306 Irrigation and Drainage 



DIVERTING WATEE BY TIDAL DAMMING 



Where lands bordering rivers leading to the sea lie high 

 enough above low tide to admit of adequate drainage, and at the 

 same time below high tide level, these may be dyked off from 

 the sea, .and then, by erecting sluices controlled by gates at 

 suitable places in the dykes, connecting with canals and dis- 

 tributaries on the land side, water may be led at will on or off 

 the fields as the tides come or go. One of the most notable 

 examples of this method of procuring water for irrigation is 

 at the mouth of the Santee river, in South Carolina, to which 

 reference has already been made, and a portion of which is 

 represented in Fig. 67. 



It will be readily understood that as the tide rises along the 

 coast, the discharge of the fresh water coming down the river is 

 prevented and the channels fill with it, it being held there by 

 the dam of salt water formed by the tidal wave. When the 

 fresh water has accumulated to a sufficient extent, the trunks 

 may be opened and the fields flooded, or they may be kept 

 closed and the water held off. The diverting of water from 

 rivers by tidal damming is only practicable where the river 

 carries a sufficient volume of fresh water to prevent the salt 

 water from ascending the channel, for were -the volume small 

 the sea would drive it back, and only salt or brackish water 

 would be found against the dykes. 



DIVERTING WATER BY THE POWER OF THE 

 STREAM 



Where rivers run too low in their channels to permit 

 the water being led out directly, many devices have been 

 employed by which a portion of the water is made to drive 

 machinery which, in turn, lifts 'another portion out upon the 

 land, where it may be led away. One of the oldest, commonest 

 and simplest devices used for this purpose is the undershot 

 water-wheel, set up in the stream and carrying buckets on its 



