The Maroochy River flats

Come out onto the Maroochy on a falling tide, at Bli Bli where a boardwalk carries you off the bank and out over the mud, and the estuary hides nothing. This is the very floor of the coast’s great slope, the flat, tidal, brackish ground the river spreads across as it loses the last of its fall and hands its water to the sea. From the bank it looks like a few hundred metres of grey wasteland. It is one of the busiest food factories on the coast.

Read it band by band and the estuary’s logic snaps into place. Seagrass out in the channel shallows; the grey-mangrove forest breathing through a thousand snorkel roots across the intertidal mud; the sun-baked saltmarsh of samphires and glassworts on the higher flats behind. Each lives about as high on the tide as it can bear, because here the tide does the sorting. None of it runs on sunlight the way a paddock does. It runs on dead leaves. The mangroves shed an enormous fall of them; the dark leaf-eating crabs drag them underground to shred; and the microbe-rich detritus out the far end feeds the prawns and the juvenile fish hiding among the roots, which feed the mud crabs and the herons and, in the end, the boats moored along the river. Step onto the open flat at low water and the ground itself seems to move, a slow tide of pale-blue soldier crabs marching across the mud, sieving it as they go, then corkscrewing under all at once at your footfall. Grey mud, and a maternity ward for the bay’s fisheries built on top of it.

The Maroochy carries a second lesson the prettier reaches do not, one buried under your feet.

Thousands of years of airless, waterlogged sediment stored up a load of iron sulfides here, inert so long as they stay drowned. For most of a century we did not know that, and drained the floodplain for sugarcane: dug the channels, dropped the water table, let the air into the mud. Air is all it takes. The sulfides oxidise to sulfuric acid, and the next heavy rain flushes it into the creeks as a slug carrying dissolved iron and aluminium, stripping the oxygen from the water and the protective slime from a fish’s skin. The “red-spot” ulcers that break out on Bli Bli bream and flathead tend to follow these acid pulses. The acid triggers and inflames rather than kills outright: the stress that lets a waterborne mould take hold in a fish that would otherwise shrug it off.

The remedy is the same insight run backwards. Keep the sulfides drowned. And on the Maroochy the coast is learning to do exactly that. This flat stores floods, filters the run-off, feeds the fishery, banks carbon in its airless mud, and keeps the buried hazard asleep. Once anyone bothered to price work the flat had been doing for nothing, the case for rebuilding it became overwhelming. So the water is being let back. The Blue Heart is re-wetting a broad sweep of the Maroochy floodplain, and just upstream at Yandina Creek the old cane paddocks turned themselves back into a bird-thick wetland the moment their failed tidal floodgates let the river creep in.

The flats we spent a century draining are, slowly, being taken seriously again. Everything drains here, and gathers, and turns into something the whole coast feeds on.