THE IRRIGA 7 IOX A GE. 313 



the height of the dam and the number of acres it will cover at that 

 height You will find few basins stand the test Most of them will 

 be too narrow or too sloping or more often both, while those with a 

 large wide bag back of a narrow gorge suitable for a safe and cheap 

 dam are very rare. 



Most of those we find suitable have a habit of having little water- 

 shed back of them, while those that have a good watershed are too 

 often very wide-mouthed. If the watershed is good enough to insure 

 filling the reservoir in ordinary years, then it is quite certain to bring 

 in considerable sediment in wet years. I have seen several complete- 

 ly filled in this way in one wet winter. It is possible to dredge them 

 out, but you had better let the other man undertake the task. It is a 

 nice amusement for a tenderfoot. 



It looks like a simple matter to build a dam fourteen feet high in 

 a narrow gorge. But when you have gone to a safe foundation your 

 dam is more likely to be twenty feet high. And the gorge that to the 

 eye seems so narrow may be fearfully wide when measured by your 

 purse. If your watershed is reliable for medium years there is certain 

 to be a large overflow in very wet winters, and to provide against 

 that with certainty is no trifling matter. It seems very easy to run a 

 pipe or a box through a dam of earth, but it is still more easy for the 

 water to cut it out. You have seen, on hillsides, sheets of shelving 

 rock covered with soil that was formed from its decomposition, and 

 about as close to it as you are likely to pa< k anything but the best 

 clay. Yet you have seen water follow the seam between the soil and 

 the rock for many yards under almost no pressure. So it will do with 

 any seam between earth and any hard material. When it once gets 

 through under pressure the fate of your dam is sealed. Many other 

 precautions make the building of a safe dam on a reliable watershed 

 so expensive that even where the proper basin exists you are paying 

 too much for the amount of water you get and will do better to resort 

 to some other means. 



A small reservoir, to be filled by a flowing stream or wells, is 

 quite another matter, especially where the stream does not flow 

 through the basin but is diverted into it. Where reasonable precau- 

 tions are taken these are nearly always well worth what they cost. for 

 there is no large overflow to provide against, and they can be filled 

 more than once a year. 



In many cases they are a necessity, as where the water is cold. 

 The temperature of irrigating water is of great importance, 3*et it is 

 constantly overlooked by novices in irrigation. For many things 

 every degree that you can add to its temperature will mean dollars in 

 the results. To be useful it must be warm, for cold water will check 

 many kinds of growth, so as to make a practical failure of the crop. 

 I have seen corn completely ruined by water from mountain streams, 

 sickly and not yet in ear in August, yet the owner did not seem to 

 



