20 



a chance foir grass and wieeds to take root in them and soon 

 become entirely obistructed. 



It seems to have been the custom in New Jersey to specify 

 the depth as thirty inches for mosquito ditches, and in most 

 localities this seems justified. The ideal ditch on the salt marsh 

 contains practically no water at low tide, or only a depth of two 

 or three inches, and it is while the ditch is in this condition that 

 the most drainage by seepag|e from the surface is obtained. To 

 obtain this emptying of a ditch, however, it is evident that the 

 range of tide in a locality where thirty-inch ditches are dug 

 must be at least three feet. In sections like Barnegat Bay, how- 

 ever, where the range of the tides is only 12 to 18 inches, it 

 seems needless to install ditches more than 18 to 24 inches 

 deep, for the ditches can seldom run dry and will soon fill in to 

 a depth corresponding tO' ^that of the average low tide. Con- 

 versely, it is theoretically advisable to dig ditches five feet deep 

 where the range of the tide is six feet or more, but in practice 

 it is found that the cost of digging ditches increases greatly 

 beyond the thirty-inch depth, and furthermore that iti most 

 meadow! the soil below this depth is not held together by grass 

 roots, and the ditches consequently cave in. 



In determining upon a system tO' be installed in a salt marsh, 

 the character of the marsh should be studied with reference to 

 the spacing of the ditches, for under normal conditions the seep- 

 age action or drainage power of the ditches should be great 

 enough to remove the shallow surface water left by rains and 

 high tides in 24 to 48 hours. If the ditches aire placed too far 

 apart this will not occur, and it will be found necessary to install 

 either intermediate ditches or cross ditches. As salt marshes 

 vary greatly in character, this distance apart of ditches also 

 varies greatly, the minimum distance usually being 75 feet and 

 the maximum 200, with an average for all meadow of about 

 125. 



It has become the custom in nearly all sections where salt- 

 marsh drainage for mosquito control is done tO' consider all 

 pools over two or three inches in depth as possible breeding 

 places, and to drain them by means of a short spur ditch from 



