BULLETIN OF THE UNITED STATES FISH COMMISSION. 35 



Dams tlirowu up here are safe from overflow, and water can be let in 

 from the race and the supply governed with precision. Both in this and 

 the preceding class, the dams need not generally be very high, since they 

 receive no freshets. 



V. Below mill-dams. — I have frequently observed level tracts of 

 laud on which good fish-ponds could be made. Generally, in such 

 places, it will be necessary to run a dam parallel with tTie creek as 

 far down as the pond is to extend, and then turn at right angles to 

 the hillside with another section of a dam, as in Class III. Dams of 

 this class need not be very high, and the water supply can be taken 

 through the dam of the mill-pond above. In the three classes last 

 named, the area of land covered by the water will be alluvial as a rule, 

 and suited exactly to the requirements of the fish. The advantage of 

 requiring moderately low dams is a great item, because it is the vertical 

 height of water that causes the majority of dams to break. As stated 

 before, they will not overflow, and the amount of water received into 

 them can be regulated and strained as it goes in and wastes out. 



]Most persons have a leading idea that all ponds must be made by 

 throwing dams across streams. This is a great mistake, for many large 

 ponds can be made on the three plans last named. 



But the value of such ponds as are made by the side of streams, be- 

 low canals and mill-dams, is apparent for other reasons than those just 

 mentioned. The land covered by these being naturally dry beforehand, 

 gives a firmer bottom to walk upon when the fish are being picked up, 

 and in the course of three or four years, when a quantity of soft mud 

 accumulates, destroying the productiveness of the pond, the water may 

 be turned out, when the mud will dry enough to produce a crop of rice, 

 German millet, or corn. One crop made on this soil will reconvert its 

 lat nt properties into fish-producing substances, and render it as valu- 

 able as in the commencement. 



It is important to have the drainage of the pond very deep. The 

 draw-gate should be below the bottom of the pond proper, allowing, when 

 desired, complete drying of the soil, which will then produce crops from 

 the rich mud soil. With two feet fall a ditch may be cut from the upper 

 end of the drainage box, and allow all the water to leach from the soil. 



Being beyond the possibility of overflow, the dam need not reach more 

 than 8 or 10 inches above the surface of the water. A dam which rises 

 but slightly above the jiond surface is less liable to attacks from musk- 

 rats, for although these animals penetrate the face of dams beneath the 

 water-level, they incline the passages ujiward, and enlarge them in the 

 dam above the water as it stands in the pond. Besides, dams look bet- 

 ter when built but a few inches above the water, though they can never 

 be safe unless the inflow is controlled. Another economic point lies in 

 the fact that you avoid large wasteways and extensive and costly screens. 



Construction of ponds. — Many persons who depend upon a limited 

 amount of water for a supply fail through poorly constructed dams to 



