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Answer: An estuary is a partly enclosed body of water, with its wetlands, where rivers meet the sea and fresh water mixes with salt water to form brackish water. The plants and animals there are adapted to salinity that shifts with tides and river flow.

Salinity in between

Fresh water has salinity near zero; the open ocean averages about 35 parts per thousand. Estuary water falls between, and the mix changes through the day and seasons.

Tides shape them

Daily tides can dramatically change estuaries. NOAA's example is Elkhorn Slough in California, a tidally flushed estuary where strong currents move large amounts of sediment.

Why they matter

• Unique communities adapted to brackish water.
• Nurseries for many species.
• Buffers between land and sea.

NOAA's National Estuarine Research Reserves study and protect many of them.

Claims

• id: C1 claim: Estuaries are usually found where rivers meet the sea. source: S1 evidence: "Estuaries and their surrounding wetlands are bodies of water usually found where rivers meet the"
• id: C2 claim: Estuary communities are adapted to brackish water. source: S1 evidence: "have adapted to brackish water"
• id: C3 claim: Ocean salinity averages about 35 parts per thousand. source: S2 evidence: "The salinity of water in the ocean averages about 35 parts per thousand (ppt)."
• id: C4 claim: Daily tides can dramatically affect estuarine ecosystems. source: S2 evidence: "can have a dramatic effect on estuarine ecosystems"

Sources

• [S1] What is an estuary?. NOAA. https://oceanservice.noaa.gov/facts/estuary.html . License: US government work, public domain. Accessed 2026-11-04.
• [S2] What is an estuary? Estuaries Tutorial. NOAA. https://oceanservice.noaa.gov/education/tutorial_estuaries/est01_whatis.html . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_ztdraa8b,
  author = {{iinventedlight}},
  title = {{Estuaries: where rivers meet the sea}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/estuaries/},
  note = {Permanent ID iil:note:ztdraa8b. License: CC-BY-4.0.},
}

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