The Great Cumbung Swamp - terminus of the low-gradient Lachlan River, Eastern Australia

The Lachlan River rises in the humid highlands of southeastern Australia and flows westward into the Murray Basin. The climate is sub-humid to semi-arid and, as it crosses the Murray Basin, the Lachlan loses most of its discharge through evaporation and infiltration. Unlike most river terminations, the Lachlan ends in the Great Cumbung Swamp without dividing into distributaries; neither are there large fresh-water lakes nor playas. Only during floods, occuring in 15-20% of years, does Lachlan water flow past the swamp into the Murrumbidgee River to the south. The swamp water remains fresh because salt is lost with water that infiltrates into underlying aquifers. This water loss causes the river to terminate because it reduces stream competence and prevents the Lachlan from forming a lake which could then overflow and breach the low topographic barriers that defeat it.  The Great Cumbung Swamp is divided into three depositional environments:  1. The Lachlan channel is sinuous and up to 40 m wide. It shows morphology equivalent to cut-bank and point-bar morphology when it first enters the swamp but becomes straight as it reaches the central, lowest part of swamp. Its most distal reach is sinuous with a slight upstream bed gradient.  2. Phragmites Marsh: Most of the Great Cumbung Swamp is marsh colonised by Phragmites australis. Very slight topography controls flood frequency and degree of desiccation, and hence Phragmites growth. The marsh displays a dendritic texture resembling small-scale drainage networks possibly formed by floodwater etching out micro-relief developed on the deep-cracking flood plain clays.  Within the Phragmites marsh are bodies of open water less than 0.75 m deep connected to the main channel through breaches in the levees. The lakes grade into the surrounding marsh with a gradual increase in the density of Phragmites clumps.  3. Overflow areas: The Great Cumbung Swamp proper is surrounded by alluvial plain colonised by scrub and eucalypts.  It is underlain by black to grey deep-cracking clays. Traversing this plain are anastomosing channels that carry water from the swamp to the Murrumbidgee during floods. These channels are slightly sinuous and up to 20 m wide, 1 m deep, and have symmetrical cross-sections.  Great Cumbung Swamp sediments are largely black clays with those deposited in the Phragmites marsh being extensively bioturbated by roots. The development of pedogenic textures and preservation of organic matter depends on the frequency of desiccation. The Lachlan channel accumulates massive black clay with only a few thin sandy beds in the upstream reach. These sandy beds probably drape the channel bottom during the falling stage.  The Lachlan River in the Great Cumbung Swamp has a very low gradient and provides an example of fluvial deposition at the lowest end of the energy spectrum. Ancient analogues for this style of river termination will probably be found in fine-grained sediments interpreted as flood plain or lake deposits.  Other deposits interpreted as deltaic, but for which no connection with lake or marine depos its can be demonstrated, may also be th e deposits of swa mps. Some sediments in the Triassic to Jurassic basins of the eastern U.S.A. may be good analogies.

Tags: Earth Sciences, GA Publication, Journal, NSW, Published_External
Organization: Geoscience Australia Data