26 MISC. PUBLICATION 336, U. S. DEPT. OF AGRICULTURE 
heavy rainfall or of high storm tides, (2) to permit free circulation 
of tidal water into low areas that are otherwise landlocked, and (3) 
to give larva-eating minnows access to the pools and ponds or other 
places where the larvae accumulate as the surface water is drained off. 
The ditch system on any marsh should be designed to give the maxi- 
mum degree of mosquito control with the least amount of ditching. 
The system for each marsh should be determined by the conditions 
presented, and stereotyped drainage systems should be avoided. Pre- 
liminary surveys are necessary to determine any natural drainage and 
to locate the water-holding, mosquito-breeding areas. The natural 
channels should then be improved and the ditches located so that they 
will extend from these channels to the breeding portions of the marsh, 
following the lowest contours. In general, the ditches should not be 
cut directly into ponds, but rather connected by short spurs, since it 
is often difficult to maintain the desired depth in the soft mud of the 
pond bottoms. On large marshes which are so flat that natural 
drainage is negligible or difficult to discern, the usual practice is to 
place the ditches in a parallel system at intervals of 100 to 300 feet, 
with main outlet ditches as needed. Even on such marshes, however, 
the amount of ditching may often be reduced by making preliminary 
surveys to locate the mosquito-breeding portions of the marsh and 
limiting control work to such areas. The installation of rim ditches 
for draining areas adjacent to the highland, where heavy breeding 
often occurs, is necessary on most marshes. 
In New Jersey, where the marshes are usually well sodded, the 
standard ditch is 10 inches wide by about 20 inches deep, with the 
sides perpendicular. In other areas, especially in the South, the 
width and depth of the ditches and the slope of the sides have to be 
modified to meet other soil conditions and problems incident to other 
types of marsh vegetation. In the Florida marshes ditches 20 to 30 
inches wide are most frequently dug. 
Various types of heavy machinery for digging and cleaning the 
ditches have been developed by workers in the North, and special 
types of ditching spades and other tools have been designed for use 
by hand labor. The type of marsh in the locality in which work 
is to be done should govern the selection of tools. Many, or perhaps 
most, of the southern marshes are not adapted to the use of such 
machine ditchers or special spades, at least of the types so far devel- 
oped, and the best tool generally for salt-marsh ditching in this 
region is the long-handled square-pointed shovel. For larger ditches 
or outlet canals a dragline or other standard machine of suitable 
capacity may be used to advantage. 
Another important method of treating certain classes of marsh is 
the installation of dikes and tide gates to prevent the entrance of 
high tides. The tide gates, opening at low tide, also provide for the 
run-off of rain water. Under special conditions the tide gates may be 
reversed to permit the entrance of high tides and to impound the 
water on the marsh. This is effective in reducing Aedes breeding, 
since it 1s the alternate drying and flooding of the marshes that brings 
about the hatching of their eggs. 
Experimental work by members of the Bureau of Entomology and 
Plant Quarantine in the vicinity of Savannah, Ga., has shown that 
the shutting off of the tides from marshes by means of dikes and tide 
