8 COMMISSION OF CONSERVATION 



"The underlying principle in the construction of fishways is the 

 retardation of the current velocity of a waterfall so as to enable fish 

 to surmount it. Innumerable devices with that end in view have been 

 invented and proved more or less successful. Certain physical con- 

 ditions in the location and a proper method of construction are the 

 important factors. 



"Of the physical conditions, the two principal ones are (1) acces- 

 sibility of the fishway free from disturbance, its outlet being located in 

 a pool at the bottom of the falls where fish would naturally pass in 

 ascending the river, and (2) an abundant discharge of water through 

 the outlet so as to attract the fish. It is to be noted that fish as a rule 

 do not ascend rivers at low-water stage, but between mean and high 

 water, and preferably during sunshine and warm weather. 



" In style of construction fishways may be classed in four systems : 



"I. The inclined plane system, in which a series of baffle or 

 deflecting plates are so arranged in an inclined flume as to cause the 

 water to follow in its descent a long sinuous route. 



"II. The pool and fall or step system, in which the water is 

 brought down to a lower level by a series of short falls with intervening 

 pools. 



"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 construction must be observed. 



"1. The slope of a fishway built on the inclined plane system 

 should not be steeper than 1 foot vertical to 10 feet horizontal; the 

 pool and fall system, as well as the counter current system, should not 

 have a slope of more than 1 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 1 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 



