FORESTS 



Hence, with the exception of 

 fragile or very steep lands, our pres- 

 ent levels of knowledge and tech- 

 nology are generally adequate to 

 minimize these sources of disturbance 

 or reduce their consequences. This is 

 true even though many elements — 

 including lack of exact prescriptions, 

 increased costs, momentum of exist- 

 ing systems, and unawareness of 

 long-run damages — may cause ac- 

 tual practice to lag well behind the 

 prospects demonstrated by research. 



Needed Scientific Activity 



As the foregoing indicates, a sub- 

 stantial body of knowledge and 

 application has been accumulated 

 through "watershed" or "watershed 

 management" research on forest 

 areas. Extension of research results 

 and at least qualitative predictions 

 to similar landscapes can be made 

 with some confidence. Greater cer- 

 tainty, exactness, and extent of pre- 

 dictions are possible simply through 

 increased funding of existing re- 

 search installations. Predictive mod- 

 els and simulation relating streamflow 

 to physical variables and precipitation 

 are being explored by hydrologists. 

 Success would bring extension to 

 forest watersheds for which numer- 

 ous data are available, and might 

 call for new modes of examining 

 factors controlling surface soil loss, 

 bank erosion, or other sources of 

 turbidity. 



Nevertheless, even within current 

 concepts, there are enormous gaps 

 in our knowledge of watersheds. 

 Many large areas are poorly known 

 in terms of exact climatic data, soil 

 units, and the hydrologic behavior 

 or response of watersheds to treat- 

 ment. In some instances, the simple 

 conceptual models derived from study 

 of soil in the laboratory or agricul- 

 tural field bear little resemblance to 

 the behavior of wildland soils, es- 

 pecially those on very steep slopes. 

 Much greater efforts at watershed 

 characterization and in study of the 

 actual functioning of small soil- 



geomorphic "systems" under field 

 conditions are badly needed. Such 

 work is not entirely lacking (see 

 Figure VII— 7), but the investigators 

 so employed are few and the number 

 of mixed-discipline investigative 

 teams far fewer, especially in the 

 light of the large areas involved. 



Three examples illustrate such 

 needs : 



1. Only within the last decade 

 has it been recognized that fire 

 on the steep California brush- 

 lands not only destroys the 

 protective cover of vegetation 

 and litter but also imparts a 

 non-wettable quality to the soil 

 itself, apparently through con- 

 densation of heat-volatilized 

 substances from the litter. The 

 result is reduced entry of rain- 

 fall, increased surface flow, and 

 erosion. This complexity has 



required new research 

 proaches, and calls for revision 

 of existing notions of infiltra- 

 tion in both burned and pro- 

 tected soils. 



2. Hewlett's variable source area 

 concept of water outflow, al- 

 luded to earlier, is still novel 

 and its consequences for water 

 quality are only now being ex- 

 plored. In certain landscapes 

 it seems to provide a mechan- 

 ism for direct overland trans- 

 port of surface materials to 

 streams without passing 

 through the soil filter, a pos- 

 sibility usually overlooked. 



3. Again, assessments of land- 

 scape stability, normal sedi- 

 ment loads, and tolerance of 

 man-made disturbance are com- 

 monly based on short time 

 periods and assumptions of 



Figure VII-7 — EFFECT OF LAND USE ON SEDIMENT 

 YIELD AND CHANNEL STABILITY 



Land use 



A. Natural forest or grassland. 



B. Heavily grazed areas. 



C. Cropping 



D. Retirement of land from 



cropping. 



E. Urban construction. 



F. Stabilization 



G. Stable urban 



Sediment yield Channel stability 



Low Relatively stable with 



some bank erosion. 



Low to moderate Somewhat less stable 



than A. 



Moderate to heavy _ Some aggradation and 

 increased bank 

 erosion. 



Low to moderate Increasing stability. 



Very heavy Rapid aggradation and 



some bank erosion. 



Moderate Degradation and 



severe bank erosion. 



Low to moderate Relatively stable. 



The table shows various land uses and their effect on the relative sediment yield 

 from the surrounding landscape as well as on the stability of stream channels. The 

 most severe sediment problems occur during urban construction, when covering 

 vegetation is removed and the flow regime in channels is changed by realignments, 

 increases or decreases in the flow, or obstructions placed in or alongside the natural 

 flowway. 



211 



