The two Cregennan Lakes, between Cadair Idris and the Mawddach, are the best lakes in Wales with plants growing from a depth of over 8 metres beneath the surface. This is double the depth of the average lake due to the clarity of the water, the oxygenation and the lack of nutrients clogging up the natural order of the system.
The land was given to the National Trust by a man who lost both his sons in the first world war. It’s a beautiful scene, but the real magic is beneath the surface with a progression of different plants, such as water lobelia as you move from the shallows, through quillworts, pondweeds, bladderworts and eventually to different types of stonewort. Have a look at the film below to see some dive footage of what it looks like.
A key thing to look at in determining the quality of a lake is the level of nutrients and the level of dissolved oxygen. Excess nutrients come from sewage or agricultural run-off and cause algal blooms which reduce the oxygen levels. In a typical lake in the summer, the waters are divided into cold water at the bottom with very little oxygen and warm water at the top with lots of oxygen. But because of the wind and corresponding wave action at Cregennan, it is constantly mixing up the cold and the warm water. Coupled with the lack of nutrients, this means the entire lake is well oxygenated throughout the year.