140 MOSQUITO ERADICATION 



that drainage for agricultural purposes is usually more intensive, 

 as well as more expensive, than drainage for mosquito control 

 purposes. 



"It seems clear," says Headlee, 1 "that the drainage necessary to 

 prepare and maintain the marsh in condition for successful agriculture 

 keeps the water-table so far below the surface that the original vegeta- 

 tion is destroyed and the marsh undergoes a large shrinkage. In some 

 cases, this shrinkage is sufficiently great to bring parts of the marsh 

 below mean tide and to necessitate the use of pumps to keep the water 

 far enough below the surface of the marsh to permit the growth of 

 crops. 



"In that type of drainage necessary to eliminate mosquito-breeding, 

 the water need not be maintained at a level lower than 12 inches below 

 the surface, even during the height of the mosquito-breeding season. 

 Indeed, at seasons of the year when breeding is not occurring, the gates 

 can be opened and the tide allowed to circulate freely. It would seem 

 that this type of drainage should not destroy the salt marsh vegetation 

 and that the meadow should not, therefore, materially shrink. There 

 is some evidence to show that, with a proper system of letting the water 

 on the marsh, the vegetation will not only not be destroyed, but maxi- 

 mum crops of salt marsh hay can be produced." 



DESIGN OF SALT MARSH DITCHES 



As a result of the investigations of Dr. John B. Smith, pioneer 

 in anti-mosquito work in New Jersey, it has been found that the 

 best type of ditching for the usual salt marsh is a ditch 10 inches 

 wide by 30 inches deep, with smooth perpendicular sides. This 

 depth normally reaches the bottom of the sod, and should be 

 maintained, except where natural slope calls for a deeper cut to 

 insure the free flow of the water. As the upland is approached 

 and the sod and underlying muck becomes thinner, the depth of 

 the ditch is decreased accordingly, neither sand nor subsoil being 

 cut into, unless the drainage of a pool or other shut-in area 

 requires it. 



Two general plans of ditching have been used in New Jersey. 

 One plan, known as the parallel system, calls for division of the 

 territory to be drained into districts on the basis of possible 

 ' outlets, each district to be drained by parallel ditches, sufficiently 

 close together to remove the surface water. The other plan, 

 known as the pool-connecting system, consists of running ditches 



1 "The Mosquitoes of New Jersey and Their Control," New Jersey Agri- 

 cultural Experiment Stations, Bulletin No. 276, 1915. 



