1044 BULLETIN OF THE BUREAU OF FISHERIES. 



III. The counter current system, in which the descending volume of water 

 is being checked by meeting a current opposing it at certain intervals. 



IV. The lock and gate system, in which a higher or lower level is reached 

 through one or more locks operated by gates. 



In all four systems of fishways certain general rules governing the construc- 

 tion must be observed. 



1 . The slope of a fishway built on the inclined plane system should not be 

 steeper than i foot vertical to lo feet horizontal; the pool and fall system, as 

 well as the counter current system, should not have a slope of more than i 

 vertical to 4 horizontal, so as to insure a current velocity of not exceeding 10 

 feet per second in any portion of the fishway. The lock and gate system deals 

 merely with a vertical lift. The width of a fishway somewhat governs the 

 slope, and the wider the fishway the more gradual the slope should be. 



2 . The available volume of water and the size of the fish must be considered 

 in the dimensions adopted for the fishway; small fish, like herring, bass, trout, 

 etc., may not require over 6 inches in the clear at the narrowest points or 

 openings in the fishway, while for large fish, like shad, rockfish, salmon, etc., 

 the clearance spaces should not be less than 9 inches in any direction. 



3. A fishway for small fish does not need to be more than 2 feet wide by 

 about I foot deep, while that for large fish ought to have a least width of 4 

 feet with a depth correspondingly large. 



4. Plenty of light should be admitted in a fishway, both for maintaining 

 therein the natural conditions of the water, and in order that the interior may 

 easily be inspected and any foreign matter removed. 



5. A fishway in all its parts should, by the action of the current of water 

 passing through it, be as nearly as possible self-cleaning of all sand, gravel, mud, 

 and rubbish. 



6. The water supply of a fishway should be ample and the same, or nearly so, 

 at both ordinary high and low water stages, avoiding thereby any regulating 

 gates or other devices calling for the services of an attendant. 



7. The top and sides of a fishway should be above ordinary high water. 



8. The fishway should be built very strong and be well protected against the 

 destructive effects of freshets, drift logs, ice, etc. 



9. The intake and outlet should be well submerged and the former pro- 

 tected against floating debris, etc., by a suitable grating. 



The location of a fishway must be such that ascending fish will not be 

 alarmed and driven off by disturbance from boats, fishermen, etc. 



The material of fishways may be wood, stone, concrete, or iron, depending 

 upon the construction of the dam, its size, the topography and nature of the 

 site, the labor and material at hand, and the funds available. 



