Dune lakes (perched and window)

Stand on the dry, dazzling sand of Cooloola in the middle of a proper drought and you would swear you had found the least watery place on Earth. You would be standing on the lid of an enormous freshwater store.

A coastal sand mass behaves like a colossal sponge. Rain sinks straight in, with nothing at the surface to hold it, and gathers below as a great lens of fresh groundwater, floating on the denser saltwater that creeps in from the sea beneath. Fresh water is the lighter of the two. Nothing about it moves fast. Water entering at the top may take years, even decades, to work its way through. On the comparable Minjerribah sand mass, groundwater has been dated at mean residence times of anywhere from thirty-seven to well over a hundred and fifty years. A raindrop that falls today may not see daylight again until a person born this week is drawing a pension.

The region’s most photographed water sits on top of all this: the dune lakes. They come in two quite different kinds, told apart entirely by how each one meets that buried store.

A window lake is the simple one, and the name does the work. The land dips far enough to slice into the top of the freshwater lens, and the lake is nothing more than a window thrown open onto the groundwater. It rises and falls with the water table; it belongs to the same body of water as the aquifer all around it. Recent mapping of the Cooloola sand mass shows how tightly these lakes, along with the perennial streams and swamps beside them, track the level and quality of the groundwater beneath. They are groundwater-dependent ecosystems in the plainest possible sense. Which is the sobering part. Draw the water table down, or foul it, and the window lake goes with it, though nothing whatever may seem to change at the surface.

A perched lake is the stranger of the pair, and the more wonderful. It sits above the water table, sometimes startlingly high in the dunes, with no groundwater feeding it at all. What holds it up is a basin sealed at the bottom by a layer of organic hardpan: the same dark, cemented coffee rock that forms deep in the sand as a podzol ages, here doing duty as a watertight floor. Nothing but rainwater fills it, caught and cupped on that seal. That buys an almost unearthly purity — very few dissolved nutrients, hardly any sediment, the water often glass-clear or stained a clean tea-brown. It also buys fragility: everything in the lake arrived as rain, and the only thing holding it there is a layer that took thousands of years to lay down. Lake Poona, in the Cooloola sand mass, is a classic of the kind; the larger, more famous examples lie on K’gari just to the north, the same sand system carrying on.

Neither lake has a bed of rock or clay in the ordinary sense. Each hangs on the one structure that divides it from a heap of sand that should, by rights, swallow it: a water table it reaches down to, or a hardpan it rests upon.

Every dune lake is the sand country shrunk to a single basin. What looks permanent is on loan. The window lake leans on a water table you cannot see and can lower without ever touching the lake itself. The perched lake balances on a floor of coffee rock you could punch through with a backhoe in an afternoon, and could not remake in ten thousand years.