GATE STRUCTURES FOR IRRIGATION CANALS. 11 



reaches the end of the lined section, sometimes as far as 100 feet 

 below a gate. 



Canals near mountain streams having a bottom paving of bowlders 

 may be protected by the liberal use of these bowlders laid as simple 

 riprap or formed into rubble concrete paving by the use of cement 

 mortar. A rough surface, gradually becoming about as smooth as 

 the earth channel below it, will aid the water to secure its " balance " 

 and to pass quietly into the unlined section. 



In parts of the West far removed from bowlders in quantity but 

 near sage-covered lands, this protection is secured by the use of 

 bundles of sagebrush securely wired to posts driven deep into the 

 canal bottom, while the California Development Co., in the Im- 

 perial Valley, has made a remarkably efficient riprap of grease- 

 wood. Where greasewood is scarce this company uses arrow weed 

 or young willows, but these are not as durable as the greasewood. 

 A structure protected in this manner is shown in Plate III, figure 1. 

 The general procedure is as follows: The erosion of the bank below 

 the gate is allowed to proceed, but carefully watched by the ditch 

 tenders until the extent of the erosion up and down the canal and 

 the depth have been determined. As a rule, the erosion is quite rapid 

 until a certain shape is washed out, when it stops so long as condi- 

 tions remain the same. The sides and bottom of the hole are then 

 trimmed neatly, the former vertical in some cases, but as a rule on a 

 slope of about \ to 1. Vertical posts or light piles then are driven 

 2 to 5 feet apart and about 6 inches back from the proposed water 

 edge of the channel. Sometimes a few posts are driven near the 

 bank side of the hole. The brush then is cut into uniform lengths 

 and bound with wire into bundles about 6 inches in diameter. These 

 are packed into the trimmed holes in layers, with their butt ends 

 forming an even wall in the channel. Each layer of brush is brought 

 to a smooth surface by puddling in wet earth, and is securely fastened 

 by wires to the posts or piles previously driven, the butt ends extend- 

 ing about 6 inches into the channel beyond the line of posts. One 

 layer after another is thus placed until the level of the protection 

 is well above that of high water in the canal. In a heavily silted 

 water this mass of brush rapidly gathers a coating of waxy mud 

 within the interstices at the water ends of the bundles and in this 

 condition lasts for years, some in this locality being five or six years 

 old and still in good condition. 



GATE STRUCTURES IN OPEN CHANNELS. 



By far the most important, and in many cases the only structure 

 on a ditch, is the gate placed near the point where water is diverted 

 from the stream. In nearly all of the Western States such a gate is 

 required by law. 



