Australia: The Land Where Time Began
Murray River see
The Murray is an old river, it was the major river in the basin over the entire time period of subsidence going back 60 Ma. It is believed it may be the southern part of drainage systems of eastern Australia at times in the past when there is believed to have been a "Congo-like" stage, about 100 to 90 Ma (White,2000). The course of the Murray from the Eastern Highlands to the Southern Ocean is 2,500 km long, passing through 5 different regions.
The Murray River has undergone some changes over its long history. About 5 to 4 Ma the lower reaches of the Murray were swamped by a marine incursion. It was again changed when the Pinaroo Block was uplifted between 2 Ma and 500,000 years ago, blocking its path to the sea. The result of the backing up of the water was the formation of Lake Bungunnia. This lake eventually had an area of about 66,000 km2, extending from Lake Menindee to Boundary Bend on the Murray in the south, with lobes extending south in the depressions between north-south trending ridges.
After Lake Bungunnia drained, there were large changes in the pattern of sedimentary deposits in the Murray Basin, reaching a maximum with the formation of landscapes of salt lakes and dunes of aeolian origin in the Mallee between about 500,000 and 400,000 years ago, that are characteristic of the region at the present. The dune systems were reworked and salt and gypsum were scattered across the landscape with each glacial phase of the Pleistocene ice age, the past 14,000 years of the present interglacial has seen the gradual recovery of the land of the Mallee from the salt desert it had become during the last glacial maximum.
Starting about 30,000 years ago, movement of the Cadell Fault raised the land about 10-12 m in the Echuca region, changing the course of the Murray. The Green Gully channel was the original course across the Cadell Block of the Murray River and the Goulburn River. After the faulting that occurred on the eastern edge of the tilt block, the ancestral Murray was diverted to the north through what is now Deniliquin, about 24,000 years ago, along the channel of the present Edward River. The blocking of the Goulburn formed Lake Kanyapella, a large lake that had beach and sand dunes of the lunette type on the northeastern shore. The dunes are the Barmah Sandhills. When the lake drained, the Goulburn River flowed to the south of the Cadell Block through Echuca. It is believed that the Murray River diverted to the south of the Cadell Block about 10,000 years ago, flowing across the Kanyapella depression and through the Barmah sandhills.
The 5 regions the Murray River passes through are
The source of the Murray is 40 km south of Mt Kosciusko, beginning with 3 small springs on the side of Forest Hill. It flows to the west across Cowombat Flat a short distance, turns north to the junction with the Tooma River, then swings west. Above the Tooma River junction it is joined by the Swampy Plain River. It could be argued that the Murray River actually starts at the junction of the Swampy Plain River, the upper section also being called the Indie River. Water diverted by the Snowy Scheme enters the Swampy Plain River from the Murray 1 and 2 power stations near Khancoban. The Mitta Mitta River used to join the Murray River to the west of the Tooma River (now diverted to flow into the Hume Reservoir). Between the Hume Dam and Wodonga the Murray is joined by the Kiewa. Almost 40 % of the Murray's water is contributed by the catchment above the dam.
The Murray enters the plains at Cowra, west of Swan Hill and downstream from Albury. The Eastern Highlands have provided the erosion material for the sediment of the plains that is distributed along the courses of ancient rivers that have been accumulating for millions of years. It forms a shallow channel as it meanders across the plains, the land is gently rolling at the start, becoming progressively flatter to the west. Some rivers from Victoria join the Murray River in this section - the Ovens River, Goulbourn River, Campaspe River and the Loddon River. Of New South Wales tributaries, the Billabong Creek is the only significant river to enter the Murray, flowing into the Murray via the Edward River and the Wakool River. The Edward River, branches from the Murray about half way between Albury and Swan Hill, east of Mathoura, flowing north to Deniliquin, where it swings back towards the Murray. About 500 km further on it rejoins the Murray, near Tooleybuc, at which point if forms a large anabranch. There are also smaller anabranches on the southern side of the Murray, such as Gunbower Creek. The Wakool River, that flows along part of the channel followed by the Murray after the uplift of the Cadell Block, is the largest of a network of distributaries and anabranches that from between the Edward River and the Murray River. Where the Murray River and Wakool River join, the Wakool has the larger flow. The Edward River and other anabranches divert more than half the water in times of flood.
A description was given of the Murray-Wakool junction in the 19th century, before it was 'tampered with', by Captain Francis Cadell "...having been augmented by so many streams falling into it, [it] becomes much larger than the Murray; it is fully 3 times larger than the main river where it joins it again; the Murray seems a perfect ditch in comparison" (White, 2000). It was Francis Cadell and his crew who removed log snags in the Murray and removed the overhanging tree branches to allow paddle steamers to navigate the river. It no doubt had a different appearance and 'feel' before Cadell go to work transforming it into a transport corridor.
Along this reach, the Murray forms a single well-defined channel as it cuts through the marine sediments of the plain that were deposited during the last marine incursion, the channel being cut during the Pliocene. Downstream of the confluence with the Wakool River, the Murray River flows in the original channel it had cut before the Cadell Uplift. It is along this section that the Murray River is joined by the Darling River, contributing about 12 % of the flow, from southern Queensland, and the Murrumbidgee River, that contributes about 11 %.
The Lakes and the Coorong
Lake Albert and Lake Alexandrina, the terminal lakes, and the Coorong, which is now separated from the lakes by barrages, retaining freshwater in the lakes, were originally part of a very large estuary system. The Murray River enters Encounter Bay near Goolwa.
There are steep cliffs along the river at this point, the channel having been cut through limestone at times of low sea level. At this point, salty aquifers in the cliff walls of the Gorge, which is of Quaternary age, enter the river which intersects the regional watertable. When the continental shelf was exposed at times of low sea level, the river flowed across it past Kangaroo Island, where deep submarine canyons were cut by turbidity currents.
Floods - lower Murray (see Source 3)
The Murray River, Australia's longest river, is an allogenic or exotic stream, rising in an area of high precipitation, it has enough discharge to continue flowing to the coast after passing through an arid zone, though it does not always have enough flow to continue past Lake Alexandrina , sand barriers forming by wave action that prevent the reduced flow reaching the sea. Twidale & Campbell suggest it would be prudent to adopt a worst case scenario when searching for a solution to the Murray's problems, convincing all those seeking to continue using water for their own needs at the same level to base a solution on science rather than on the needs of the water users is an ongoing problem. Like all rivers, before all the construction of the engineering structures and the use of the water for irrigation the Murray River was subject to natural variations of flow.
Since European settlement, but before major interference with river flow, there where times when the river was so low it was possible to wade across it at Morgan. There have also been floods such as the devastating floods of 1956 in which large parts of towns along the river such as Renmark, Morgan, Mannum and Murray Bridge were inundated. Though the 1956 flood was the highest level reached by the Murray since European settlement it was not the highest known of. Excavations in 1963 of cave deposits at Walkers Flat (Fromms Landing) found riverine sand at a height of 6.4 m above the level of the river in 1963. At a depth of 3 m (10 ft) it was backed against a low cliff cut by the flooding River, and consisted of a mixed sequence of colluvial sand and occupation layers containing hearth sediments with shells, etc. Zone 11, the youngest strata cut by the river, and the oldest riverine sand deposited later and datable (Zone 8), indicate that the flood represented by the sand and low cliff occurred between 1220 ± 94 and 1000 ± 91 BP.
In the same section of the River the 1956 flood reached a height of 6.86 m. Below Blanchetown the height of the river had been raised by about 0.6 m by the Goolwa Barrage (1940), the pre-European settlement flood was actually more than 7 m above the normal local river level. After recent sea level changes and related river level adjustments were allowed for, the pre-European flood would have been about 25 cm higher than the 1956 level. It is widely believed that the shaping of river valleys are very largely the result of flood effects. Each drainage basin is peculiar to itself, floods in some systems not achieving what might be expected.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|