Mark the general shape – oval or kidney-shape look more natural than round, although edges can be planted to soften the outline. Make the pond bigger than you think you want it – it’s amazing how quickly it will fill up after you add mud to the bottom and your water-loving plants start to grow in the water and around the edges.
Although stepped sides create a shelf for pots of plants or for attractive rocks, toads, frogs, birds, and insects will find it easier to use your pond if the sides are gently sloped. Why not have the best of both worlds by making one side sloped, the other stepped?
Digging is hard work. To minimize excavation and avoid disturbing the roots of nearby trees and shrubs, you may want to use the soil you remove from the middle of the pond to build up the edges. Use a level and a long straight piece of wood to check that the top edge of the final hollow is even all the way around.
To calculate how big the waterproof liner must be, measure the hollow carefully and allow at least an extra foot on all sides to prevent excessive water loss through seepage. Line the hollow with landscaping cloth first to prevent roots from puncturing the liner. Spread the liner out on top – 35-mil polyvinylchloride is a good buy.
There are experienced, reputable contractors who would be very happy to construct a pond to meet your requirements. Tours of existing water gardens, such as those organized by the Greater Ottawa Water Garden Horticultural Society, are great opportunities to get ideas and inspirations.