John and Liz Yelland: Lake Alexandra wetland
Bottle number: 73
About the water
Collected at: A wetland on the south-western shore of Lake Alexandrina, [near Point Sturt], South Australia
E: 314652, N: 6073276, Zone 54
Collected by: John and Liz Yelland
This wetland is part of my farm.
Since there was no water to collect, I have tested the soil instead, using the protocols used by the CSIRO supervised Acid Sulfate Soils monitoring program.
1 to 5 Electrical conductivity: 920 uS
pH: 5.5 (Merck strips) Slightly acidic, typical of dry surfaces around the Lakes. The vegetation and frogs don't seem to mind.
ACT Waterwatch says:
The lower lakes are under terrible threat, and having landholders with such long memories is crucial to helping us restore them (if that is even possible). While this site is pretty salty, it's certainly not the worst out there.
About the site
This wetland is part of my farm. Settled in 1862 by my great-great grandfather and family, it is part of the lands of the Ngarrindjeri people, around the shores of Lake Alexandrina, Australia's largest natural freshwater lake. They considered this part of the Lake shore to be a particularly rich place to gather freshwater mussels, swan eggs, ducks and fish. Many camp sites are nearby. My father described people coming from Raukkan (Pt Mcleay mission) in the 1920s to fish and camp.
The shore is low lying and the water level in the Lake changed depending on the flow in the Murray River. Floods were common. This particular wetland was a boggy flood-way leading to an ephemeral lagoon. Sometimes it would have had up to half a metre of water, other times it would have been dry. On the northern side of the floodway was a slightly higher piece of ground, used by the farm as grazing land. Access was difficult over the mud, so a short ford was made by the family, using logs laid side by side – a 'corduroy' road.
All this was flooded by the completion of the Barrage system across the Murray Mouth in 1940. The Lake level rose to a new 'pool' level of about 750 mm above sea level, and was maintained there by the operation of the gates in the barrages.
The reason for this was the ever-increasing salinity of the Lake water. Having moved to the area in the 1850s to 1870s for the fresh water supply and abundant vegetation on the Lakes, the early settlers were dismayed to find the water becoming more salty as the river flows were reduced by the then-new irrigation schemes and diversions to towns in Victoria and New South Wales. Despite the ending of the 'federation' drought, things had not improved in the Lower Lakes. By 1912, parliamentary inquiries had begun to decide on a solution. The locks, weirs and barrages were being built, to control the flow.
The new Lake level and return of regular fresh water in 1940 was a boon to the local farmers who embraced the irrigation of pastures with gusto. All seemed to be well. However, in 1981, the Murray Mouth closed due to lack of river flow. By 2006, the Lake level was noticeably reduced. By 2007-8 it was very low. In 2009, it reached more than a metre below sea level. It now seems that it might die, becoming a vast salt lake.
The receding waters left a vast plain of mud and dead shell fish. On this I found the remains of the 'corduroy' road. Gradually, the mud has naturally grown reeds, grasses and other vegetation. This year, there was sufficient rain to fill this area with water to ~200 mm. Again, it became an ephemeral wetland. A boggy mud and vegetation became a home for a vast chorus of frogs and many thousands of tadpoles. The herons flew in, the terns hovered. The frogs survived.
I had hoped to collect some water, but the fierce heat of the last couple of weeks has dried it out, so the sample bottle contains only some damp sand.
What's going on:
The Lake shore is generally naturally vegetating. That is certainly the case in this area where there has been no attempt at planting or aerial seeding.
There are plans to do a lot of planting of natives mainly to control wind erosion and to assist in the reduction of acid production by the drying soils.
Regular supervised monitoring of acid sulfate soils is being done at more than 30 sites around the Lakes. This site is near one of them.
The main problem I have is feral cattle eating the grass and disturbing the ground. The shoreline of the Lake is gradually being fenced to stop stock wandering, but this has only recently started in earnest.
Further active vegetation planting near this area, on the exposed Lake bed, by the local Landcare group, is planned for next year.