake Eyre, though it rarely reaches the lake, and often doesn't even reach the southern part of the channel system, soaking into the parched soil and evaporating. The evaporation rate in the desert regions of northern Australia are among the highest anywhere. The vegetation of this region must be capable of surviving a yearly cycle of flood and drought, an example is the coolabah tree (Eucalyptus microtheca).
The rivers of the Channel Country are inward draining, flowing into Lake Eyre, though water only reaches the lake during exceptionally large floods, that occur only rarely. During the 20th century Lake Eyre filled only a few times. Nearly all the alluvial area in the floodplain of the Channel Country has an altitude of less then 150 m above sea level, very few parts rising above 300 m.
The average gradient of the Channel Country rivers in western Queensland is only 1 in 6000 - 17 cm per kilometre. The inward-draining Murray-Darling system also has very low gradients. 2000 km from the mouth, at Walgett, in northern New South Wales, the elevation is only 140 m. So the average gradient is 1 in 14 000, or 7 cm per kilometre.
The rivers mainly carry mud, though sand is present in minor anastomosing channels, the dominant channel pattern being braided. The bars and channel floors are mostly mud, made up of 10-20 % sand, 25-35 % silt and 55-65 % clay. This clay includes extremely fine fractions.
The very low gradients of the system leads to a predominantly anastomosing type of river planform. The Cooper and Diamantina-Warburton flow into Lake Eyre along single channels. Floodplains in the upstream sections of the rivers and valleys of the tributaries are typically a few kilometres wide. South of Windorah, the Cooper floodplain is 70 km wide.
In the headwaters of the Lake Eyre Basin, rainfall is 400-500 mm/yr, reducing to 120 mm/yr in the Simpson Desert. The flow rate of all these rivers is extremely variable, the degree of fluctuation of the Diamantina is among the highest ever recorded. In the highest floods the amount of water flowing in this river can reach higher in a week than the average for a year. Upstream from Birdsville, the Diamantina and nearby channels can reach up to 500 km wide. 220 000 km2 of land was submerged in the Lake Eyre and Murray-Darling basins in the floods of 1990.
These floods move very slowly across the flat land, taking months to reach Lake Eyre, even in times of high floods. Downstream from the headwaters the water volume can drop off drastically as it moves along the dry channels, from evaporation and seepage. The evaporation rate in this area is among the highest in the world, and the dry soils can absorb enormous amounts of water. Losses of up to 75 % have been estimated for the Cooper, depending on the magnitude of the flood.
Channel Country rivers stop flowing completely in droughts. and no discharge is seen in some years, though stagnant water is found in the expended waterhole sections of channels. Between Windorah and Nappa Merrie there are more than 300 waterholes in the Cooper system, generally 2-3 times wider than the associated channels. They are often straight, but may have meanders along their length.
The nature and form of the river systems are greatly dependant on the geology of the area. Cretaceous mudstones, siltstones and sandstones of the Rolling Downs Group are widespread in the Queensland part of the area. The Tertiary quartzose Eyre Formation overlies them. Kaolinised (weathered down to clay) bedrock is covered by a silcrete duricrust formed on the Tertiary sediments. This kaolinised layer is very susceptible to erosion, supplying large amounts of clay-rich sediments to the lower rivers. The anabranching of the rivers is associated with the cohesive, clay-rich sediment forming the stable banks of the multiple channels.
A characteristic of the floodplains of the Cooper and Diamantina during drought is complex networks of cracks that can be up to 1 m deep. The cracking clay soils are the source of the mud lining the channels when the Channel Country is flooded, the clay carried in suspension and the silty sand-sized aggregates forming the surface layers of the floodplains. The Channel Country distributive floodplains have a large reserve of fine sediments.
Along much of the lower reaches of the Cooper and Diamantina the channels are flanked by aeolian dunes, in many places invading the floodplains. These dunes are easily mobilised by the wind. Sediments deposited during floods remain in the channels, but once they dry out, which is soon after the flood has passed, they become susceptible to wind erosion. Ephemeral rivers have large quantities of these sediments in their channels, which is soon available for distribution by the wind.
These clay soils are said to be self-mulching, after they dry out they have a crust on the surface that prevents them being picked up by the wind, but as soon as this surface layer is broken the underlying soil is soft and powdery and can be easily be picked up by the wind. It is now known as 'bull dust'. Patches of bull dust on the unsealed roads of the outback can be hazardous to drive through. After the wet season, when the soil has dried out, the surface of the road can be hard but when a patch of bull dust is hit at high speed it can be difficult to keep the car travelling in a straight line. I found out just how 'exciting' it can be, hitting a patch of bulldust on a slight bend on the Urandangi-Mt Isa road in the 60s.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|