Australia: The Land Where Time Began
A result of being the driest vegetated continent, Australia doesn't have many high-volume large rivers or large permanent lakes. Of the rivers it does have, the inland rivers are unusual compared to rivers found around the world. With the exception of some similarities to rivers in southern Africa, they differ from all other rivers in a number of ways. One of the most obvious ways is that, unlike rivers elsewhere, they often diminish along their course, whereas most rivers in other parts of the world increase in volume along their course when they flood. Another feature of the inland Australian rivers is their erratic nature. Whereas rivers in other parts of the world tend to have a regular pattern of flow, low times and flood times, the Australian rivers have a flow pattern that varies widely. Many flow mainly, or only, when floods come down from far upstream, and these flows can be years, or even decades apart, in some places.
It does have many groundwater systems, which many surface water systems rely on to varying degrees. Subsurface water contributes baseflow to the flow of some rivers in areas where the rainfall is considered to be reasonable. Large volumes of water can be exchanged between a river and the groundwater from the hyporheic zone (water below a river bed and below the banks of the river). The amount of exchange depends on the nature and porosity of the riverbed and can vary along the course of a river.
One of the ways in which the flora of Australia's arid zone have adapted to the dry, erratic climate over much of the continent is by putting down deep taproots to the water that lies beneath the surface over much of the arid areas, often in the form of rivers that flow on the surface only intermittently after heavy rain upstream but continue flowing as subsurface rivers.
The most important agent of erosion on the surface of the Earth is water flowing over land, whether in the form of wash, rills, streams and rivers, being active, at least in the time of the rare floods, even in the deserts. It also has had a major role in its frozen form, ice, flowing across the land as glaciers at a number of times throughout the geological history of Australia.
Erosion (Source 3)
Rivers flow downhill under the influence of gravity, the surface always sloping down, though often imperceptibly, whatever the irregularity of the river bed. There are a number of features of any particular river that determine its available energy, the volume discharged, the length of time it flows, which is dependent on the gradient of the stream and the characteristics of its channel. Each river has its own level of potential energy that flow downhill under the influence of gravity converts to kinetic energy, most of which is dissipated in the heat of friction, that is proportional to half the water mass times the square of the velocity:
Ek = M/2 x V2 (1) (Source 3)
where Ek is kinetic energy, M is mass and V is velocity. Up to 96-97 % of the energy of some streams is dissipated as the heat generated by friction. The result is that a stream's energy varies according to its mass (volume) and the velocity at which it is flowing, which in turn varies according to the gradient of the stream, volume and water viscosity and the characteristics of its channel. From the kinetic energy formula (1) it can be seen that stream energy is largely determined by velocity.
Velocity variations in cross-section (Source 3)
It is usual to show, in all the empirically derived formulae, velocity to be varying with slope, channel cross-sectional form, hydraulic channel radius, and roughness of the river bed and straightness of the channel. It has been found by measurements that the centre of a stream channel is where the water flow is fastest, the velocity at the margins being slowed by friction with the bed and banks. About 1/3 of the depth from the surface, in the centre of a symmetrical channel, the water flow reaches its maximum velocity, though it is above the deepest part if the channel is asymmetrical. This velocity pattern had been expected from purely deductive studies. The flow velocity of streams has been found to vary over time and along the longitudinal stream profile, and according to the authors (Source 3) some confusion and debate has been caused by the variation that has been found along the longitudinal stream profile.
Variations of velocity along stream channels
A concave upwards longitudinal stream profile has been developed by many rivers. The longitudinal profile of a stream is the bed traced from its source to wherever it flows to, the sea at the continental margin, a lake, etc. It is referred to by the German term Thalweg. The steepest part of rivers is in their upper reaches near the the source. To the early investigators it seemed so obvious that the fastest flowing part of a stream would be found at its source where its gradient was the steepest that it became an "established fact", supported by the observations that when a stream flowed across a plain some distance from the source its flow was apparently slower than at the source.
When stream velocity measurements were eventually taken at various points along the profile it was found that the only part of the streams that were as had been expected, faster flowing, was a short part of the headwater zone. Measurements show that the flow velocity generally increases downstream.
The authors (Source 3) pose 3 questions - What is the reason for the unexpected result? How can reconciliation be achieved between the observations that apparently supported the old beliefs and the those resulting from the measurements? How can landforms that were explained by decreasing flow rates downstream now be explained in light of the results of measurements?
According to the authors, the answer to the first question is uncertain, appearing to probably be related to characteristics of the channel, and the dissipation of a high proportion of stream energy in the heat of friction in some channels. In mountain streams, the beds and banks tend to be rocky and rough. The wetted perimeter, the contact length between channel and water, is very long when compared to the cross-sectional area, with the result that losses due to friction are very high, more than 90 %, and possibly as much as 97 % of the potential energy being dissipated as heat of friction. There are many turbulent eddies in such rough channels where protruding outcrops or large blocks or boulders disturb the smooth flow of the water, and in many places the flow downstream is impeded by many local reverse flows. The authors suggest the effects of steep gradients and confined channels may be outweighed by losses resulting from friction.
The rivers are of high volume on the plains, and the channels are largely cut into the alluvium that was previously deposited by the same rivers, resulting in river beds that are much smoother than in parts of the river where the bed is rocky. There are not many obstacles that project into the water and compared to the cross-sectional area of the stream the wetted perimeter is small. Hydraulically, this situation is much more efficient, lower energy losses resulting from friction that, together with the high volume, compensates for the reduced gradient compared to that of the headwaters.
The apparent sluggishness of the section of the river flowing across plains is at least partly illusory, the high velocity zone of the river in this sector is below the surface, a factor that makes swimming and diving in large rivers more hazardous than is apparent. In the surging mountain torrent part of the river, a considerable proportion of the flow is backwards, the result of eddy development.
It has been found that the depositional plains result in part from the way rivers, especially those close to their base level, move their curses across the surface of the plain, meandering from side to side, the banks on the outward side of bends being eroded by the faster flowing water and depositing sediment on the inner side of the bend, in point bar deposits, where the water is moving more slowly, not as a result of having insufficient capacity to carry the sediment load. Sediment wedges are left as the river migrates from side to side, move from side to side, as well as downstream as the channel forms, eventually occupying the entire valley floor. This is the manner of formation of many floodplain deposits.
There are a variety of reasons for the dominance of fine debris in the sections of rivers flowing across plains, none of which are connected with the velocity of the stream. The mass of the largest particles that can be carried by the stream, the stream competence, is determined by the velocity of the flow, the competence increasing with increasing velocity downstream. The reason for the scarcity of coarse debris downstream is the debris undergoes attrition or breakdown as it travels along the length of the stream. Debris derived from the Rocky Mountains or the Appalachians in the US moves hundreds of kilometres by the time it reaches the Mississippi, most of it being reduced to sand or silt. In Australia, coarse detritus entering the headwaters of the Murray River in the Snowy Mountains has already been reduced to fines before it reaches the lower reaches of the river. In the humid tropics, weathering is very effective, such that in rivers such as the Congo River and the Amazon River, few coarse materials survive the chemical weathering in the regolith, and those that do are quickly reduced in size. At some time in the past a block that was calculated to weigh about 20 tonnes was carried in the Nile River as far as Cairo, but this is considered highly unusual, most coarse debris disintegrating long before it has travelled more than a few hundred kilometres. Though rivers flowing across plains have the capacity to carry coarse debris, there is rarely enough present for this capacity to be demonstrated.
Stream flow variation through time
Over time river flow varies, leading to variation of river energy. River can be permanent, change flow rate regularly, according to the season, or intermittently, others only flow at irregular intervals, the latter are episodic rivers. Many of the arid zone rivers of Australia are episodic, their flow rate ranging from no surface flow to high floods. East coast rivers are intermittent, most work being achieved by the rivers at flood times, as that is the time when the volume and velocity of the flow reaches its peak, and because the flood conditions are disequilibrium flows, the run-off affecting a land surface that is not adjusted to experience such a high level of attack. Load transport varies through time in each stream as a result of the stream energy temporal variations. At times when all flow is subsurface, the surface flow having dried up, solid transport stops. When floods occur any given sand particle may be transported in suspension, being carried by saltation (a jumping movement) as volume and velocity declines, the particle then being rolled along the river bed, part of the traction load, when the water level is low. At this point sand ripples or dunes form that appear to migrate upstream. The apparent movement against the current is caused by the sand particles moving from the downstream face of one ripple and being deposited on the gentle upslope face of the next ripple downstream. The result is that although the sand particles do in fact move downstream, the ripple mounds move upstream against the current.
Stream energy use
As a river erodes the channel walls and its bed it loses part of its potential energy. The load the river carries is composed of various grades of material, coarse, fine and in solution, that have been eroded by the river as it flows along its course, the water flow carrying the material in various ways. There is also debris that is added to the river by tributaries and rain wash, soil and rock creep, gullying and landslides, all of which are part of the river's load.
There are a number of ways in which river beds and banks are eroded by the river water. In rivers weathering, alteration or disintegration, of the ricks in the banks and bed of rivers occurs, the resulting material being removed by erosion, both processes taking place simultaneously in rivers, some of the erosion resulting from material being removed in solution (corrosion). All of the minerals that are commonly found to constitute rocks are soluble in water to some extent, even quartz (SiO2) dissolving very slowly in water. Some rocks and minerals, such as limestone, rock salt and gypsum, dissolve easily in river water, solution being at a significant level in outcrops with a high content of these materials. Fragments loosened by weathering are removed by the hydraulic action of running water. The size of these fragments ranges from individual crystals, aggregates of these crystals, and even to rock plates at places where the strata are fissile or the strata are very well layered.
Scouring by the abrasive action of sand and gravel carried in the river water is the main agency of erosion. In some areas where pebbles and cobbles are whirled around localised in a limited area, scouring can occur at a very marked level to form potholes, though abrasion or corrosion act generally. In running water abrasion is an analogous process to sand blasting by wind. The potential for erosion taking place in a stream varies with the square of its velocity, making the effects of storm or flood important, doubling of the water flow velocity when the river is in flood has the effect of increasing erosion by 4 times. As more water and debris strike the banks and bed of the river in a given time there is an increase of erosional impact. As particles of sand and larger fragments are moved downstream they are abraded, being reduced in size by attrition.
Cavitation is another process taking place along the river channel, exposed rocks in the bed and banks of the river being broken down by this means. As a direct result of water being incompressible, in parts of the river channel where the cross-sectional area of the river is reduced the flow velocity increases. Though the kinetic energy is increased locally, the specific weight, density, head and total energy of the river remain constant, there is a decrease in water pressure in the narrowed section of the stream. Bubbles form if the pressure is reduced to that of the vapour pressure of water, that generate shock waves when they implode that spread to the bed and banks with sufficient force to shatter rocks. This process is of significance in the upland sections of streams where the bed is rocky and there are narrows and basins that are determined structurally.
One of the process by which erosion can take place in rivers is by slumping of wet banks, a process that is significant in places where the river flows through areas where the soil has a high clay content, such soils becoming unstable and collapsing when they are thoroughly wet, most of the resulting debris being immediately carried downstream. Collapse may occur in intermittently flowing rivers after the flow has stopped, in which case the erosion is the result of collapse, not direct erosion by the river water.
Irregular secondary movements and velocity fluctuations comprise turbulent eddies in flowing water that are superimposed on the average flow. Hydraulic left is provided by these eddies that carry sand and gravel from the bed of the channel into the part of the water column that is flowing more rapidly than than occurs adjacent to the channel bed where there is a zone of lower velocity and frictional drag.
The load carried by rivers is effectively sorted, as with wind, the finer, lighter debris being carried more rapidly than the coarser, heavier material, the heavier material being bypassed by the lighter material, partly because it travels faster, but also because the heavier material is not moved as frequently. Because of the higher density and viscosity of water compared to air, very much coarser, heavier material can be transported, by rolling or by being carried, by rivers than by wind. A grain of quartz is about 2.5 times heavier than an equivalent volume of water, and about 2000 times heavier than an equivalent volume of air. Material such as cobbles and boulders can be carried by rivers because they roll along the bottom of the channel, thus the term for them 'bed load', that is carried by traction. The upper parts of large fragments are in the zone of more rapid flow while their bases are in the slower flowing bottom zone, the fragment being rolled along as a result of the pressure variation being applied to the upper and lower parts of the fragment. When coarse debris is carried over peaks of adjacent large fragments the forces of turbulent updraught become important.
Debris is also carried by rivers in suspension, solution and saltation. The suspended fraction of the load is of lighter material that is carried in the water column of the river, that mostly doesn't touch the bottom. The fraction of the load that is carried by saltation is of heavier material that is carried into the higher velocity flow by turbulent updraughts, eventually falling back to the channel bed where it either bunces up again or is carried up again by turbulent updraughts, gradually moving downstream in a series of jumps along the bed of the river. It is not possible to distinguish the suspension fraction form the saltation fraction of the load because particles in sediment traps can be either those travelling linearly or in parabolaa.
Among the material carried in suspension are clay particles, microscopically small and with a plate-like structure, both of which causes them to settle slowly even in still water, that come out of suspension only if the flow of water stops or they become trapped among coarser particles. In still water fine sand particles settle after about 2 hours, clay particles taking about 1 year to settle 30 m.
The most important fractions of the load are part in suspension or saltation, and the portion in solution, are the most important. 340 million tonnes of material in suspension enter the sea from the Mississippi River each year, as well as 136 million tonnes in solution and 40 million tonnes in traction. The load in solution is by far the most important in the lower Murray River, 1.3 million tonnes of salt, on an average total of about 3.2 million tonnes being carried to the sea per year.
The volume and velocity of any particular stream determines the mode of transport that varies over time. When a high velocity section of a stream carries a cobble in suspension it may be deposited in a section of the stream that is wider and slower. The transport of all fractions of the load ceases when surface flow ceases, though in the channel alluvium or fill underflow may continue, the solution portion of the load being carried in this subsurface flow. Flushing of fine sediment can also be induced by heavy run-off.
Transport and flood effects
The capacity of a river, the weight of debris it can carry, is determined by its discharge volume. The competence of a river, the size of the largest particles it can carry, is a function of velocity at any given time, with the result that small fast-flowing streams can carry fragments that are quite large. Both capacity and competence are high in rivers in flood, as the rivers are flowing high and fast. The form or rivers, as well as the features associated with them, are imposed by floods, not the usual non-flooding river activity level, making floods an important factor in the form of any given river. An example of the changes that can be wrought by floods on a river system can be seen in the northwest of the US where Lake Missoula was formed by ice-damming in the Late Pleistocene. When the ice barrier broke an enormous volume of water flooded to the southwest scouring about 7250 km2. As the soil was scoured away down to the basaltic bedrock there remained a large complex of winding interlaced channels, some of which reached up to a few kilometres across, as well as large waterfalls and scoured channel basins. It is known as the Channelled Scablands. At the height of the flood, boulders as large as 20-30 m across were carried downstream, the flows, that moved at up to 16.5 m/s, forming giant ripples up to 7 m high. The debris from the flood buried an area of 2330 km2. This flood is believed to be one of the largest floods known, and possibly the largest, the estimates of the length of time it took range from 1 day to 1 month. The torrents, estimated to have been as much as 21.3 x 106 m3/sec, formed a new landscape.
The debris from erosion eventually reaches the ocean basins, usually spasmodically, very little of the material being carried directly from the source to the sea. The interior drainage of Australia, such as the rivers flowing to Lake Eyre, deposit the sediment in the lower parts of the basin, forming broad alluvial spreads, an example of which is the Channel Country in southwestern Queensland, an area that is extremely flat, comprised of river channels that intertwine, and that are mostly covered by water when floods reach the area.
It is believed that alluvial veneers on the floodplains store the total sediment load delivered over several years as valley fills, as well as shoals and bars in the channels.
Where the river becomes less efficient, some sediment is deposited in fans, as when it breaks its banks as it emerges from an upland, spreading its sediment onto a plain, though most of the load of the river reaches the sea, the sediment being deposited to form deltas, or on drifting along the coast, it is formed into spits, bars, and beaches by the waves, or is deposited on the floor of the sea. New sedimentary layers form as the detritus from the land is recycled.
Flooding - northwest Queensland (see Source 3)
An example of the difference from river to river in what is achieved by floods occurred in January and February 1974 when floods affected the Carpentaria Plains of northwest Queensland. Over November and December 1973 and early January 1974 the area received about 500 mm of steady rain. Normanton, situated near the Gulf coast, averages 958 mm/y, but at this time it received 488 mm in 24 days in late 1973 and early 1974, that included 292 mm in the first 2 weeks of January.
Rivers such as the Leichardt, Cloncurry, Flinders and Norman Rivers essentially flow to the north across flat plains towards the heavier rainfall country. At about 200 km from the Gulf, Canobie averages 497 mm, a bit higher than at Normanton's mean. In conditions ideal for flooding continuous heavy rain in early 1974 the countryside was inundated in the worst floods in living memory. The 1974 floods are believed to have been worse than those of 1869-70 when Burketown went under. Leichardt Falls were submerged by the floodwater.
It was seen from aerial photos and Landsat images that the landscape was not scored by the floods, and the transport of huge amounts of detritus to the sea did not occur. There were 2 types of flooding. In rivers such as the Nicholson, Flinders, Gilbert, Leichardt and Einasleigh that rise in the hill country of the Isa Highlands and the Einasleigh Uplands, the water was dirty and discoloured. They ran high, the discoloured water flowing in or close to the channels. During the floods some secondary channels came into use, but the dirty water stayed below the level of the plain within the shallow river valleys in nearly all, not much overland flooding occurring. The flat depositional Wondoola Plains, with a gradient of 1 in 3000, stand higher than the valleys, the rainwater or dumped water on them standing clear. As the river levels fell this dumped water ran into the channels causing gullying, and more widely, scalloping, erosion and recession of the low cliffs marking the edges of the shallow valleys.
Almost all the tidal flats of the Karumba Plain were inundated by dirty floodwater when the rivers reached the coastal zone, with widespread overbank flooding and deposition. Overall the amount of debris carried into the Gulf was unexpectedly small, and the small amount of erosion that occurred on the plains was also unexpected.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|