The alternation of ebb and flow is one of the most fascinating phenomena on earth. When children go on holiday to the North Sea for the first time, their amazement at the disappearing and returning water can sometimes be seen on their faces. Probably it was no different for people in primeval times, when they had no idea of the attraction of the moon, the sun and the influence of the hovering earth, which keep the water masses moving around the globe.
The tidal range on the North Sea is still manageable. On the German North Sea coast it is between two and three meters. That is little compared to the water movements that are occurring in the region in the satellite image above. The picture is from the east coast of Australia. Here the difference between ebb and flow is up to ten meters. It is the largest tidal fluctuation in the region.
The picture shows Broad Sound, a funnel-shaped bay a good 50 kilometers long and around 20 kilometers wide. It is located in the state of Queensland, the southern end of the Great Barrier Reef is right in front of the bay. And it is almost 700 kilometers to the capital Brisbane, the most important city in the state.
A researcher in the early 19th century noticed that the bay is something special. British explorer Matthew Flinders reported that the tide “reached at least thirty, maybe even thirty-five feet” and came hours later than expected. In fact, the water here rises about six times higher than anywhere else on the east coast. Flinders was puzzled.
It has now been resolved. The unusual tidal range is due to special local geographic conditions. The tidal forces on Broad Sound are enhanced by the shape of the bay and the shallow waters of the nearby Great Barrier Reef.
The reef initially inhibits the movement of the water. The current is concentrated in two nearby channels – Flinders Passage in the north and the Capricorn Channel in the southeast. The water movements are particularly strong here. The water masses converge in the bay and lead to a significant increase at high tide. Because of the reef, the water basically has to make a detour through the two channels to reach the bay – that explains the time lag. Due to their shape and size, different areas may have higher tides than others.
Record suspicion in Canada
The picture above was taken on October 29, 2020 at 10 a.m. Australian east coast time by NASA’s “Landsat 8” earth observation satellite. It is a mixture of art and science, writes NASA. Because the researcher who edited it worked like a photographer who adjusts the lighting and uses filters to bring out fine details in the water.
The lush colors make it easy to see and differentiate between sediments and phytoplankton in the water flow. The light brown tones near the shore are probably blown mud. In contrast, coarse-grained carbonate is moved further out. No significant rivers flow into the bay, the tide alone provides the color mix.
With the ebb and flow of the tide, things are even more violent elsewhere: in the Canadian Bay of Fundy on the Gulf of Maine, the tidal range is up to 21 meters.