Cruising down the Daintree River

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By Jenna Gersie

On my second visit to the Daintree National Forest, part of the Wet Tropics World Heritage Area, the rain was so heavy on the canvas roof of our cabin that it drowned out the noise of any nearby wildlife. It rained on and off for the duration of our four-day trip, but at least when it was raining, the mosquitoes were a little less ferocious. It rained so much that when we took a river cruise on the Daintree River on our way home, the water was the color of heavily creamed coffee. There had been 400 milliliters of rain in the past forty-eight hours, and clay from riverbanks further upstream had been washed into the water.

Our riverboat was driven by its captain, Mick, and his loyal companion, Jetta, a beautiful old dog who sat faithfully by his side as the boat floated downriver. The Daintree River, 140 kilometers long, is fed by a consistent supply of fresh water from the surrounding 1,200 square kilometers of rainforest. Our location, about 11.3 kilometers inland from the Coral Sea (“Where the Rainforest meets the Reef!”), was a brackish environment, where salt water from the ocean meets the river’s fresh water.

The Daintree River, with its riverine mangrove environment through which our boat drifted, is the most mangrove diverse river system in the world. Worldwide, there are only about eighty different species of mangroves. In Australia, there are a total of forty-one species. In the state of Queensland, there are thirty-nine species, and on the Daintree River in Far North Queensland, there are an incredible thirty-three species of mangroves.

What fascinates me most about mangroves is the “sacrificial leaf.” Perhaps the name for this leaf is a bit anthropomorphic, as if the leaf itself is making a conscious decision to sacrifice itself for the sake of the tree. But the actual ecology of this system is remarkable. On the tip of one twig on each branch, the oldest leaf, (the first to grow), is yellow, while each of the other leaves is a shiny green. These yellow leaves are between two and three years old. When these leaves reach their second birthday, they lose their waxy membrane and begin to trap whatever salt is being taken up through the tree so that usable water passes on to the rest of the leaves. (The roots themselves take up about 90% of the salt, passing mostly fresh water on to the rest of the tree.) Eventually, the sacrificial leaf will die from salt saturation, at which point it will fall from the tree to be eaten by crabs, shellfish, and fungi on the water, until it is completely broken down and returned to the river system.

After admiring these brave yellow leaves, we were lucky enough to witness one of the river’s great creatures: a large saltwater crocodile. This particular crocodile, affectionately named “Scooter” by regular passersby such as Mick, was a fourteen-year-old male. Scooter unfortunately only has a few more years to enjoy his territory on the Daintree River, because when he reaches the age of about sixteen years and begins to take an interest in lady crocodiles, the larger, older, and more territorial male “Scarface” will ensure that Scooter leaves.

Saltwater crocodiles, larger and more aggressive than Australia’s second crocodile species, the freshwater crocodile, are not the smartest creatures around. A four-meter crocodile has the brain the size of a walnut! But naturally, these ancient animals know what they need to do to survive. The cold-blooded reptiles prefer to live in warm water, around twenty-eight degrees Celsius, so with the river at only about twenty-two degrees Celsius, Mick expected to find crocodiles on or near the bank. The highest part on a crocodile’s body is its eyes, so that even if it is mostly submerged, it can still observe its surroundings, and that is how we found Scooter. If you’ve ever heard of crying a “crocodile tear,” it’s because these reptiles, who absorb salt through their skin, expel that salt again through their eyes in the form of tears. In the brackish environment that we visited, both the flora and fauna have adapted to deal with salt, and each of their adaptations is a beautiful glimpse into nature’s cunning.

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