434 Transactions of the 



seventy-two thousand acres. "When running only four feet full the dis- 

 charge would irrigate in the same way thirty-five thousand acres. 



I have described these works at some length, because they constitute 

 in some sort model works, and because they make by no means the most 

 favorable showing for irrigation enterprise. They have been costly for 

 their extent, owing largely to the fact that the bead-works (dam, etc.) 

 were constructed during the AVinter with the river running full; and 

 second, becauso the greater part of the canal, from the beginning to Cot- 

 tonwood Creek, passes through a hard pan cement rock. The total cost 

 of the works approximates to sixty thousand dollars, while for the same 

 character of works, constructing the head-works during the dry season 

 and with ordinary earthwork in place of bard-pan, forty thousand dollars 

 would suffice. The work on the extension of this canal beyond the Cot- 

 tonwood will be of this lighter character. 



Some idea of the part to be played by irrigation in the internal econ- 

 omy of the State may be formed by observing some of the results 

 rendered possible by this comparatively small work. Suppose the sev- 

 ent}--two thousand acres which it irrigates to be cultivated to wheat. 

 The result is a crop of one million eight hundred thousand centals, or 

 say-, ninety thousand tons; value at the railroad, which passes through 

 the land, two million five hundred thousand dollars It is not likely 

 that any crop less valuable than wheat will be raised. Five thousand 

 acres planted to alfalfa would more than maintain all the dairies in 

 Marin County. If it shall prove that the land is suited for cotton, a 

 crop to the value of five million seven hundred and sixty thousand 

 dollars should be annually taken from this area. And yet the dimen- 

 sions of this irrigation system, in comparison with the works in pro- 

 gress elsewhere, and these in comparison with the comprehensive 

 works projected, are very small. The imagination refuses to follow the 

 effects of general irrigation as displayed by the experience already had, 

 to their legitimate consequences. 



The dimensions that ought to be given to a canal, in order that it may 

 irrigate a particular area, or the area that may be irrigated from a 

 canal of particular dimensions, may be deduced from the following 

 data: the "duty" of one cubic foot of water per second flowing 

 through a canal is to irrigate two hundred acres of land, giving to the 

 same, during the season, a depth of eleven and one half inches, or one 

 thousand five hundred cubic yards of water per acre. In many places, 

 if not usually, one half this amount is expected to suffice; and, in that 

 case, the "duty" of one cubic foot per second would be four hundred 

 acres. 



The highest velocity permissible where the canal banks are of loose 

 earth, as in the California canals, is two feet per second (say one and 

 one third miles per hour). In a canal of favorable section — flowing say 

 five feet of water on a twenty-foot base, and with side slopes of two to 

 one — the rates of flow would be approximately as follows: on a fall of 

 nine inches to the mile, two feet per second; on a fall of six inches to 

 the mile, one and three quarters feet per second; on a fall of three 

 inches to the mile, one and one quarter feet per second. 



If the water be shallower, the width less, or the, slope of the banks 

 flatter, the rate of flow, at the above rates of fall", 'would be somewhat 

 less; but the above figures may be taken in order to get at approxima- 

 tions to the practical work that may be expected of a canal of given 

 dimensions. 



