Designing the Dam
When to Build
You can build a pond any time of the year, but summer is usually the best because weather and soil conditions allow use of heavy equipment. Then, fall and winter rainfall fills the new pond and lets you stock fish at the right times.
Deep ponds are not necessary for productive fisheries in Mississippi and may lead to water quality problems, such as low dissolved oxygen, which can kill fish. Most ponds have low oxygen levels at depths greater than 4 feet in summer.
Ponds in Mississippi should have an average depth of about 5 to 6 feet and be no more than 10 to 12 feet deep. About half of the pond should be in water that is 4 to 5 feet deep. This lets fish forage on the bottom, even in summer when low oxygen concentrations are common in deeper water, while maintaining enough depth to sustain the fish during drought.
About 20 percent of the pond bottom should be at least 6 feet deep to provide winter refuge and summer refuge in extremely dry years. During summer, evaporation can reduce water levels at a rate of up to ½ inch per day, and ponds may lose 2 feet or more in water depth. The pond banks should drop quickly to at least 3 feet deep to minimize risk of aquatic plants becoming established.
Dams should be at least 8 to 12 feet wide at the top, depending on the height of the dam. Dams fewer than 12 feet high require an 8-foot top width. Dams between 12 and 15 feet high require a 10-foot top width, and those higher than 15 feet require a 12-foot top width. If you plan to use the dam as a road, it should be at least 16 feet wide across the top.
In many areas of Mississippi, soil types are such that dams must be cored with clay to prevent seepage. The slope of the dam should be no steeper than 3:1 on the waterside. On the backside, a 4:1 slope allows it to be safely mowed and maintained. For example, a dam with a 3:1 slope will have a 1-foot rise for every 3 feet of horizontal measurement.
A combination drain and overflow pipe, as well as an emergency spillway, are necessary for good management. It is very important that you place the drainpipe on the pond bottom so you can completely drain the pond. Controlling the water level is important for weed control and fisheries management. A drain is necessary to manage the pond efficiently. The overflow pipe is the outlet for normal water flow through the pond. The emergency spillway is an area lower than the top of the dam on one side of the dam to safely release excessive runoff from heavy rainfall.
Overflow and drainpipes may be corrugated metal, aluminum, steel, or polyvinyl chloride (PVC). Some materials are more durable than others and may be preferred. For example, PVC pipe, although inexpensive, is prone to breakage and vandalism. Be sure the pipe meets the standards for use in a pond dam. You can add drains to existing ponds, but you will need professional assistance.
It is also a good idea to fit a larger pipe over the standpipe, starting 18 inches off the bottom and extending 12 inches above the surface. This pipe will draw water from the deeper areas of the pond, where there is no oxygen. During periods of water flow through the pond, this improves the water quality in the pond, increases usable habitat, and reduces the risk of a fish kill. The outside pipe must be at least one and a half times the interior width of the standpipe to allow for maximum flow.
Establish suitable perennial vegetation on the dam as soon as possible to prevent erosion, muddy water, and maintenance problems. Nonnative grasses such as Bermuda and Bahia are commonly planted because they establish a sod quickly and tolerate frequent mowing to just a few inches in height. A good, deep-rooted native grass (such as switchgrass or big bluestem) is an alternative perennial cover that provides better wildlife habitat, requires less mowing, and tolerates drought better than nonnative grasses. If native grasses are established as cover on the dam, do not mow them shorter than 6 inches in height.
If dam construction is completed before or after the recommended planting dates, plant a temporary cover to limit soil erosion until the permanent cover establishes. Sowing a temporary cover of browntop millet (25 pounds per acre broadcast from May to July) or winter wheat (90 pounds per acre broadcast from August to November) protects exposed surfaces and provides wildlife food and cover.
Lime, fertilize, and seed the dam with an appropriate grass as soon as construction is complete. We recommend mulching the dam and other sloping areas. It is very important to prevent erosion of the dam. Complete the pond in summer before stocking fish in the fall. If practical, do not let the pond fill with water until just before stocking. This prevents the pond from becoming contaminated with unwanted fish species, lets grass cover the pond bottom, and helps prevent muddy water.
Do not let trees or shrubs grow on the dam, since they weaken the dam and increase the likelihood of leaking or failure. However, if large trees are growing on an older pond dam, do not remove them, because their decaying root systems may weaken the dam.
For more technical information on dam and water control structure design, see NRCS publication Ponds- Planning, Design and Construction.pdf.