SELECTING THE SITE
There are several good reasons why you should use care in selecting a pond site. Construction costs, fish production, usefulness, and length of life of the pond are all affected by its location.
A good pond site should have the following features: (1) soil containing enough clay to hold water, (2) land lying so as to permit the most economical construction, (3) a drainage area of the right size to assure a good water supply, (4) a location suited to the purpose the pond is intended to serve. Avoid sites receiving drainage from feed lots, barn lots, or septic tanks. In case the best site available is somewhat less than ideal, you may find it possible to overcome the deficiencies by careful planning.
Ponds one acre or more in area are better for fish production than those of smaller size; also, for a fish pond it is best to have 15 acres of watershed area for each surface acre of water. Larger ponds also provide more stable sources of water for livestock.
Soil Quality:
The soil over much of Missouri contains enough clay for good pond construction. If you are in doubt about the soil of a particular site, sample the subsoil by using a soil auger or posthole digger in several locations along the proposed dam site.
A simple test for suitable clay content is to squeeze a handful of fairly moist soil into a firm baU. If the ball doesn't crumble with a little handling, the soil contains enough clay for use in the dam core. If the ball crumbles easily, the soil is not suitable and you must locate a better site.
Topography:
There are several things you should consider about the topography of a pond site. Construction cost is less if the dam can be located where the banks of the proposed pond basin come close together. The area to be flooded should be as flat and wide as possible to obtain the most water volume in relation to dam height, but sufficient depth to prevent excessive growth of water weeds in the sha11ower areas. The banks of the proposed water line should be fairly steep to avoid shallow water in which these weeds may become a problem. If the ground below the dam slopes gently, you can use a sod spillway; if it is steep, you may have to make a more expensive concrete spillway. Avoid sites requiring dams more than 20 feet high; construction costs will be high. It is unwise to build ponds by damming creeks. Usually creek watersheds are too large and the runoff too great to control except by large, expensive dams and spillways. The runoff also is likely to be muddy and the pond soon would become silted. This kind of pond is a poor producer of fish.
A spring is not a good source of water for a fish pond. The water temperature in spring-fed ponds is too low for good production and growth of pond fish, and spring water increases the probIem of weed control. If the spring is large enough that the pond overflows, suitable water fertility and a stabilized spillway are hard to maintain. The ideal watershed will provide enough silt free water to keep the lake full with water seldom running over the spillway. Use of the Bottom Withdrawal Spillway may make a spring source a more visible situation (see Aquaguide).
Drainage Area:
Size and condition of the drainage area are very important. The ratio of watershed area to pond surface recommended by agricultural agencies and the Conservation Department ranges between 10 to 1 and 15 to 1. The type of soil, steepness of the watershed slope, the amount and kind of vegetative cover, and the proposed use of the pond all make a difference in determining the best ratio. The watershed should be proportionately larger if the pond is intended for irrigation or for watering large herds of livestock. If you find a good dam site where the watershed is too large or too small, you may be able to get the proper proportion by building a terrace to divert water either from the pond or into it to control the water level.
For fish ponds a minimum drainage area (10 to 15 acres per surface acre of water) is desirable because the pond will fill more slowly and the total water volume will change much less often. A low rate of water change is important to fish production. If the watershed is too large, even normal runoff will flush out much of the microscopic plant and animal life a fertile pond supports, and much fish food can be lost. Large fish can escape or unwanted fish can get in by swimming up a stream over the spillway during heavy overflows. Too small a watershed will not provide adequate water to keep the pond reasonably full (see Aquaguide on Bottom Withdrawal Spillway).
The watershed must be protected from erosion. Permanent grass cover or unburned woodland free from erosion is best. All gullies and bare soil in the watershed should be revegetated. Locate ponds so that they do not receive barnyard, feed lot or septic tank drainage which may stimulate the growth of undesirable filamentous algae ("moss" or "pond scum"). Avoid sites which permit drainage from roads. Road drainage adds greatly to the watershed area, is a source of contamination, especially silt, which keeps the water muddy.
Location
Locate the pond where it will serve the most uses, bearing in mind that soil and drainage areas must be suitable. Ponds located near the house generally are used more for recreation and are cared for better than those some distance away. You may be able to locate ponds so that they furnish water not only to livestock, but also the barn, house, or truck garden. These are also good reasons for building a larger pond. For wildlife production, locate the pond near permanent wildlife cover, or where you can supply such cover by plantings. Fishless ponds are a good possibility where adequate depth for fish life is not present.
Diversion Terraces:
A diversion terrace may solve the problem of a deficient pond site. If the watershed is too large, part of the runoff can be diverted with a terrace: if it is too small, the watershed could be increased by terracing in more area. Terraces often can be operated to suit different weather conditions: in times of drought, the entire runoff from a large watershed can be diverted into the pond; in wet years runoff can be by-passed after the water reaches spillway level.
Constructing The Pond
Ponds located on good sites may fail because of faulty construction. Securing a good pond requires special attention during construction.
Brush, trees, stumps, etc., should be completely cleared from the dam site and borrow area, and partially cleared from the pond basin. There should be no roots and tree limbs in the dam fill: they leave holes in the dam when they rot and may cause it to leak. A few stumps, or brushpiles, if not in the vicinity of the dam, will provide extra fish cover.
Building the Dam:
Figure 1 shows the construction detail of a dam. The dam site should be cleared and staked off before beginning excavation. Remove the topsoil in the staked area to a depth of a foot or more (depending on its thickness) and stockpile it. Also remove and stockpile the topsoil from the borrow area. When the dam is finished, use this topsoil to cover the back slope, top and front slope down to the water line. This will be a big help in getting the nurse crop and sod started. Since topsoil usually is too porous to hold water, it should not be placed in the dam itself.
Figure 1: Construction detail of a dam
After the topsoil is graded aside, excavate a trench (core cutoff ditch) along the dam centerline for the entire length of the dam. Extend the trench up the side of each hill to spillway level. Dig the trench at least seven feet wide, down into (but not through) the clay subsoil. Next, dig a ditch at right angles to the cut-off ditch, for the water tank supply line. This ditch must be deep enough for the pipe to be below the frostline on the downstream side of the dam. Every pond used for watering livestock should have a pipe no smaller than 11/4 inches in diameter.
The water supply pipe should be bedded firmly with anti-seep collars installed. Extended one end to the stock tank site, and the other end along the pond bottom to about the center of its deepest part. At that point, build a concrete platform 6 inches thick for a solid base for the filter, or use large flat rocks for this. Use a 50-gallon metal barrel for the filter box. Knock out both ends and place it on end on the base over an upright extension of the pipe. The upright pipe should be capped, and must have several rows of 1/4-inch holes bored in it to allow water to flow to the tank. If a 1-1/4-inch pipe is used, drill about 40 holes to give a full flow of water. Cutting several holes in the side of the barrel will also improve flow of the water. After the pipe and barrel are in place, fill the barrel with coarse gravel.
All ponds should have a drain pipe in addition to a water supply pipe. It may be installed separately or as a combination drain and water supply pipe. As a fish management tool, the drain pipe may be used to empty the lake for destroying undesirable fish or to draw down the water level, thereby forcing the small bluegill from the shoreline weeds where they may be more easily preyed upon by bass. This pipe should be large enough to drain the lake rapidly. A 6 to 8 inch pipe is recommended for ponds up to approximately 3 acres and a 8 to 12 inch pipe for larger lakes. A gate valve on the downstream end of this pipe is recommended. Anti-seep collars must be installed on this pipe also.
Once the pipe is properly installed, you can begin building up the dam. It is best to make the entire fill of the same quality clay for the core. If there is not enough good clay, use the best for the core, the next best to fill the front (water) side, and the poorest material on the back side. The front side of the dam should be built to a 3 to 1 slope, and the back side to a 2 to 1 slope. Steeper slopes will erode badly or will slough off and ruin the dam. The top of the dam should be about 3 feet above the floor of the spillway or waterline when the pond is full. This space between the waterline and the top of the dam is call the freeboard, and is very important in protecting the dam from overflow and from muskrat damage. The dam should be slightly crowned in the center to allow for settling. It should be at least 12 feet wide at the top to avoid muskrat damage and to provide a roadway across it.
The dam can be protected from wave action in at least three ways. A dense sod is protection enough where ponds are not exposed to strong winds. One of the best grasses for this purpose is Reeds canary grass. It should be sown with a nurse crop, or plugs of canary grass sod may be set out at intervals of about 2-1/2 feet across the slope of the dam about at waterline. If the threat of wave-cut banks is serious, you can riprap the banks. On Missouri prairies it's a good idea to locate the pond so the prevailing southwest wind won't blow waves against the dam.
Spillway Construction
The most common cause of dam failure is poorly designed spillways. Frequently, they are built too narrow and with not enough freeboard. A well designed and constructed spillway is important for good fish management and muskrat control. The spillway should be located on solid soil next to the dam and should be wide and level enough to carry the overflow in a slow, shallow stream not more than 3 to 6 inches deep. With such a shallow outlet flow, large fish are less likely to leave the pond, and undesirable fish have less chance of swimming up into the pond. To prevent erosion by flowing water, the slope of the spi11way should be as gentle as the topography allows. The floor must be level from side to side. Even the slightest depression can lead to serious erosion. The spillway should be fertilized and seeded as soon as it is finished to provide a tough protective sod. Don't make the mistake of erecting a screen across the spillway "to keep from losing fish". The spillway is the "safety-valve" of the pond. Screens inevitably get plugged with trash, causing water to back up, overflow, and damage the earth fill. You may want to build an auxiliary spillway at the other end of the dam if the slope behind the dam is steep.
A pipe spillway system can be an important alternate (See Bottom Withdrawal Spillway Aquaguide). It may be desirable to install a pipe spillway in conjunction with the vegetative spillway. This may be particularly true if the watershed is larger than that recommended. Under normal conditions the pipe inlet should be placed approximately 1 foot below the elevation of the vegetative spillway. In this manner the pipe spillway will handle the burden of all but the very heavy rains. This will limit the amount of erosion on the vegetative spillway and will also help keep out undesirable fish that might otherwise enter the pond from a gently sloping vegetative spillway. The size needed will depend upon the volume of outflow but a 10-inch size is recommended as a minimum to prevent clogging. This pipe should have a concrete apron or other erosion control device at the lower end. The upper end may be open or it may have a riser or enter a concrete box drop inlet. Drop inlets or risers provide for more efficient drainage of the water. The pipe spillway should be placed in undisturbed soil to one side of the dam if possible and should have anti-seep collars. If placed in the dam, the soil should be well compacted.
The diversion terrace should have the same gentle slope as the spillway. Its design depends on the amount of water it is expected to carry. It must be well sodded to prevent erosion that might ruin the terrace and muddy the pond. The opening through which water is diverted into the pond should be at least 6 feet wide. The opening may be blocked if you want to run the water around the pond instead of into it.
If terracing is needed, consult the local County Extension Agent or Field Service Agent for advice on staking out the terrace and engaging equipment.
Deepening the Pond Edges:
One important construction feature in fish ponds is to deepen the edges of waterline to eliminate very shallow water. Most weed problems start in shallow water. By grading the edges down at a rather steep slope (2 to 1) to a depth of 4 feet, you can avoid later trouble with shallow water plants.
The water level should be staked out at the same time as the dam and spillway. The excess dirt removed in edge-deepening should be used on the surface of the dam or spread above the waterline to give a higher bank. The entire marginal area should then be smoothed down and planted with grass to protect the shoreline. Another way of using any extra dirt is to make an island, but islands should not be constructed in small ponds. They increase the amount of shallow water and take up too much of the area.
