Australia: The Land Where Time Began
Base-level and valley deepening
It is now widely accepted that most river valleys are eroded by the rivers that flow along them, having been excavated by flowing water, by solution, the hydraulic action of flowing water, abrasion and by the slumping of wet banks. River banks and beds are scoured by the rivers, and the valley-side slopes are eroded by tributaries and other agencies. A relatively few rivers occupying rift valleys are exceptions, though in these rives the channels are eroded by the rivers occupying them.
Narrow chasms with steep sides would be the result if valleys were eroded solely by rivers. Where the country rock resists weathering this sometimes occurs. Active agencies such as wash, mass movements, rills and small streams widen the valley, though it is uncertain whether major rivers or valley-side wasting is more effective at eroding the surface.
The base level, the projected plane that extends from sea level beneath the continents, is the lower limit of erosion that determines how deep the river can cut. Base-level is a plane inclined enough to allow streams that have attained base level to continue flowing under prevailing conditions of climate, vegetation and weathering.
Local and temporary base-levels are also important, though sea level is the ultimate base level of erosion. Particular streams or sectors of streams have these points that impose lower limits on them that are local and temporary. Upstream from a hard rock stratum it may provide a local base level. For 1.3 million km2 of central Australia a local base level is provided by Lake Eyre that is below sea level. Rivers flowing into the lake are able to cut down to 15-16 m below sea level, the variable level of the bed of Lake Eyre. For all tributary streams their junction point with the main stream is their temporary base-level of erosion, the level lowering as the main stream cuts further into its bed. Theoretically, assuming there is little or no change in the relative position of land and sea over a long period, all streams will eventually flow at or near the ultimate base-level, as all local and temporary base-levels will be eliminated.
Base-level changes cause changes in the behaviour of rivers and the form of valleys, whether it results from earth movements or a change in the sea level. When the base-level is lowered the rivers incise further, the old valley floors remaining as river terraces, that are beyond the reach of the highest floods. Rivers extend to the new shoreline when sea-level is lower. Valley floors are built up if the base-level is raised, with the drowning of the lower valley to form an estuary if it shallow. Sydney Harbour is an example of a ria, a structure that forms if it is deep. Cut and fill is caused by alternating rises and falls of base-level. In valleys such as those of the Clyde River and the Manning River in the Sydney Basin evidence has been found of accretion that occurred over a period of 100,000 years, after which the fill was evacuated by catastrophic floods.
This is followed by the rebuilding of floodplains by the formation of levees and overbank deposits, true river energy being encountered in the channel area, with erosion taking place during floods. There is renewed vertical accretion within the incised channel.
Widening of valleys
Pronounced vertical incision with relatively slow widening of the valley occurs if a stream is well above the local base-level, such as where a surface with low relief has been newly uplifted, the incision tending to be relatively deep with steep sides. Such valleys may eventually develop a wide V-shaped cross-section after the valley is widened, with a floor that is relatively flat. The valleys that result from the winding plan form of most rivers, as a whole, display interlocking spurs.
There is a tendency for vertical distances to be exaggerated. Gorges, also called canyons, are deeper than they are wide only in very special conditions, most gorges being wider than deep. The Werribee Gorge in Victoria is 180-200 m deep and 450 m wide. Kings Canyon in the George Gill Range in central Australia is 150 m deep and 300 m wide near the mouth of the gorge. Exceptions to this rule are Standley Chasm and Simpson Gap, as well as other gorges cutting though the central Australian quartzite ranges. Most valleys are wider than deep, even when they appear to be deeper than wide. The effectiveness of agencies of erosion on the various parts of the slope determine the morphology and behaviour of valley-side slopes, the climate, vegetation type, bedrock type and soil conditions all contributing to determining the effectiveness of the erosional agencies.
Of special significance is the location of the river with respect to a particular sector of the slope, lower slope erosion occurring more rapidly than the upper part, steep slopes being developed and maintained, the slope moving back by scarp retreat. Where a slope has a capping of resistant material these conditions are present, and where a major river attacks the base of a slope, as well as places where attack of high points in the relief is slow enough to be classed as negligible, resulting from the weathering and erosion conditions, and comparatively rapid attack on the lower slopes occurs. These conditions tend to occur in arid and semiarid areas.
Slopes are worn down or decline where the upper slope is attacked more vigorously than the lower slopes, as in the absence of effective mechanisms of evacuation, material either accumulating at slope bases or removal is slow, and in places where the weathering and erosion are such that there is strong attack on the upper slopes, as is seen in the sub-Arctic, as well in temperate areas where weathering, rounding and lowering of even the highest parts of the relief tends to be induced by soil and vegetation cover. The relative rate of erosion of the upper and lower parts of the slope, and the comparative loss or gain of the various slopes units, or the slope budget, determine the form and development of the slope. In South Australia, the Murray River Valley, that is cut in flat-lying limestone of Miocene age, the location of the river at the base of the cliffs, that is removing a very large proportion of the eroded material that it is producing directly by erosion or slumping from the top of the cliff or bluff, thus maintaining steep slopes on the outside of the curves. As the river meanders or winds the inside of the curves, opposite where the cliff is being undercut, deposition of debris weathered from the slope occurs at the base of the cliff. As the exposed rock face is worn back an inclined bedrock slope is gradually developed. The cliff top is rounded as the upper slope evolves, becoming apparent as the rate of cliff recession is reduced. This slope unit eventually develops to the point where the angular cliff is consumed from above and below, being replaced by a smooth sigmoidal slope that has a flat convexity above and a gentle concavity below.
In South Australia, the Murray River is structurally simple, cross-sectional variations of river form result from riverine processes. Structure exerts a large influence on the cross-sectional form of valleys in many areas. In some areas, such as karst (limestone) areas, for various reasons the processes involved in the widening of valleys are retarded. These are areas with narrow gorges. Structural benches are present in areas where flat-lying beds and resistant beds alternate, as occurs in the Blue Mountains to the west of Sydney, parts of the Hamersley Range and on the Kimberley Plateau in Western Australia. Valleys are of asymmetrical cross-section where the strata are dipping, though scarp slopes are typically benched, or ribbed where the strata are thin.
The valleys of rivers are extended both seawards and headwards. In the headwater regions oversteepening causes slopes to become unstable as a result of the incision and lowering of streambeds, leading to slumping, the stream extending backwards, by regressive or headwards erosion, to the divide. The capture of one stream by another is the result of headward extension, a process that is also called stream piracy, and it has commonly been found in many fold mountain belts that typically develop as trellis and annular stream patterns. 'Elbows of capture' are the sharp turns in the courses of rivers where capture has occurred.
In the river valleys there is some deposition of detritus, and according to Twidale & Campbell, not all agree on the form of this deposition. Rivers develop winding courses, and on the outer sides and downstream sides of curves, bank erosion occurs that results in undercut bluffs and point bar deposits are formed on the inside. Chutes are formed when the river cuts across the bars at high tide, though essentially deposition occurs on the inside of the curve. In most river valleys point bar deposits that result from lateral migration, the rivers continuing to migrate laterally, eventually spreading over valley floors. Detritus layers are spread over the floodplain when rivers overtop their banks, accumulating in vertical sequence, resulting in some deposits being due to lateral accretion while others are due to vertical accretion.
Point bar deposits with thin beds resulting from vertical accretion are present on many floodplains. Many point bar deposits are present in both modern floodplains and ancient alluvia in the stratigraphic column. Lateral accretion deposits in the Mississippi River, a river known for meanders and point bars, are restricted to the meander belt, most of the valley floor being occupied by overbank deposits, as occurs in many river valleys. Point bar deposits form in times of average flow and the overbank deposits form when the river floods.
A portion of the debris carried by rivers is deposited in coastal estuaries and deltas, and at lake margins. Some debris is deposited where rivers debouch from uplands in arid and semiarid areas, and especially in cold mountain areas, as alluvial fans, some merging to form alluvial aprons. Streams flowing in the uplands are confined to narrow gorges they have excavated and when they debouch onto the plains they spread out into fan-like patterns over the plains. The wetted perimeter, the area where the water and land surfaces come into contact, increases, the stream possibly depositing some of its load, the coarser material first. The wetted perimeter is further increased as a result of these deposits and energy is lost, that leads to more deposition with the formation of shoals and debris islands that further retard the flow of the stream. A large part of the load is deposited by the stream as a low-angle cone or an alluvial fan, which it flows over, dividing into many distributory channels. Streams spreading over the plains can erode pediments instead of depositing fans (see Pediments), in circumstances that are very similar but different in some way.
A continuous apron often forms by the coalescence of alluvial fans formed of layers of badly sorted debris. At any place where the sudden increase in wetted perimeter is permitted by the topography alluvial fans form. This occurs in the Snowy Mountains of New South Wales, such as the outlet of Club Lake cirque, and the junction of the Fish River and the Mersey River on the Tasmanian central plateau.
During the development of river valleys much of the debris produced by erosion is transported to the ocean margin, then deposited, accumulating into sedimentary strata. Much of the deposition occurs in either estuaries or very close to the mouths of rivers, where it develops in various forms, some of which are fan-shaped, others with a pattern in the shape of a bird's foot. In deltas the surfaces gradually build up until they are exposed at low tide. The main way in which a river is lengthened is by the formation of meanders, though it is also lengthened by the addition of new land in the form of deltas, and by regressive erosion, headwards.
see Source 1 for more detailed information on fluvial landforms and illustrations
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|