Qualla: Llyn Alaw

Llyn Alaw: Introduction

Sun, 24 May 2026 00:00:00 GMT

Photo credit Phil Williams, CC BY-SA 2.0. It is the largest body of water on Anglesey, but only by area, not by volume. Llyn Alaw stretches 4.3 kilometres long across a flat northern landscape and covers 3.6 square kilometres, yet at its deepest it is only 5.2 metres - which is to say, shallower than a typical Olympic diving pool. The reservoir was made in November 1965 by closing a dam across the existing marsh of Cors y Bol; filling completed in January 1966, and the official opening came on 21 October the same year. Because the site was already marshland, no farms or hamlets had to be abandoned. The reservoir supplies drinking water to the northern half of the island at a rate of 35 million litres a day, run by Dwr Cymru Welsh Water. But what makes Llyn Alaw remarkable is what its shallowness invites: a winter sky filled with ducks, swans and geese arriving from the Arctic.

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Image: Phil Williams | CC BY-SA 2.0

Llyn Alaw: A Lake Filled by Rain

Sun, 24 May 2026 00:00:00 GMT

Photo credit Eric Jones, CC BY-SA 2.0. There are no major rivers running into Llyn Alaw. Its catchment is mostly agricultural, and the lake fills almost entirely by trapping winter rainfall. In summer the level is drawn down to meet demand, exposing mud flats around the margins and creating low islets where the bed li...

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Image: Eric Jones | CC BY-SA 2.0

Llyn Alaw: The Winter Visitors

Sun, 24 May 2026 00:00:00 GMT

Photo credit Phil Williams, CC BY-SA 2.0. Llyn Alaw is designated a Site of Special Scientific Interest because of the variety and numbers of waterfowl that visit. The biggest concentrations come in winter. Whooper swans arrive from Iceland - the wild swans that breed nowhere south of the Arctic and pass through Anglesey...

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Image: Phil Williams | CC BY-SA 2.0

Llyn Alaw: Edge and Margin

Sun, 24 May 2026 00:00:00 GMT

Photo credit Talsarnau Times, CC BY-SA 3.0. What was flooded in 1965 was Cors y Bol, a marsh of considerable ecological interest in its own right. The reservoir replaced one wetland with another, in effect; the difference is that the engineered lake fluctuates more sharply and supports rather different birds. The shore is ...

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Image: Talsarnau Times | CC BY-SA 3.0