There are no industries to fill the estuaries with darkness and poisons, the Tryweryn and Dee are as clear as crystal and Llyn Tegid as a sea of glass.
Trans. Sir O M Edwards, Cymru Vol 4, Page 13
Llyn Tegid is fed by three main rivers – Afon Dyfrdwy (River Dee), Afon Llafar and Afon Glyn. Although allegedly, the water of the River Dee doesn’t mix with that of the lake as it flows through it. In certain weather conditions, and from a certain point on Garth Fawr, it is said that the river can be seen snaking through the lake maintaining its identity.
The River Dee springs on the slopes of Dduallt above the village of Llanuwchllyn and is 70 miles (110Km) long. The course of the river flows to the east from Bala, through Llangollen under and over Thomas Telford’s Pontcysyllte aqueduct. It then flows northerly to Chester before going back over the border to Wales and out to sea in an estuary between Wales and the Wirral peninsula.
Llyn Tegid’s water levels are artificially controlled by the River Dee Regulation System to meet the water demand of some areas of north west England. A little downstream to where the River Dee leaves the lake are sluice gates to control the levels. Surplus water is held back during the winter so that there is enough water to meet the water demand during the low-flows of the drier summer months.
As the water demand in Liverpool and the Wirral increased, in 1957 work began on creating another reservoir on the course of the river Tryweryn, which joins the Dee not far from where it leaves Llyn Tegid.
Once the Tryweryn Bill had been passed by parliament, which gave Liverpool City Council a compulsory purchase order to buy the land of Cwm Celyn valley, a dam was built across the valley and the village of Capel Celyn was drowned. The village of Capel Celyn was a Welsh cultured community, which had a chapel and cemetery, school, post office, twelve farms and land owned by four other farms. This development was highly controversial, which generated national fury, resentment and protest. In October 2005, the Liverpool City Council apologised for drowning the valley.
Llyn Tegid was formed at the end of the last Ice Age – around 14,000 years ago.
During the Ice Age the Arenig and surrounding area, which is to the north west of Llyn Tegid, was under hundreds, even thousands of feet of ice and glaciers flowed from there to all directions.
Llyn Tegid lies in a valley which follows an old geological fault line – Bala Fault, which is a rupture in the land. As glaciers passed through the valley the fragile rocks of the fault eroded – this is why the lake is so steep, and can be up to 43 metres (141ft) in places.
As the ice thawed it left behind deposits that had been embedded in the ice. In time, the deposits formed a dam (moraine) which prevented water from flowing out of the valley.
Just after the Ice Age, the lake was much larger than it is today; but as the River Dee ate away at the moraine the lake level dropped. It is possible that the lake, for a while, also drained through the valley of the river Wnion on the south western end of the lake.