104 MOSQUITO ERADICATION 



considerable magnitude may be required to eliminate it . . . In case 

 drainage is decided upon, it will probably take the form of channeling 

 the stream below the swamp to lower the water and increase the 

 flow. . . . 



"The third case mentioned — that of a swamp fed by a seepage 

 outcrop — presents the most difficult problem of all. Here we have, 

 not only an area of standing water, probably full of vegetation, but also 

 a series of tiny puddles in the form of hoof-prints, etc., along the outcrop 

 margin. Each of these is a potential breeding-place of the worst kind. 

 The treatment of such an area requires a special procedure, and, since 

 the proposition is one that is apt to be found in most any locality, it may 

 be considered in some detail. 



TREATMENT OF SEEPAGE OUTCROPS 



"Seepage water usually appears on the hillsides, etc., at the outcrop of 

 a stratum of water-bearing sand or gravel underlain by an impervious 

 stratum of clay, shale or other material. The outcrop may be in the 

 nature of more or less distinct springs or simply a gradual oozing out 

 through the soil. In either case, it is fed by a water-table below the 

 surface, and treatment must be aimed particularly at this water-table. 

 It does not suffice to dig ditches directly away from the springs and down 

 the hillside. Such a method would require a separate ditch for each 

 spot from which water is issuing, and would mean, in many cases, a 

 series of ditches about 12 inches apart along the whole hillside. The 

 only effectual way of collecting the water in such places is by means of 

 ditches dug at right angles to the flow of the seepage water, or, in other 

 words, across the exposed end of the water-table. Such ditches may 

 then be connected to one or more main ditches, if necessary, and the 

 water carried down the hillside parallel to the seepage flow. These 

 points are illustrated roughly in the accompanying sketches of actual 

 swamps. 



"In No. 1, there was a seepage on both sides of a narrow valley, the 

 water coming out of two hills opposite one another, as shown in the 

 sketch. As a result, the bottom of the valley in this region was a 

 typical cat-tail swamp, with water from 1 inch to 2 feet in depth. Since 

 the source was somewhat up on the hillside, it was useless to dig a 

 ditch through the bottom of the swamp and down the valley. This 

 would simply carry off the deep water and leave the seepy marsh as it was. 

 Instead, a deep, narrow ditch was dug along the margin of each hill, 

 just at the upper edge of the seepage outcrop and at right angles to the 

 flow of the seepage water. In this manner, the water-table was inter- 

 cepted, and all the water that formerly oozed out down the hillside now 

 seeps into the ditch and is carried off. As a result, the swamp, no longer 

 fed from the hillside, has dried up. 



