PART VII — WATER RESOURCES, FORESTRY, AND AGRICULTURE 



Figure VII— 6 — RELATION OF SEDIMENT PARTICLE- 

 SIZE TO FLOW RATE 



hon 



"i r 



10 



i r 



PERCENT 

 J I L_ 



10 20 



SAND 



30 



40 



50 60 

 SILT 



70 



20 



30 

 CLAY 



40 



The graph shows lines of best fit for measurements of sediment particle-size dis- 

 tribution made from 1961 to 1964 in the Scott Run basin, Fairfax County, Virginia. 

 There appears to be no change in particle-size distribution with time. The low-flow 

 regimes show high concentrations of silt and clay. As the flow decreases and the 

 speed slows, the silt particles — being heavier — drop to the stream bed, leaving the 

 fine clay particles to become the greater portion of the load. As the flow increases, 

 there is an increasing concentration of the larger, sandy particles. 



hydrological behavior and disturb- 

 ance of vegetation on the other. (See 

 Figure VII-6) 



On the majority of forest water- 

 sheds, the principal cause of erosion 

 and stream turbidity outside of flood 

 periods is exposure or disturbance 

 of the mineral soil surface. This may 

 come about through any of a number 

 of causes — excessive grazing, tram- 

 pling by livestock or humans in large 

 numbers, roads and skid-trail con- 

 struction, and, as mentioned, some- 

 times after severe fire. 



Current overgrazing and the legacy 

 from even more severe overgrazing 



in the past poses severe problems in 

 some low-rainfall forest areas of 

 western United States. Reducing fur- 

 ther damage by livestock, and occa- 

 sionally by big game, is more of a 

 political-economic problem than one 

 of technical know-how. Repair of 

 past damage, however, is handi- 

 capped by the large area and low 

 values of affected lands, the slow 

 pace of natural recovery, and limited 

 funds for both research and applica- 

 tion of known principles. 



Increasing recreational uses — in- 

 cluding human traffic on trails and 

 campgrounds, development of roads, 



ski runs, facilities, and now the 

 large numbers of off-the-road ve- 

 hicles — create an array of new prob- 

 lems for forest land management. 

 Obviously, hazard to water quality 

 is only one of these, though often 

 significant. Less obviously, new kinds 

 of use conflicts are being generated, 

 and research in behavior and values 

 is likely to be as important in ad- 

 dressing these as is that in economics 

 and watershed management. 



Contrary to popular belief, the 

 mere cutting of trees, even com- 

 pletely and over large areas, seldom 

 leads to any surface erosion, espe- 

 cially if regrowth occurs promptly. 

 The critical factor determining 

 whether logging operations will or 

 will not influence stream turbidity 

 is how the felling, skidding, and 

 hauling are conducted. There is now 

 a substantial body of research and 

 experience in several forest regions 

 demonstrating that the mechanical 

 operations and necessary road con- 

 struction can be carried on with 

 minor or no impact on watershed 

 values and stream turbidity. 



Several essential principles of road 

 design, construction, and mainte- 

 nance, as well as for protection of 

 stream channels, have emerged that 

 minimize soil exposure and arrest 

 sediment transport. These principles 

 are readily translated into practice 

 in many landscapes, though the op- 

 erational details and controls are 

 known for only a few. In some steep 

 mountains or slide-prone areas, how- 

 ever, geological structure and topog- 

 raphy impose unforeseen hazards and 

 extremely high costs. Greater avail- 

 ability of soil and geotechnical in- 

 formation might reduce both, though 

 the resources for providing informa- 

 tion to large wildland areas are 

 meager. In any case, cost factors as 

 well as watershed considerations have 

 dictated new attention to harvesting 

 and transport systems, including the 

 long-used aerial cable methods and 

 feasibility tests with balloon and heli- 

 copter logging. 



210 



