Clay

For many years clay was the lining material used for ponds, lakes and waterways; the canals of Great Britain, for example, were mostly made waterproof with puddled clay. In some instances it is still the most cost-effective and best way to line ponds, especially where it is present in the subsoil and there is a plentiful water supply. That said, it has its limitations, as all clay ponds leak to a greater or lesser extent. This may be caused by plants, which put their roots through the clay lining, leaving fissures when the organic materials in the roots rot. It may also be caused by drought: if the water level drops the clay cracks, and when the level rises again water escapes through the cracks and erodes into channels.

Another possibilty is that the clay has been mixed with some other unsuitable material that allows water to leach out. To minimize this, clay should be carefully compacted in layers. This is generally done with the tracks of excavators while the clay is still damp. In addition, it is difficult to raise the water level of a clay pond above its natural level, dictated by the surrounding land and water tabel.