DRAINAGE MEASURES. 45 



Francisco on the north to San Mateo on the south, a distance of 

 about 10 miles. The salt-marsh area included consisted of a narrow 

 strip along the San Francisco Bay shore, varying from one-half to 2 

 miles in width and 10 miles long. No part of the area was continually 

 covered with water, and it is all above the lowest high tide. The 

 higher tides, however, particularly those accompanying full moon, 

 almost completely submerge the area. The operations, as described,*^ 

 are quoted as follows: 



What was done on the marsh. — The actual work of control was commenced February 

 27, when a gang of men was started to work at ditching on the salt marsh. This work 

 was started near the Blackhawk dairy, where the marshes begin north of Burlingame, 

 it being the intention to work northward toward San Bruno, and make the work 

 permanent as far as we would be able to go in a single season. However, the work 

 went rapidly and the troublesome areas north of Millbrae were not so numerous as was 

 figured, and consequently practically the whole area was covered during the past season . 



The ditching in the Blackhawk area consisted in connecting the pools and areas of 

 standing water with the tidal creeks in order that they might drain more rapidly and 

 before a brood of mosquitoes would have time to develop. The largest of these ditches 

 were 12 inches wide and about 15 inches deep, and these served as main channels into 

 which smaller laterals were cut. These laterals, and, indeed, the greater part of all of 

 the ditches, were but one spade wide, and one or two spades deep, according to the 

 depth of the pool to be drained. Only where the pools were very large and a great 

 quantity of water to run off in a short time was it necessary to make larger ditches. 

 By "a spade "here is meant the common California spade, which is about 6 inches wide 

 and 10 inches high. The eastern drain spade has not yet found its way to California; 

 undoubtedly it would be preferable for the deeper ditches in this kind of work. In 

 addition to the well-defined pools, there was a considerable area in the Blackhawk 

 region which was covered with but a few inches of water for a considerable time after 

 each high tide, and before the rains ceased in the spring water stood over this area 

 almost continuously. Such areas had to be treated by making a number of parallel 

 ditches from 50 to 75 feet apart, in order to permit of sufficiently rapid drainage. 

 Rather extensive ditching was done here to make the area safe while the rains were 

 still continuing, while later in the season, when the rains ceased, it would have been 

 safe with much less ditching. Small pools that were far from tidal creeks were made 

 safe by filling in rather than draining. The size of the pool, and the length of ditch 

 necessary to drain, will determine which of the methods is to be followed. In this way 

 the marsh area was gone over, doing away with all the places where larvae were found 

 or were likely to be found for a distance of about a mile along the bay northward, 

 where the diked area was met with. 



This part of the marsh presented a more difficult problem. The dike, having been 

 neglected for ten or twelve years, was in poor condition, and there were several breaks 

 in the upper end near Millbrae. The gates were not in working order, and their floors 

 were too high to drain the area enclosed. 



The breaks in the dike at the upper end permitted the water to back up at the op- 

 posite side, and this, together with the fresh water from the hills, kept the water level, 

 at almost high tide, over a large part of the area. To make matters worse, the dike, 

 just after it was built, was in effective operation just long enough to thoroughly dry the 

 ground and cause it to crack. These cracks, which are 4 or 5 inches wide and 2 or 3 

 feet deep, still exist, forming a complete network over most of the area. Mosquitoes 

 were found breeding in this area, and it was next to impossible to get over the ground, 



oBul. 178, Univ. of Cal. Exp. Sta., pp. 15-21, 1906. 



