156 MOSQUITO ERADICATION 



tides, throughout the summer. This allows the fish supply to 

 be kept up. 



"Moreover," says Gies, 1 "by raising the gates on one side of the 

 marsh and allowing water to enter, while keeping the gates on the other 

 side of the meadow closed, except for discharge of water, it is possible 

 to get circulation of the water through the ditches across the marsh, and 

 sweep out breeding by this mechanical action. The gates are all closed 

 during periods of monthly high tides or storms, so the unusually high 

 tide-water will not back up over the meadows . . . 



SALT MARSH SHRINKAGE 



"There is one other point that has not been touched on — that is, the 

 matter of meadow shrinkage, if the gates remain permanently closed. 

 Most salt marshes are composed of a top layer chiefly of decayed vegeta- 

 tion or peat on a foundation of blue clay, mud, sand, etc. The elevation 

 of the meadow surface is kept up to a point slightly above mean high tide 

 by the continuing growth and decay of the salt grasses, sedges, rushes, 

 etc. This class of vegetation needs plenty of salt water to attain maxi- 

 mum growth, and, hence, if the tide-gates are kept continually closed 

 and the salt water excluded from the marsh, the growth of the surface 

 vegetation stops and the spongy top layer of peat dries out and begins 

 to pack down hard, and the marsh settles down lower and lower. In 

 parts of Hudson County, New Jersey, this shrinkage has amounted to 

 several feet. 



"Finally, a point is reached where the elevation of the meadow has 

 become so low that it can no longer be drained by ditches or tide-gates, 

 and it is necessary to install pumps, which increases the cost of mos- 

 quito control . . . 



" . . . We are keeping the tide-gates open in our own district, not 

 only for about 2 weeks out of every month during the summer, but dur- 

 ing the entire non-mosquito-breeding season, from November until 

 March. We hope, in this way, to keep up our marsh elevation by con- 

 tinuing our vegetation growth, so that we will not be forced to the extra 

 expense of pumping." 



PUMPING 



While small areas of marsh, so low that they will not drain at 

 low tide, may be filled, large areas of this kind probably will 

 require pumping. 



Whether a given area shall be pumped, what type and what 

 capacity of pump shall be employed, what ditching must be used 



1 Transactions of the First Annual Conference of Sanitary Engineers, 

 U. S. Public Health Service, 1919. 



