36 



MISCELLANEOUS PUBLICATION NO. 10 Go, U.S. DEPARTMENT OF AGRICULTURE 



watersheds ever reaches the ocean. Thus, total sedi- 

 ment production in the United States runs about 

 4 billion tons a year. 



Sediment production from representative basins 

 in the United States varies from 51 to 10,000 tons 

 per square mile (19). Sediment delivery varies 

 widely according to soils, geology, topography, 

 precipitation, vegetative cover, and applied soil 

 conservation practices. 



Measurements of soil erosion from field plots 

 leave no doubt that cultivated fields can be major 

 contributors to the sediment burden. Forested 

 cover provides excellent protection against sedi- 

 ment delivery, but burned out forest land can be a 

 tremendous contributor of sediment. Logging 

 roads also become sites of erosion and sediment de- 

 livery. 



Highway construction sites, barren roadbanks, 

 mining operations, construction sites for industrial 

 and suburban development, and eroding stream- 

 banks are also conspicuous sources of sediment. 



Sediment suspended in flowing or impounded 

 water may have a harmful effect on (a) water sup- 

 ply, (b) recreation, and (c) power generation. 



Potable water must be free of sediment. Many 

 industrial uses, for example, food processing, re- 

 quire sediment-free water. Sediment deposited in 

 condenser tubes used in industrial cooling may 

 cause costly incrustations. Cost of clarifying water 

 increases with degree of turbidity. High turbidity 

 of water adds costs through the need for greater 

 use of chemicals as flocculants, and more frequent 

 cleaning and disposition of silt from settling 

 basins. 



People like clean water for swimming and other 

 recreational activities. 



Fine suspended sediment has caused heavy losses 

 of commercial fish and shellfish yield from both in- 

 land and tidal waters (118) . 



Coarse sediment passing through power plants 

 has caused serious abrasion of turbine blades. 



Deposition of sediment in stream channels or 

 aggradation of flood-plain lands may impair 

 drainage and cause channels to overflow more fre- 

 quently. Floodflows carrying high sediment loads 

 inundate a much larger area than comparable flows 

 free of sediment (19). Floodborne sediment may 

 damage growing crops; and sands, gravel, and 



such coarse debris when deposited on fertile al- 

 luvial soils may reduce their productivity. 



Reduction of reservoir storage capacity is an- 

 other devastating consequence of sediment. About 

 1 million acre- feet of sediment is deposited in arti- 

 ficial reservoirs of the United States each year. 

 Loss of reservoir capacity to sediment has partic- 

 ular implications for programs of water resources 

 development, Indeed, prevention of such losses is 

 a primary justification for land treatment meas- 

 ures and watershed protection programs in many 

 upstream tributary areas of the country. 



Estuaries, ba} T s, and coastal harbors tend to be- 

 come vast sediment traps where continuous dredg- 

 ing and other operations are required for handling 

 sediment. Commingling of fresh sediment-laden 

 water and saline water, plus the influence of tides, 

 waves, currents, and shipping traffic, complicate 

 the depositional processes in such coastal areas, and 

 we should recognize that sediment is a major con- 

 taminant of these areas. 



We have become accustomed to thinking of the 

 consequences of sediment in engineering terms; 

 that is, as rates of reservoir silting, dredging of 

 harbors, or channel systems. But we are now be- 

 ginning to recognize other possible influences of 

 sediment on our environment, We are concerned, 

 for instance, about sediment as a carrier of pesti- 

 cide residues and the significance of phosphorus 

 adsorbed on sediment in the eutrophication of lakes 

 and estuaries. There are few complete and definite 

 answers to these questions. 



Studies in various parts of the country have 

 shown that erosion control practices in the forest, 

 on the range, and on the farm can and do reduce 

 the sediment delivery. The amount of sediment 

 coming from watersheds having good conservation 

 treatment at the Blacklands Experimental Water- 

 shed, near Riesel, Tex., is only 12 percent of that 

 from watersheds farmed without soil conserving 

 practices. Sediment yield from a 400-acre water- 

 shed receiving good conservation practices at the 

 Central Great Plains Experimental Watershed, 

 near Hastings, Xebr., was only 5 percent of that 

 from a comparable 400-acre watershed that did not 

 receive soil conservation treatment. Application of 

 streambank erosion control measures on the 145- 

 square-mile Buffalo Creek Watershed, near Buf- 

 falo, N.Y., reduced sediment delivery from that 

 watershed to Buffalo Harbor by 40 percent. The 



