Australia: The Land Where Time Began
Lake Bullen Merri & Lake Gnotuk
This is a cloverleaf-shaped twinned crater, the other twin being Lake Gnotuk. It is believed it may have originally been 2 or 3 maars that coalesced as the their perimeters were enlarged by subsequent eruptions. Its surrounding wall is of tuff, and there is a lava flow exposed discontinuously in the crater wall. There is also an exposure of Tertiary limestone sediments in the wall. Its sediments span the last 16,000 years. There is a steep cone-shaped floor, that is 80 m above the sea level at the base. The rim is 130 m above sea level. It is separated from Lake Gnotuk by a saddle, which is the overflow point, any excess water flowing into Lake Gnotuk. The latest know overflow was in 1841. Around the lake there are a number of well-defined terraces, demonstrating high water levels in the past. These and Lake Keilambete all erupted through the Port Campbell Limestone.
In the strata around the lakes the groundwater derives from 2 sources. The lower source is a low-yielding confined aquifer in the Port Campbell Limestone. There is also an unconfined aquifer at the base of the tuff, which forms a perched watertable surrounding the crater. Both aquifers merge near the lake, supplying the springs around the edge of the water.
Aquicludes (aquifuge), solid impermeable layers, underlie the sediments of the 3 lakes, so there is no significant seepage into the into the lower layers. The soils and tuffs of these 3 craters are very permeable and absorbs most rain falling on it. This absorbed water infiltrates down to the watertable or is transpired by the vegetation. Runoff is minimal and infiltrated soil moisture makes up a baseflow.
The lakes have been thought of a very large evaporimeters, where the lakes are mostly dominated by surface processes, and the input from the groundwater. The present climate in the area around the lakes is subhumid, rainfall averaging 815-825 mm/year.
Prior to the arrival of Europeans the lakes were surrounded by grassy woodlands to open forest. There were also river red gums (Eucalyptus camaldulensis), E. viminalis, Allocasuarina srricta, and several Acacia species. Themeda triandra and Poa labillardiera, perennial tussock grasses, dominated the grasslands. In the area around the lakes there were many large shallow lakes and densely vegetated swamps. On the basaltic soils to the north there were grassy woodlands that merged with grasslands. On the calcareous soils to the south, there were open forests.
This all changed following the arrival of Europeans around the lakes, the native grasslands have been replaced with exotic grasses, and cypress (Cupressus spp.) and sugar gum (E.cladocalyx) dominated tree belts. Most of the swamps have been drained for agriculture and areas subject to annual floods are now being used for farming throughout the year. Grazing is the main land use.
The sediments from these lakes provided evidence for the climatic history. After the maximum of the last glacial, in the Late Pleistocene, temperature and rainfall were rising. At this time Lake Bullen Merri was the only lake that didn't dry out. It was shallow, from a few to 10 m deep, from 16,000-14,500 BP. At the present it is 60 m deep. At Lake Keilambete and Lake Gnotuk there may have been swamps.
At about 14,500 BP - Lake Bullen Merrie was very shallow, producing carbonate platelets at its deepest point, indicative of dry times.
10,500-10,000 BP - the region was wetter and the lakes all held water.
8,500 BP - Water levels began rising and by 7,000 BP all 3 lakes were overflowing.
7000-5000 BP - The lakes were overflowing with brackish water. Then followed a period of sustained drying, the lakes water levels were approaching their minimum and the salinity of the water was rising.
3100-2000 BP the water level in all these lakes was low and the salinity high. There were several periods of recovery during this time before the drying returned.
After 2000 BP the water levels in the lakes underwent a gradual rise. Lake Bullen Merri water levels were as high or higher than present levels after about 1700 BP. Evidence for this deep water level can be found in the form of submerged trees 27.4 m below the present surface of Lake Bullen Merri.
Today Lake Bullen Merri is a brackish to fresh lake, while Lake Gnotuk is twice as salty as sea water. Salmon are released mainly into 2 Victorian lakes, Lake Bullen Merri and Lake Purrumbete - to the east towards Lake Corangamite.
Algal blooms have been reduced in Lake Bullen Merri by the use of an aerator that mixes the warm surface water with the cooler bottom water.
Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|