Oyster Shells from 5 Million Years Ago

When the lower Murray River was part of the ocean

My father was born in the southeast of Australia, not far from the Murray River that is the longest river in Australia which is the driest inhabited continent on Earth. My father would tell me how the river began in the mountains as a stream feed by melting ice, dropping down to wide fertile plains his homeland; where my cousin Steve Henty still milks cows in the old dairy every morning, early.

Further south the Murray River narrows as it meanders through a limestone gorge until in most years, almost exhausted as there is little freshwater remaining, the river eventually reaches Lake Alexandrina — a vast shallow expansive of water stretching to the horizon, not far from the Southern Ocean.

The mulloway fishermen, with shacks along the sandy beaches facing the Southern Ocean would say before the dams and weirs were built along the river to hold the water to irrigate the farms, that the river would flog down from September until maybe Christmas, filling the lagoon, then out the mouth. By Christmas, flow had usually slowed and water levels in Lake Alexandrina often dropped right down particularly in drought years.

Then when the southwesterly wind picked up, the sea would pour in through the mouth and work its way across the lake. So, Lake Alexandrina was fresh in spring and summer, but salty by autumn.

Before the barrages, built in the 1930s as sea dykes across what was once the Murray River’s estuary, the sea tides would push in. For sure, when the river was running strong, the waters of this vast estuary were sometimes fresh and filled with river fish, including Murray cod. But most of the year and in most years, the sea tide won, and it would push the freshwater back across the wide estuary, and so the water was salty with the largest fish including mulloway from the Southern Ocean, coming to spawn especially on the highest tides in autumn.

Twenty years ago, in 2006, water levels in Lake Alexandrina fell precipitously from 0.85 metres above sea level to -1.10 metres below. There was simply not enough water in upstream dams to keep both Lake Alexandrina and the adjacent smaller Lake Albert supplied with adequate freshwater. In earlier times, for example, during the Federation drought (1895-1902), as flows from upstream slowed, the seawater pushed in taking over the entire lake and extending many kilometres up the main channel of the river. But during the Millennium drought (2002 to 2009), the 7.6 kilometres of concrete sea dyke engineered and built in the 1930s, held back the Southern Ocean and the lake began to dry out.

The South Australian government could have opened the 593 gates within the five sea dykes to let the Southern Ocean in, but instead the government kept the gates shut tight. That the government chose not to open the gates during the drought and let the Southern Ocean flood in, was never reported in the national media which instead focused its television cameras directly on the receding lake waters. Never once did we see an image of the massive sea dykes.

All the while politicians from both the left and the right of Australian politics and the Australian Greens, they blamed the climate for changing; as though this was new and unprecedented — and your fault, and the fault of upstream irrigation including my cousin Steve Henty. It is as though the river had no natural upstream flood plains. Yet before irrigation, even before colonial times, a first European explorer, Charles Sturt, he wrote:

“Australian rivers fall rapidly from the mountains in which they originate into a level and extremely depressed country; having weak and inconsiderable sources, and being almost wholly unaided by tributaries of any kind; they naturally fail before they reach the coast, and exhaust themselves in marshes or lakes; or reach it so weakened as to be unable to preserve clear or navigable mouths, or to remove the sand banks that the tide throws up before them.”

Meanwhile, the South Australia Department of Water will explain the sea dykes were not opened during the drought because: “The State Government is committed to maintaining the Lower Lakes as a freshwater system.”

The governments, they rarely tell even half a true story. The misinformation was not justified on the importance of the lake as a supply of freshwater for irrigation, or as drinking water for the city of Adelaide, but based on the false claim that Lake Alexandrina has always been a freshwater lake and that letting in seawater would spoil its natural ecological character.

Much nowadays is justified on the basis that a particular natural habitat should be of a particular character, as though change is not intrinsic to ecosystems. In the case of Lake Alexandrina it is never written that the vineyards to the west of the lake don’t want an estuary, nor the local property developers, and on it goes – and so Australia’s longest river is denied an estuary.

As though Australia has not always been a land of drought or flooding rains, and as though every river does not deserve to feel the sea at its mouth.

In modern times, that I’m defining as perhaps the last 400,000 years, Australia has experience droughts and sometimes rivers fail before they get to the ocean. But long ago, there was a time when it was always hot and wet and sea levels were maybe 70 metres higher.

Upstream, beyond Lake Alexandrina, where the limestone cliffs tower over the Murray River, my boot once scraped a ledge, and there—embedded in the limestone—were oyster shells, some as big as a dinner plate. These fossil bivalves, cemented into the cliff face, evidence the ledge was once part of a seafloor.

Those who read the rocks agree that from 35 to 5 million years ago (Eocene to Miocene), Earth was a hothouse—there were no ice caps. That was when these limestone cliffs upstream from Lake Alexandria were formed. Global temperatures were warmer, perhaps on average 10°C warmer than today. Sea levels were higher, perhaps as much as 70 meters higher than today. Carbon dioxide levels were much higher, perhaps 2.3 times higher than today. For sure, the climate changes, and the composition of the atmosphere has changed even more.

To be continued.