60 



form flow of water, gradual ascent and absence of high barriers; 

 (2) a minimum sacrifice of water in the interest of the dam 

 owners; (3) an entrance into which the fish are readily directed ; 

 (4) a firm, solid construction, resistant to freshets, or one which 

 may readily be removed when not in use. 



Existing Types. There are three kinds of fish ways in use on 

 the alewife streams at present, each of which has several modi- 

 fications : 



(1) Natural Stream. The most successful type is not, strictly 

 speaking, a true fishway, but merely a small side stream of 

 gradual slope, connecting the pond with the main stream below. 

 Projecting rocks, or even concrete bars (Fig. 11), help to check 

 the current, and afford resting places for the alewives in their 

 forward progress. This type is the most easily surmounted of 

 all, but has the drawbacks of excessive waste of water, impossi- 

 bility of installation in many locations, and difficulty in directing 

 the fish into its entrance, since alewives follow the greatest flow 

 of water, and therefore tend to pass up the main stream to 

 pocket at the foot of the dam unless directed by a screen or 

 barrier into the smaller stream. 



(2) Pool. This form consists of a series of pools with a 

 1-foot drop in the level between each pool. It is usually 

 of wood, concrete or stone construction, the number and size 

 of the pools varying with the height of the dam. The old 

 Lawrence fishway on the Merrimack River was of this type. 

 Wherever this type of fishway is installed, its size, shape and 

 length must be altered to correspond to the physical require- 

 ments of the dam. 



(3) Inclined Plane. Various types of inclined plane fish- 

 ways are in use, the most important being the Brackett. 

 These fishways consist of a long, narrow box inclined at an 

 angle equivalent to a 1-foot rise for every 10 feet, with vari- 

 ous arrangements to check the flow of water and to afford rest- 

 ing pockets for the alewives. This type is the most popular 

 form on our alewife streams, and seems especially suited for 

 small dams, owing to the convenience of installation and its 

 cheapness. Though occasionally of cement, as on Red Brook, 

 it is usually of wooden construction. The most primitive 

 yet effective type is made by placing 1-foot cross boards per- 



