36 BULLETIN OF THE UMTED STATES FISH COMMISSION. 



hoid enough to keep the pond full. Others, who have au abundant 

 supply, usually receive into the ponds entirely too much when the rain- 

 fall is great. The ponds which are in danger are those which receive 

 the floods. 



To secure the desired result, the food products of the water must be 

 given up exclusively to the carp as the properties of the soil are given 

 to the cotton plant. Therefore, when it is intended to construct a pond, 

 there are several questions which should be determined beforehand. 



Evaporation. — If the supply of water is small, too large a pond 

 will expose so much surface in dry weather that the level of the water 

 will be lowered by evaporation, and by filtration through the porous 

 soil forming the basin. It is difQcult to estimate this loss, but I do not 

 believe that it would be safe to regard it as less than | of an inch per 

 day in dry, hot weather in shallow ponds. At this rate an acre pond 

 would lose at such times 6,783 gallons per day. or 282.6 gallons per 

 hour. In other words, if the loss by evaporation is approximately ^ of 

 an inch of the surface a day, it will require a constant supply of spring 

 water, amounting to 282.6 gallons per hour, or 4.7 gallons per minute, 

 to keep the pond full. Ponds half the size would lose but half as much. 

 Eain-water must not be depended upon to supplj" fish-ponds. 



Management of overflow. — A carp pond to be of value must 

 be arranged in a manner that all the water coming in and going out 

 can be passed through screens. Labor and money invested in any at- 

 tempt to pass the floods from heavy rains through screens may be re- 

 garded as thrown away. A volume of water a foot in diameter, running 

 with the usual velocity of streams after rains, contains enough floating 

 and suspended matter to fill several yards of screen in a few hours, and 

 ofteuer in a few minutes. The earlier this is realized the better. If it 

 is the purpose to build a large pond by building a dam across the stream, 

 it will be best to cut a canal around the dam at the outset, through 

 which the floods may pass without entering the pond at all. Such a 

 canal should begin a few yards above the head of the pond. By using 

 a level you can stand at the site of the dam and determine the upper 

 begiuning point before the dam is built. But it may also be determined 

 after the water is raised, since the surface will indicate the line along 

 the side of the pond above which the canal must extend. The fall in it 

 should not exceed 1 inch in 20 feet, and if it passes close along the 

 pond side, its bottom should not be lower than the water surface of 

 the pond. To determine its required dimensions necessary to waste the 

 floods, you must ascertain as nearly as possible the acreage of land which 

 sheds rain water into the basin or valley above. A rain-fall of 1 inch 

 amounts to 3,628 cubic feet, or 27,138 gallons, to each acre. Ascertain 

 the rain-fall of your region, in order to serve as a guide for making 

 wasteways on dams and for regulating the size of canals around them. 

 Note the extremes in the rain -fall, for it is the heavy rains that test the 

 construction of ponds. The canal should be two or four times wider 



