274 ECONOMIC MAMMALOGY 



other vegetation found near the water, and in winter chiefly the bark 

 and twigs of shrubs and trees. In most localities small, softwood trees 

 of little value, such as aspens, cottonwoods and willows, are preferred. 

 Orchard and other valuable trees may be protected from them by 

 woven-wire fences and cylinders. 19 Where other trees are scarce they 

 sometimes cut oaks, 20 conifers and other trees not usually used by 

 them, but damage done by cutting valuable timber is seldom serious. In 

 Norway, beavers live largely on aspen, birch, oak and alder. 21 



Sometimes damage results from the construction of beaver dams, 

 which cause the overflow of fields, pastures, forests and roads. It is 

 said that this may be prevented and the water in their ponds kept at a 

 nearly uniform level by the skillful insertion of iron pipe in the dams. 22 

 Where this is impracticable or ineffective, the beaver may easily be 

 controlled by trapping under permits from the proper authorities. 

 Beavers also sometimes cause breaks in irrigation canals, 23 



Though the flooding of lands because of beaver dams is sometimes 

 detrimental to human interests, it is also sometimes beneficial. Beaver 

 ponds help equalize the flow of storm waters and water from melting 

 snow, and thus minimize the damage from floods in small streams, 

 storing the water and thus keeping up the flow of trout streams and 

 streams upon which the ranchers are dependent for water for their 

 stock, during dry seasons. 24 Occasionally the ponds make it possible to 

 obtain irrigation water needed to save crops from ruin. "Colorado 

 ranchers recently opened beaver dams in the mountains, pouring mil- 

 lions of gallons of water down stream beds and out through irrigation 

 ditches. On one stream alone crops valued at $15,000 were thus 

 saved." 25 



There has been considerable controversy over the possible effects 

 of beaver ponds on fishes, 26 Such ponds afford quiet water favorable to 



19 Bailey, N. Amer. Fauna, No. 4, p. 108, 1926; U. S. Dcpt. Agric., Technical Bull., 

 No. 21, 1927. 



20 Warren, Cutting of oaks by beavers, Jonrn. Mammalogy, ix, 253, 1928. 



21 Salvesen, The beaver in Norway, Journ. Mammalogy, ix, 99-104, 1928. 



22 Bailey, Beaver habits, beaver control and possibilities of beaver farming, U. S. 

 Dept. Agric., Bull. No. 1078, pp. 11-12, 1922; Tech. Bull., No. 21, pp. 13-14, 1927. 



23 Dixon, Rodents and reclamation in the Imperial Valley, Journ. Mammalogy, 

 in, 136-146, 1922. 



24 Dearborn, The maintenance of the fur supply, U. S. Biol. Surv. Circular No. 

 135, p. 10, 1920. 



25 American forests and forest life, xxxiv, 3/0, 1928. Honks, When beavers aid 

 irrigation; emergency use of the water impounded by the industrious animal, Sci- 

 entific American, March, 1924, p. 161. 



26 For example : Bean, Fish planting in public water, New York State Conserv. 

 Comni., pp. 1-24, 1916. Embody, A study of fish-producing waters of Tompkins 



