1110 EEPOET OF COMMISSIONER OF FISH AND FISHERIES. [IQ] 



inundations and consequent injuries to the dike by weirs or ditches for 

 carrying off the superfluous water. 



The weirs (Plate V, Figs. 14, 16), are generally constructed at one 

 end of the dike where, owing to the less depth of water, they are not 

 exposed to any high pressure. According to the size of the pond and 

 the probable quantity of superfluous water, the breadth of these weirs 

 varies from 3 to 10 meters or more. The edge of the weir should, ac- 

 cording to circumstances, lie ^ to 1 meter lower than the crest of the 

 dike. The scarps, which in that part of tlie dike where the weir is 

 constructed should not be near as steep as in the rest of the dike, meet 

 in the edge of the weir in the shape of a roof, and are covered with 

 thick boards closely joined together. The side walls of the weir must 

 also be supported by firm buttresses. If the top of the dike is to be 

 used as a road, a bridge should be thrown across the weir. 



To prevent fish from being carried over the weir in high water a grat- 

 ing is placed upon it, or in frontof it(PlateV, Fig. 14(%), generally of wood 

 or galvanized iron (Plate V, Fig. 16), which in l)road weirs runs par- 

 allel with the edge of the weir, but in narrow weirs is generally x>laced 

 at a sharp angle against the current (Plate Y, Fig. 14 h) so as to give 

 more openings for the water to flow out, and to prevent leaves, &c., 

 from stopping up the passage. If this nevertheless should happen, a 

 floating beam (Plate V, Fig. 14 c) or a strong board is fastened in front of 

 the grate obliquely against the current. Such a beam, or board, will 

 stop all objects floating on the surface, and drive them towards the 

 shore, where they will accumulate and whence they can easily be re- 

 moved. 



Wherever rivers or large brooks flow through ponds, it will be well 

 to construct weirs in them before they enter the pond, so that the high 

 water and the mud, &c., which it is apt to carry with it, do not enter 

 the pond at all. To prevent the fish from escaping from the pond into 

 the river or "brook, its mouth should be closed by a grating or by a brush 

 weir. Brush-weirs are made of fascines of thin brushwood measuring 

 from IJ to 2 meters in length, which are laid in the direction of the cur- 

 rent, and must protrude about one -half meter above the highest water- 

 mark. It will also be well, in order to prevent any stoppages, to place 

 a floating beam in an oblique direction in front of the brush weir. 



Ditches for carrying off the superfluous water are particularly re- 

 quired in ponds located in glens or valleys, so as to catch the snow and 

 rain water flowing down from the heights, which generally carries with 

 it sand, gravel, &c., and would soon fill the pond with its accumulations. 

 Such ditches should, therefore, as a general rule be constructed paral- 

 lel to the edges of the valley and of the pond, and should be broad and 

 deep enough to contain and carry oft" all the superfluous water which 

 may reasonably be looked for. Such ditches may also be required for 

 catching and carrying off injurious water from places where flax is 

 rotted, or from tanneries, dye establishments, and other factories. 



