Tidal Spectacle

One of the most impressive tidal landscapes on earth is to be seen on the northwest coast of Australia in the Kimberley region. The tidal range there reaches 10 to 12 metres in height, the highest in the southern hemisphere.

Wide tidal channels, lined with mangrove forests, meander through endlessly wide flood plains dotted with rock crests of the ancient Kimberley Mountains.

These gigantic masses of water flood the land and withdraw every six hours permanently transporting the detrital material of the ancient mountains, washing huge amounts of sand away from one place and depositing it in another.

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The most spectacular phenomenon in this tidal landscape stretching between the Indian Ocean and the Australian continent is to be found at the open sea 20 kilometres away from the coast - it is the Montgomery Reef.

It is a gigantic rock plateau . Its total area is 400 square kilometres and it is 80 kilometres long. It is more than double the size of the Duchy of Lichtenstein.

When the tide is high, the platform is under water. Only a few rocky islands can be seen on the periphery of the reef, including three islets surrounded by sandbanks and covered by mangroves in its centre.

However, when the tide is out, the reef emerges from the ocean like a gigantic see monster.

The sea water gushes from its edges in frothy cascades.

Deep channels fed by the roaring waterfalls open in the reef and, like torrential mountain streams, rapaciously flush into the Indian Ocean at the speed of 3 metres per second.

Why the water on the reef edge starts to roar …. During the outgoing tide the water level around the reef falls by 10 metres within 6 hours. Thereby its level sinks much faster around the plateau than the water that covered the reef during the high tide can flow back from the lagoon onto the plateau.

As with an overflowing swimming pool, the water permanently rushes down from the platform, plunges and splutters off the edges of the reef into the retreating sea. The reef with its islets, sandbanks and lagoons seems to be rising higher and higher out of the water. At the tide’s lowest ebb, the steep edge of the reef can rise up to 4 metres over the sea level.

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The Montgomery Reef is not a coral reef in the traditional sense of the word. For a start, it is considerably younger than the famous Great Barrier Reef on the east coast of the Australian continent, which has been moving upwards for millions of years while the ocean floor has been slowly but steadily subsiding due to the tectonic movements of the earth crust.

… it is comparatively young. The Montgomery Reef is just 6,000 years old. It was formed during the last Ice Age, when the ice mass at both Poles melted, which resulted in the rising of the sea levels by 120 metres worldwide. The ocean flooded the coastal areas of the Kimberley Mountains including the plateau on which the Montgomery Reef grounds.

…. and it is mainly formed by red algae. It contains only a small amount of corals. The biggest part of it is formed by the red algae, which filter out limestone from the water. They do not form any hard skeletons as it is the case with the corals stuck to the bedrock. The red algae, which float on the sea surface, absorb the lime particles into their cell walls, encase in them, sink to the bottom and form thick mat floors, which cover the bedrock. They build up garland-like structures, which – when seen from the air - create bizzare patterns on the sea floor.

This was discovered only a few years ago, when the Australian research scientist Barry Wilson from the Western Australian Museum and his colleagues from the neighbouring Institute of the Oceanography in Perth thoroughly examined the rock shelf under the reef and the reef’s structure.

Such a tidal range of 10 to 12 metres in height or higher - similar to the Kimberley Coast - can be observed only in Fundy Bay in Nova Scotia, Canada (up to 15 metres); in Bretagne, France; in the Bristol Canal connecting England and Wales, and in Penzhina Bay off the northwestern coast of Kamchatka, Russia.

What still might be of interest: Over thousands of years and until the beginning of the twentieth century, the central isles on the Montgomery Reef and some of the rocky islets had been inhabited the Aborigines taller and of a stronger stature than the ones we know from today’s Australia – some of them were 2 metres tall. They had adapted very well to the extreme living conditions in the tidal zone. However, they disappeared suddenly in the 1930s. It is hard to imagine how a tribe of more than 300 people could suddenly be swept away from the surface of the earth. It has remained an enigma ever since.

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