The lake has required major conservation work: two things happen to lakes - they fill with sediment, and vegetation develops in the shallows.
Half the surface area had been lost due to the accumulation of silt. The lake in its previous state could not support the diversity of wildlife for which it had the potential, e.g. more birds, marginal plants and dragonflies. If the silting had been left unchecked, the lake would have been lost completely. Sediment had been building up since the lake was constructed about a hundred years ago, and its removal was an enormous and expensive task which required professional help.
Trees, saplings especially willow, and scrub, had grown up along the margins. The roots were causing the stone walls of the lake to disintegrate and the dam to crack.
Leaves falling into the lake cause a build up of methane and loss of oxygen in the water, with the loss of variety and richness in aquatic life. Experts on lakes and ponds say there should be no more than 30% shade for a healthy lake, since too much shading by trees prevents sunlight falling on the water which causes a further reduction in aquatic plant life. Any area of woodland should be at least 20m back from the water's edge.