Llyn Dinas is a lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 near Beddgelert

Beddgelert

Beddgelert, or in older English spelling often Bedgellert, is a village and community in the Snowdonia area of Gwynedd, Wales. It is reputed to be named after the legendary hound Gelert. Population 617.- History:...

, Gwynedd

Gwynedd

Gwynedd is a county in north-west Wales, named after the old Kingdom of Gwynedd. Although the second biggest in terms of geographical area, it is also one of the most sparsely populated...

 in north Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. It is formed by the River Glaslyn

River Glaslyn

The Afon Glaslyn is a river in Gwynedd, north-west Wales. Whilst not of great significance in terms of its length , it is one of Gwynedd's primary rivers, and has greatly influenced the landscape in which it flows....

.

Llyn Dinas lies on the valley floor a few miles north of Beddgelert at an altitude of about 55 metres above sea level. It has an area of 60 acres (242,811.6 m²) and is fairly shallow, with the maximum depth only 10 metres. The lake offers good fishing for salmon and trout.

It takes its name from the nearby Dinas Emrys

Dinas Emrys

Dinas Emrys is a rocky and wooded hillock near Beddgelert in Gwynedd, north-west Wales. Rising some 250 ft above the floor of the Glaslyn river valley, it overlooks the southern end of Llyn Dinas in Snowdonia. Little remains of the castle structures that once stood here, save its stone...

, a rocky and wooded hill just downstream of the lake where the remains of both medieval and older fortifications have been found. A rock near the lake named Carreg yr Eryr (The stone of the eagle) was said in a charter of 1198 to mark the spot where the boundaries of the three cantrefs of Aberconwy, Ardudwy and Arfon met. According to Giraldus Cambrensis

Giraldus Cambrensis

Gerald of Wales , also known as Gerallt Gymro in Welsh or Giraldus Cambrensis in Latin, archdeacon of Brecon, was a medieval clergyman and chronicler of his times...

an eagle used to perch on it once a week, anticipating battle between the men of the three cantrefs.