Successful dam building is a comparatively modern achievement. While ancient engineers performed marvels in building on dry land, they were dismal failures when it came to dam building.

They made the natural mistakes of thinking that great size and weight were necessary to hold back water successfully. They built their dams as they built their magnificent pyramids and temples, of cut stone laid in huge piles. Those dams simply overturned, crushed by their own weight and the pressure of the water behind them.

Scientific dam construction was discovered and put into operation by the French some eighty years ago. They learned that size has little to do with the problem. The secret was in the proportion of the dam. The application of that principle is responsible for the magnificent achievement of Hoover Dam. The same principle is used in making the tiny crib dam needed for a pond.

The crib dam is nothing more than a pyramid of poles or lumber laid crisscross at regular intervals. The spaces between the poles are filled with various sizes of stones and rocks. The upstream face of the dam is sheeted with planks of wood to hold the water within the confines of the pond. It is as simple as that.

The proportion of the dam is very important, even for a small one. The length of the dam (the distance between the banks) is immaterial. It is the width of the base that counts. The width is calculated from the upstream foundation (called the "toe") to the downstream foundation (called the "heel"). Two and one-half feet of base must be allowed for each foot of height to which the dam will be built. So, if the dam is to be but three feet high, its base (from toe to heel) must be seven and one-half feet long. See the sketch on page 17.

The crib itself is not hard to make. Suppose this dam is going to be four feet high. That means its width at the base (from toe to heel) should be ten feet. Its finished shape is pyramidal, achieved by successively shorter rows of poles laid on top of those poles that go from bank to bank.

The first row of poles or 2 x 4's is laid across the stream, about two feet apart. They rest on the stream bed and their ends are firmly embedded in the banks. Some of the excavated dirt and rocks may be used to secure the cross-stream poles. The longest of the toe-to-heel poles (ten feet long) are laid at right angles to the first row and spiked firmly to it. Since this is a short dam (distance from bank to bank), the second and all subsequent rows should be laid about six inches apart. The spaces between row I and row 2 must be filled with stones, small rocks and coarse gravel, before the third row of poles is laid and spiked down. The diagram on page 17 shows a cross section of a crib dam. Notice the way the poles are alternated, and the spaces between them filled.

CRIB DAM.

Partially finished crib dam showing leakage. When face of dam is completely sheeted with wood planks, water will rise behind dam to overflow point.

Cross section showing arrangement of poles.

Some dams have only the upstream side sheeted. This saves work but reduces life of dam.

Continue laying poles, spiking them down and filling the spaces with stones until the desired height is reached. The spacing should have been so gauged that the top of the dam consists of but a single pole going from bank to bank. It is now ready for sheeting with planks of wood. All the time you have been building the dam, the brook has been finding its way through the bottom. The wood sheeting, which prevents the water from running through the dam, thus forcing it to accumulate and fill the pond, consists of a number of inch thick boards nailed to the top pole and to the bottom pole on the upstream side of the dam. They must be set as closely together as can be managed in order to prevent leakage of water. Even so, some leakage will occur until the wood has swollen and become silted up with dirt washed against it by the brook.

So now the dam is done and a noble piece of work it is. The size of the pond behind that little dam is immaterial. It could be fifty feet or fifteen hundred feet long. That dam could hold it!

Suggestions For Other Concrete Projects

Stepping Stones for Gardens: use a 1-3 mix; pour two inches thick into wooden molds. These are made by nailing I" x 3" boards together into frames of desired size. Grease inside of mold with old crankcase oil before pouring in cement. This allows stone to be slipped out easily when dry.

Boat Anchors: use 1 -3 mix. Pour into empty gallon size paint cans. Just before concrete hardens, set a 2" screw eye into top center for linking on cable or chain.

Bird Baths: draw your own pattern on brown paper; round, oval, shell-shaped, etc. Have it at least 18" wide. Cut out pattern. Take to tinsmith and have him cut, bend and rivet together a five inch strip of galvanized iron to conform to outline of pattern. Grease inside and pour in cement mix to a depth of four inches. When concrete has set hard enough to hold imprint of hand, carefully scrape out the center of the bird bath to form a shallow basin. The sides of basin should slope up to rim. Do not make more than three inches deep. This type of bird bath has a perfectly flat bottom which makes it completely stable when set on the ground. It may be set into the earth a couple of inches, if desired.