CALIFORNIAN SALT-MAESHES 395 



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 liigh. 

 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. Eather 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 opposite 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, even with waders. A con- 

 siderable part of the area was submerged to the depth of a foot or more, thus 

 concealing from view the cracks and tidal creeks, which one was likely to fall 

 into at any step, and which made any attempt at rapid progress somewhat dis- 

 couraging. 



" It was at once evident, under these conditions, that if the area was to be con- 

 trolled, the dike must be either cut through in a number of places in order to 

 allow a freer circulation of water, or the breaks must be repaired and the gates 

 put in operation, and the water kept out. The latter scheme was the one fol- 

 lowed, because it would be possible to make the area thoroughly dry, and thus 

 the results would be more certain. In attempting to operate the gates we were 

 made to appreciate the effect of a ten or twelve years' coating of rust on the 

 large screws by which the gates were manipulated. After the gates were put in 

 operation the breaks in the dike were repaired and the weak places strengthened. 

 The largest break repaired was immediately joining the upper gate. This was 

 30 feet wide, and by the action of the water had worn down so that at high tide 

 there was a depth of 10 feet of water. A double wall of sheet piling about 6 feet 

 apart was sunk here and the space between filled in with earth. The other breaks 

 were repaired by sinking a single wall of sheet piling in the center and filling in 

 on both sides with dirt. 



