After Lesjaverk (23 miles) the eastern end of Lesjaskogsvatn is reached. This lake has the distinction of being a watershed. From the east end flows the Lågen, the great river that runs down Gudbrandsdal to Lake Mjøsa, while from the west end the Rauma River emerges to flow down to the Romsdalfjord.1956 September, M. D. Greville, “Through the Romsdal by Rail”, in Railway Magazine, page 594:
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Geography, Hydrology, Natural sciencesCanada, US A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin.
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Figuratively A critical point marking a change in course or development.
watershed moment — watershed event — watershed ruling
In the minds of many, 1847 was a watershed year during which Ceres, Pallas, Juno, Vesta, and Astraea were reclassified as asteroids.2009 [2007], David A. Weintraub, Is Pluto a Planet?: A Historical Journey through the Solar System, Princeton, NJ: Princeton University Press, →ISBN, page 115:
2 more examples
Coronavirus has been a watershed for the railways. It has accelerated the decline of season tickets and reduced business travel after years of steadily rising passenger numbers.2021 November 17, Anthony Lambert, “How do we grow the leisure market?”, in RAIL, number 944, page 34:
In a 2,000-word blog post published ahead of the press event, Brad Smith, Microsoft’s president, called this a “watershed year” and acknowledged the potential downsides, calling for “wide-ranging and deep conversations” on the issues.2023 February 7, Cade Metz, Karen Weise, “Microsoft Throws a Coming-Out Party for A.I.”, in The New York Times, →ISSN:
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Broadcasting, Media, Radio, TelevisionCanada, UK The time during the day in which material of a more adult nature (e.g., violence, swear words, or sex) may be broadcast on television or radio, either one permitted by law or one contrived from convention (e.g. when children are not watching).
Etymology From water + shed. A calque of German Wasserscheide.
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Serving to mark a significant development, change in direction, etc.
Green's Dictionary of Slang is a watershed publication in the annals of slang lexicography, being, beyond doubt, the most comprehensive scholarly dictionary of slang ever published.2018, James Lambert, “Anglo-Indian slang in dictionaries on historical principles”, in World Englishes, volume 37, page 251: