design and plants for a wildlife pond
The base needs to be covered with about 5 to 10 cms of soil, which will turn into mud, and perahps 5cms of gravel over that, and this layer needs to be allowed for when excavating.
A natural flow of water from a stream is best, but the water can be freshened by diverting roof water into the pond and allowing for an overflow, which can be used to create a boggy area beside a pond, or diverted into a sump to take the water away to a deeper level in the soil.
Plant marginal plants such as bullrushes, flowering rush and marsh marigold. Milfoil is a submerged plant. Waterlily can be planted and gives good shade for part of the water surface. The allow things to develop naturally. It takes a couple of years to settle fully.