To determine the size, depth, and contours of your stormwater garden, think about the kinds of flora and fauna you want to see. A small pond with a surface area of about fifty square feet and a depth of twelve to twenty-four inches will do for most aquatic plants, although they may spread out to fill it within a year. To maintain an area of open water, consider a slightly larger pond with some portion at least three feet deep. Varied edge contours will attract different kinds of life. Curves, outcroppings, and inlets along the bank will increase the amount of shoreline and provide more covering for fish and frogs.
Steep slopes protect fish from pawing predators, while sloping edges promote plant growth.
Ideally, your pond will host floating-leaf plants in deep water, emergent plants in shallow water, and submerged plants, for they are all important parts of an aquatic ecosystem. Now consider the pond's bottom. Heavy clay soils like we have in Lake County hold water pretty well, so you probably won't need to buy a liner. This is particularly true if you are running the sump pump to the pond. Without a liner, you can plant directly into the soil.