:Llyn y Fan Fawr

{{Short description|Lake in Powys, Wales}}

{{Use dmy dates|date=April 2022}}

{{Infobox body of water

| name = Llyn y Fan Fawr

| image = Llyn y Fan Fawr.jpg

| caption =

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|pushpin_map=Wales Powys

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| location = Powys, Wales

| coords = {{coord|51|52|52|N|3|42|0|W|type:waterbody_region:GB|display=inline,title}}

| type = natural

| inflow =

| outflow = Nant y Llyn, tributary of River Tawe

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| basin_countries = United Kingdom

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| elevation = 605 m (1815 feet)

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{{langnf|cy|Llyn y Fan Fawr|great lake of the peak|italic=no}}{{cite book |last1=Owen |first1=Hywel Wyn |last2=Morgan |first2=Richard |title=Dictionary of the Place-names of Wales |date=2007 |publisher=Gomer Press |location=Llandysul |isbn=9781843239017 |page=300 |edition=First}} is a natural lake in the county of Powys, Wales. It lies at the foot of Fan Brycheiniog,{{cite book|author1=Andrew Davies|author2=David Whittaker|title=Walking on the Brecon Beacons|url=https://books.google.com/books?id=f5Qo1yy7xFYC&pg=PA191|date=9 September 2010|publisher=Cicerone Press Limited|isbn=978-1-84965-346-6|pages=191–}} the highest peak of the Black Mountain ({{lang|cy|Y Mynydd Du}}) range within the Brecon Beacons National Park. Created as a result of glacial action,{{cite book|author=Geological Survey of Great Britain|title=Memoirs. England and Wales: Explanation of Sheet|url=https://books.google.com/books?id=0SEeAQAAMAAJ|year=1932|publisher=H.M. Stationery Office}} it is one of the largest glacial lakes in southern Wales.{{cite book|author=Derek Ratcliffe|title=A Nature Conservation Review: Volume 1: The Selection of Biological Sites of National Importance to Nature Conservation in Britain|url=https://books.google.com/books?id=YPQaFxH-AZYC&pg=PA236|date=26 January 2012|publisher=Cambridge University Press|isbn=978-0-521-20329-6|pages=236–}}

File:Llyn y Fan Fawr from a small beach.jpg

Shoreweed and quillwort are found here whilst greater water-moss occurs on boulders at the water's edge. Freshwater limpets, caddisflies and leeches are also recorded. However, as regards fish, a C17th manuscript asserts that it "hathe no fishe attaile in't nither will any fish being put into it live, but as soon as they have tasted of this water turne up their silver bellies and suddenly dey."{{cite book |last1=Mullard |first1=Jonathan |title=The New Naturalist Library: Brecon Beacons |date=2014 |publisher=HarperCollins |location=London |isbn=9780007367696 |pages=226-227}}

The surface of Llyn y Fan Fawr lies at about 1815 feet or 605m above sea level.{{cite book|author1=Hans Luther|author2=Julian Rzóska|title=Project Aqua: A Source Book of Inland Waters Proposed for Conservation|url=https://archive.org/details/projectaquasourc0000luth|url-access=registration|year=1971|publisher=International Biological Programme|isbn=978-0-632-08730-3}}Ordnance Survey Explorer map OL12 'Brecon Beacons National Park: western area' Its primary outflow is to the River Tawe.{{cite book|title=Bye-gones: Relating to Wales and the Border Counties|url=https://books.google.com/books?id=md4GAAAAYAAJ&pg=PA504|year=1894|publisher=Printed at the Caxton works.|pages=504–}} Much of the land around the lake is peaty and thus wet underfoot. The main route of the Beacons Way runs along the ridge above the lake whilst its low-level alternative wraps around the lake's eastern margin.