袁泉

China's Qiantang River is famous fora unique natural phenomenon， a wavy patterntide that was only observed for thefirst time in 2021 and has come to beknown as fish scale tide.

The Qiantang River's estuary （江口）in Zhejiang Province has long been famousfor having the strongest tidal bores in theworld. At times， they can get as tall as ninemeters， which means the area regularlysees trains of large waves moving upstreamagainst the normal current. However， in2021， during a scientific expedition， researchersdiscovered another interestingnatural phenomenon unique to this estuary.When certain conditions are met， thetide comes in spiraling （螺旋） waves thatlook like fish scales on the water's surface.The phenomenon becomes known as fishscale tide.

Because this unusual tide pattern wasonly observed a couple of years ago， theexact reason for its occurrence is apparentlystill up for debate. Different sourcescite different reasons for the formation offish scale tide. For example， some netizensbelieve that the phenomenon is the resultof a series of factors.

According to INF News， astronomy，topography and wind direction are themain factors that produce fish scale tide.Apparently， the moon and sun play a bigpart in the phenomenon， as does the shapeof the estuary—very narrow at the rivermouth， but then very wide where it meetsthe sea. Whatever the exact necessary conditionsfor the formation of fish scale tidemay be， one thing is for sure—they rarelyoccur at the same time， hence the rarity ofthe phenomenon.

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What is fish scale tide according to the text？

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