roughbarked said:
Kingy said:
There are many lakes in the deserts around the world. In this case, I have been looking at the ones near the Murray-Darling river system.
The Menindee lakes, Lake Victoria etc. They are fed by ephemeral rivers and fill every 5 to 10 years(Climate Change may adjust this).
Given that they collect blowing sand in each wind event, and the silt from the inflowing water in each flood event, they become a sump with no outlet except the lighter/cleaner water that overtops the spillway/sluice.
My question is: Why haven’t they filled up and become part of the desert?
Is there an underground subsidence under them that has continued for many millennia?
In the case of the Menindee lakes; The lakes were originally a series of natural depressions that filled only during floods. As the flow receded, the water in the natural depressions drained back into the Darling River.
The flows in the Darling are sparse. Between floods, the Darling River can dry out with practically no inflow at all. Even though large floods can occur at any time of the year, they most frequently occur in March following late summer rains (wet season) in the northern headwaters in Queensland, and from July to September due to late winter rains in the Border Rivers or NSW tributaries.
When floods do occur, it can take several months before the water makes its way along the system to the Menindee Lakes. In addition, a considerable portion of the water is absorbed by the floodplain or simply evaporates as it flows slowly over very flat terrain.
As such, not a lot of sediment makes it to the lakes to fill them up.
This begs the question of where the natural depressions came from in the first place.
It has only been a short time, geologically speaking, since the end of the last ice age, when rainfall was much less than it is today. Since the end of the ice ages there has not been nearly enough time for natural depressions to fill with sediment. Similarly, there has been nowhere near enough time for the sea cliffs along the edge of the Nullarbor to form. So we have to look before the ice ages for an explanation of where the depressions came from..
Before the last ice ages? What happened to create the depressions?
Possibly underground subsidence due to the movement of faults.
Posibly underground subsidence due to the removal of underground water as the continent dried.
One alternative viable mechanism is that the depressions are the remnants of a much larger inland sea, cut off from other lakes by alluvial fans. For example Lake Torrens and Lake Eyre cut off by alluvail fans starting at the Flinders Ranges.
Or that the lakes are remnants of previous water courses, scoured out under water by water turbulence on uneven strength bedrock and then left behind when the river changed course. The change in river course being due to alluvial deposition in the previous channel leading through the lakes.
I don’t think that wind-blown scouring by sand would suffice to create the depressions, but perhaps.
The depressions that resulted in the many lakes of Canada were created as a continental ice sheet scraped over the surface. Australia used to be near the South Pole 30 million years ago.
So all I can offer is a collection of options. Michael V would be able to select the best option.