Australia: The Land Where Time Began
Typically rivers flow along single channels, the structure of the banks changing to suit the rate of flow and the sediment load carried. Anastomosing rivers are more common in arid Australia than in other parts of the world. These rivers have multiple channels that are separated by varying numbers of ridges and islands that are often vegetated. The component channels are much narrower and deeper than would be required to carry the same amount of water in a single channel. The Channel Country is an example of anabranching rivers.
Recent research has shown that a number of factors contribute to adoption of this form of channel flow. Where the gradient of the river bed cannot be easily increased by cutting into the substrate, as is the case in long distances of flat plain, the forming of multiple channels that are narrower but deeper increase the flow rate of the water.
In these circumstances the amount of both water and sediment carried is increased by forming multiple channels, in spite of roughness of the substrate in some places.
These rivers are thought to be closer to the most efficient sections for moving water and sediment, than a wider, shallower single channel.
Some channels may continue to operate, even if their efficiency decreases. Some may disperse water and sediment across floodplains with a low gradient by forming a distributory system.
Some characteristics found in anabranching rivers are flood dominated flows, erosion resistant banks, relative to flow energy, and mechanisms leading to channel damming and channel avulsion.
Anabranching can result from a one-off event, such as a channel dividing to pass an obstruction, but others may form with no obvious triggering event.
The nature of arid regions of the Australian continent and its climate predispose it to forming anabranching rivers like nowhere else on Earth. Very low relief, heavily weathered, fine-grained sediments, and an arid environment. Arid country rivers have declining flows downstream, as a result of high levels of evaporation and seepage downstream. This also leads to increased sedimentation towards the downstream end of the rivers. The combination of cohesive banks, which are often fringed with trees, tends to encourage the streams to stay narrow, leading to the movement of water and sediment for the maximum distance.
Anabranching rivers are the dominant river type in parts of arid and semi-arid regions of central Australia. There are many different types of anabranching river. In places anabranching rivers are superimposed on stable or laterally active rivers of many types.
The 2 main drainage systems, the Murray-Darling and Lake Eyre, have many anabranching rivers. Many anabranching rivers are also present in other, smaller drainage systems. The abundance of this type of river is a result of the geological conditions of the Australian continent, and is much less common on other continents. Australia is the flattest continent, by a long way. With an average elevation of 340 m above sea level, it is less than half the average for the world.
Being the driest vegetated continent has meant that there is plenty of sediment to be moved down arid zone rivers, combined with decreasing river flows, this has led to the accumulation of sediments downstream. Deep weathering and shallow gradients have led to much of their alluvium being fine-grained and cohesive, especially in parts of the Channel Country and Murray-Darling Basin, the famous 'bull dust'. Often these river flows are of low volume or ephemeral, and a number of riparian-adapted trees grow along the banks of arid zone rivers, growing down the banks of the streams, protecting them further from erosion, stabilising the channels. In contrast, large rivers in wetter areas the lower banks are often less protected by vegetation because of frequent inundation.
On the Kimberley Plateau, researchers have found a type of river that has been found nowhere else in the world. Alluvial reaches of the Durack River, alternating with narrow stretches confined by bedrock, sand ridges that are approximately parallel, steep-sided and heavily vegetated sandy ridges divide the stream into very straight, canal-like anabranching channels. The ridges, that have a dense growth of trees, are about the height of the surrounding floodplain. Riparian (vegetation associated with, or living on the banks of rivers or streams) vegetation is abundant, and in places it grows in patches over wider parts the stream bed.
Near Karunji Homestead, the vegetation in the sandy area is grassy savanna woodland. Adjacent to the streams, tall trees and a dense understorey grow on the sandy alluvial soils. The rainfall regime is the summer monsoon that is usually about 700 mm/year. Evaporation can be as high as 3000 mm/year, so the water that falls as rain in the area doesn't remain on the surface long after the rain stops. Tropical cyclones bring heavy rain to the area, with severe flooding. Upstream and down stream of the sandy reach the stream is cut into the bedrock. The presence of very large boulders in these sections of the river show just how strongly the floodwaters can flow.
Streams in the region often flow in bedrock and boulder channels, especially along the downstream steepened reaches. Stream gradients are much lower in the middle reaches which are dominated by sandbed alluvial channels, but they alternate with bedrock reaches. Alluvial stretches are wider and often straight, and are stable laterally and vertically, and have bedrock outcropping in places along their beds. Floodplains are often associated with tributary junctions, are discontinuous, and often occur in the widest section of valleys. Shrubs and large trees, e.g., Melaleuca leucadendron, often grow in the channels. They establish on the river beds and banks during the dry season by putting down taproots to the groundwater that lies beneath the channels.
A multilayered planform is typically revealed, often of the ridge-forming anabranching type, with up to 10 parallel anabranches, along channels formed in alluvium on larger streams. Large permanent waterholes are often found at the downstream ends of many alluvial channel reaches. They are transitional areas separating alluvial channels upstream from bedrock channels downstream. The well-defined anabranches between the ridges are about 10-15 m wide. They can continue for several hundred metres, with flat beds that usually have no growth.
The reason for the development of this planform, and the mechanism of formation, is still uncertain. It is thought the ridge formation reduces the flow resistance as the depth increases. The ridges act to confine the flow between them, resulting in increased flow and increased sediment movement. The more efficient flow in these sections compensates for the less efficient flow in the alluvial sections, with their often heavy vegetation cover. In the higher energy channels, the trees usually don't survive in the beds that form between ridges. The development of double-flow helices in each anabranch, may lead to the formation of ridges separated by channels.
Channel avulsion was observed to form some of the anabranches, new channels being scoured across adjacent floodplains. This occurred particularly in the widened sections of valleys, often near tributary junctions. The flow from the tributaries may be the cause.
Riparian vegetation and sedimentation appears to be organised into a linear pattern and the flow resistance limited, speeding up sediment transport, by the ridge-form channel type of stream structure. The unique conditions of northern Australia have led to the development of this type of river system that is a unique type of flow pattern.
Another unusual feature of this stream type is the stacking of very large slabs of rock on steps in the bedrock channels in the Durack River. They have been studied at Jack's Hole, and on another creek between the Durack River and the Ord River.
This type of feature occurs when the bedrock of the channel is jointed and dips downstream at a low angle. The steps form in the bedrock, the slabs accumulate, singly or in clusters, leaning against the upstream step. They can also extend up- and downstream past the bedrock step. Some of the slabs at Jack's Hole and the unnamed creek are very large, some having an intermediate axis of greater than 8 m. The fact that such huge slabs are moved in these streams requires the floods be of very high volume and velocity. The conditions necessary to move these slabs occur very rarely, so for most of the time they reduce cutting of the bedrock by the streams.
Many successive upstream-facing steps, where they extend past the banks of the channel, are common in places where the bedrock strata dip downstream along the channel slope. Hydraulic plucking of blocks from upstream of the step produces individual slabs. The succession of the steps results from the dip of the strata.
The spacing of the truncated horizontal joints in the bedrock of the channels strongly influences the spacing of the steps, and the height of the steps, downstream. The vertical joint pattern of the bedrock of the channel controls the planform of bedrock steps. If an approximately parallel set of vertical joints traverses the channel, straight or linear steps result, non-linear steps, often of a zig-zag pattern, result from intersecting sets of vertical joints.
Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|