Here are some important points to consider when planning your pond or lake.
Why not build your pond away from overhanging trees? The leaves they drop will break down and produce nitrates which will encourage algae growth. Build your pond or lake in a location that is in sunshine for about half the day.
Even a small pond should be at least 60cm deep at one point to prevent oxygen levels falling too low when the surface freezes (to protect invertebrates / amphibians). If you intend to keep fish in your pond the deer areas are vital to protect the fish from Herons and cats.
Have at least one shallow side, some of which should be well planed, for amphibians to leave the water. Shallow sides allow more marginal plants, which in turn shelter more wildlife. Gently sloping sides allow hedgehogs to escape, and are less dangerous in the event that a child toddler falls in. Toddlers or young children should always be supervised near the water.
Before digging your pond or lake, ensure there are no sewage, water, or power cables running through your garden, check with your local utility companies / local council. Planning permission may be required, depending on the size of the lake or pond and the area.
Be aware that any tree roots close to your pond or lake bed may find their way through your linings, causing future leaks.
Before introducing fish be aware that fish eat invertebrates and tadpoles, and discourage damselflies and dragonflies, generally speaking the widest variety of natural life can be found in ponds that do not contain fish.
Do not site your pond on marshy ground, otherwise water will tend to rise behind the liner causing future problems. If you have no choice but to dig a small pond in an area with a high water table one or more sumps can be dug around the pond with pumps inside to artificially lower the water table, (a complicated and expensive solution though).
Once the lake has been dug out, it is common practice to use a layer of soft sand to smooth out the surface. For the underlay a geotextile membrane is best (you could cut costs with polypropylene carpet, but remove any nails / tacks first). There are many different liners that are now available, butyl is a popular choice because it is durable and long lasting, but polythene is sometimes used. The liner is usually covered by more soft sand and fine washed gravel before the pond or lake is filled. Some pond and lake beds are constructed with concrete, often incorporating liners. The traditional method is to use puddled clay, however this is difficult and expensive, so is rarely seen on new ponds and lakes. Lake building techniques are constantly changing as technology moves forward, be sure to ask the advice of several different contractors to decide the best system for your project.
Please note, this article is for advice only, always consult a trained landscape designer before attempting groundwork.