There are dozens of different considerations that determine the perfect site for a pond. Many of them are a matter of preference or fixed by circumstances, such as proximity to your home for convenient swimming or the distance between the pond and fields needing irrigation. Other factors are more specific to your property, such as ground slope, soil composition, and available sources of water.
Slope and Depression
Farm ponds are best located at the lowest points of slopes and along the bottom of natural valleys and gullies. Slopes direct water into the pond to keep it naturally refilled, and capturing this runoff in a man-made structure prevents downhill erosion and damage. Locating a farm pond in or below a valley or slope is especially important for embankment-style ponds. These ponds involve containing water behind a dam or levee, so the valley or slope is necessary to direct enough runoff water to keep the pond filled all year round.
Fully excavated ponds can be located on flat or even raised areas, but this often creates opportunities for damaging erosion and flooding when the pond inevitably overflows and needs to drain. Ponds supplied by springs, streams, wetlands, and other available sources of water are almost always located at the lowest point of a ridge or slope since that’s where water naturally gathers. Manually filled ponds and those connected to wells that act as reservoirs are more flexible in their location and relation to runoff sources. However, you still need to calculate any water that can flow into the pond during rainfall, even if you’re not relying on it alone, since this will increase the water level and lead to overflow.
Soil has three major components, aside from the loam generally found primarily at the surface. All of the soil below the surface where your pond is being shaped or excavated consists of a mix of sand, clay, and silt. Sand and silt particles don’t fit together tightly enough to hold water, so the soil below a pond needs to be at least 20% clay to hold water naturally. Even at 20% or slightly above, many natural-bottomed ponds still develop leaks eventually as the soil particles allow water to wick away.
Unlike a crop field, you can’t mix clay or other ingredients into the soil below the pond to improve it. However, you can completely coat the bottom and sides of the pond with a layer of imported clay, either from elsewhere on your property or from a supplier. Bentonite clay is the most widely used and recommended type of clay used for layering into a pond. This material costs more than synthetic pond liners in most parts of the country, especially when you consider that you need two inches or more of clay on all surfaces to get a solid seal.
Soil testing is the best way to determine how suitable a particular site is for pond installation. Professional testing can add hundreds of dollars to the project budget, but there is a DIY test you can try. Take a few samples of soil from the depth of excavation from across the planned pond area and test each of them separately. Remove any grass or roots and mix the soil thoroughly. Put a handful of each sample into a quart-sized jar and fill it with water. Cap and shake thoroughly, then allow the jar to stand and settle for 24 hours. Three to four layers will emerge, with the sand at the bottom. Clay forms on the top, with the silt on the middle. If the clay layer on top doesn’t measure 20% or more of the total height of the soil in the jar, you’ll need to import clay, pour a concrete bottom, or use a flexible liner material.