its correct name, being Llyn Tegid in Welsh) is a lake in Gwynedd, Wales. The name Tegid may be related to Welsh teg, meaning "fair". It was the largest natural body of water in Wales before its level was raised by Thomas Telford to help support the flow of the Ellesmere Canal. It is 3.7 miles (6.0 km) long by 0.5 miles (0.8 km) wide, and is subject to sudden and dangerous floods. The River Dee runs through it and the waters of the lake are famously deep and clear. The town of Bala sits at its north-eastern end and the narrow gauge Bala Lake Railway runs for 3 miles (4.8 km) along the lake's south-eastern shore, and continues south-westward to the village of Llanuwchllyn The lake lies in a glacial valley which follows the Bala to Tal-y-Llyn fault line. The valley was blocked by a terminal moraine in the area of the village of Bala, thus forming the lake, which has remarkably straight and parallel sides.Llyn Tegid has abundant pike, perch, brown trout, roach, eel. It also contains the gwyniad, a fish unique to the locality and listed as critically endangered by the IUCN due to the introduction of the invasive and non native ruffe;and the very rare mollusc Myxas glutinosa (the glutinous snail). According to legend, whilst the Dee itself flows through the lake, the waters never mix. However this was not confirmed by the detailed limnological work undertaken from the 1990s, to understand and manage the occurrence of algal blooms on the lake.The lake now forms part of the River Dee regulation system and the level at its outflow is automatically controlled. Depending on flow conditions and the level of water in Llyn Celyn, water can flow either into or out of the lake at the normal outflow point.source:wikipedia