The original "riddle of the rivers" was the mystery about the destination of the westward flowing streams of the Murray Darling Basin. They did not appear to have an outlet to the sea. The Murray, unlike most other great rivers of the world, was not  "discovered" by Europeans from the sea. Despite draining one seventh of Australia,  the outlet of this vast river system is so small that it is easily concealed by waves and surrounding sandbars.

The myth of an inland  sea began well before the Blue Mountains were crossed in 1813 and wasn’t finally  dispelled unit 1845, many years after the last piece of the Murray Darling  system jigsaw was put in place. Ironically, one of the last people to concede  that an Inland Sea didn’t exist was Captain Charles Sturt who is generally  credited with solving the "riddle of the rivers"! Sturt’s Central Australian Expedition of 1844-45 carried with it a portable boat in preparation for a "voyage".

Joseph Banks, the famous botanist on Cook’s voyage to eastern Australia sowed the seed from which the Inland Sea myth grew. As a respected adviser on Australia, Banks wrote in one of his reports to the Colonial office in 1798:

"It is impossible to conceive that such a large body of land, as large as all Europe, does not  produce vast rivers, capable of being navigated into the heart of the interior…"

Captain Matthew Flinders,  an excellent navigator and map maker, encouraged the myth after he charted  the entire southern coastline of Australia in 1802 without finding a significant  river mouth. Flinders speculated that the rivers might flow to an inland sea. While Flinders was charting the coastline he met the French explorer Nicholas  Baudin, also exploring the south coast of Australia, in what is now called  Encounter Bay. This meeting was within a few kilometers of the Murray mouth but incredibly, neither of them detected it.

Credibility for the Inland Sea theory grew after the crossing of the Blue Mountains. Explorers, including Evans, Oxley, Hume and Cunningham, recorded a number of large westward flowing rivers. Charles Sturt fully expected to find a sea when he set off in 1829 on the expedition, which was to record the existence of the Darling River. The expeditions led by Charles Sturt to the Darling River, and down the Murrumbidgee and Murray to the sea, between 1828 and 1830 finally solved the riddle of  the rivers. However, for Sturt, the Inland Sea theory was not completely dispelled  for another 15 years; it was simply pushed further inland.

Thomas Mitchell ridiculed  the idea of an Inland Sea and suggested that the westward flowing rivers simply joined a larger river, which he believed flowed northward into the Gulf of Carpentaria.

His expeditions between  1831 and 1836 confirmed that the network of westward flowing rivers in fact drained into the Darling and Murray rivers of the Murray-Darling Basin emptying into an inland sea was not as naïve or ridiculous as it may seem today. It was just several million years too late! The rivers once did flow into an  Inland Sea which helps to explain their drainage pattern.

The drainage pattern of the Murray-Darling Basin has remained relatively unchanged for millions of years. This contrasts with most major river systems in the world which  were erased by the last Northern Hemisphere ice age and have generally only been in their present form for less than 15,000 years.

For around 20 million  years, the southern part of the Murray-Darling Basin was covered by a broad arm of sea which geologists named the Murravian Gulf. At its maximum, the  sea extended eastward beyond the present site of Swan Hill and the ancestral Murray, Murrumbidgee and Lachlan rivers flowed directly into it. The sea had receded by around 12 million years ago but invaded again 6 million years ago  and made several advances and retreats before finally disappearing 3 million  years ago.

Even if the explorers  had set out 700,000 years ago, a blink of an eye in geological terms they would have discovered the rivers draining into a huge freshwater lake: Lake  Bungunia. It was formed when an earth uplift dammed the ancestral Murray south of the present site of Swan Reach in South Australia. The Lake backed up the Murray Valley to present Murrumbidgee junction and extended north to include  the Menindee Lakes and south to include Lake Tyrell. At its maximum extent, this megalake covered 33,000 square kilometers and teemed with wildlife including  waterbirds, murray cod and giant 3 metre lungfish. The "riddle of the rivers" did not end with the unraveling of the Murray-Darling drainage system. The land west of the Great Dividing Range holds many intriguing anomalies, both  natural and man made.