In most instances, the maximum height of a dam should be 20 to 25 feet. Dams higher than this are expensive to build and frequently stratify in summer resulting in deep water areas unsuitable for aquatic life. Dams in Virginia should be high enough to provide a minimum depth of six feet year-round. Otherwise, dams must be high enough to compensate for continuous evaporation and seepage. Because a pond is simply a depression for holding water, the dam and bottom must be composed of soil which minimizes seepage.
Clay soils are best for lining ponds because they minimize leakage.
Sites containing gravel or sandy soils are unsuitable, often requiring costly earth moving. Limestone or shale areas are unsuitable because of possible fractures which create leaks. Swampy areas are poor sites because they are difficult to drain and costly to maintain. The water supply must be sufficient to rapidly fill the pond and maintain a relatively constant water level-one that does not fluctuate greatly throughout the year. Ponds with large overflows of water flush essential nutrients and allow fish to escape.