SOIL EROSION HALL 309 



tread bare tracks along which streams will begin to flow during the 

 rains and to acquire sufficient velocity to eat into the soil. In how- 

 ever small a way such gullying starts, if it is not checked it extends 

 itself from season to season, both fanwise and by eating uphill, until 

 eventually, by a sort of geometrical progression, the large area may 

 become impassable to man and beast. Though agriculture generally 

 is indicted as the initiator of erosion, more properly it is the reckless- 

 ness or the want of foresight in man that should be blamed. 



In a recent book, Rich Land, Poor Land, 2 there is an account of a 

 gully with a history: 



The land fell away almost sheer for 200 feet. We stood over one of the gully's 

 arms and far down caught a glimse of the central basin. The guide took up the 

 tale. "Do you know what started him? A trickle of water running off a farmer's 

 barn about 40 years ago. Just one damn little trickle, and now a third of the 

 county's gone — 40,000 acres." 



One should realize that in certain regions and climates the soil is 

 so lacking in any binding power save its skin of vegetation that all 

 handling of the soil has to be watched with the same sort of com- 

 munal care that is accorded to fire risks in a crowded city. 



Nor is agriculture the only culprit in initiating the destruction of 

 the vegetation. At a place called Ducktown in Tennessee, a gullied 

 area has been created through the destruction of the vegetation by 

 sulphur dioxide from a copper smelter. "In a great circle about the 

 smelter, measuring perhaps 10 miles in diameter, every living thing 

 had been destroyed by the sulphur fumes." 



The loss of land by soil erosion is not the only loss the community 

 suffers. The soil that the rain starts moving reaches the rivers and 

 has to find a resting place somewhere. It is in regions liable to 

 erosion that irrigation becomes of so much importance, and dams are 

 thrown across the rivers to store the floodwater of the rains. But no 

 one had foreseen what masses of sand and silt would begin to choke 

 the reservoirs when erosion was going on at the headwaters. I quote 

 from the same book, Rich Land, Poor Land: 



Elephant Butte Dam was built by the United States to guarantee New Mexico's 

 quota of water "forever." "Forever" will end in a few years if the Rio Puerco, 

 which chiefly ravishes the Rio Grande, is not controlled. Every flood season it 

 pours 9,000 acre-feet of silt into Elephant Lake. 



Even if there are no dams, the eroded material is deposited on the 

 river flood meadows of the lower valley, a fertilizing deposit as long 

 as the topsoil is coming down, but as gullying proceeds the river is 

 laden with larger quantities of the sterile subsoil material. The bed 

 of the river steadily rises through accretions of silt, and the flood 

 banks, like the levees of the Mississippi, have to be raised to keep the 



3 Chase, Stuart, Rich land, poor land, McGraw Hill Publishing Co., New York, 1936. 



