A Forest Management Nonpoint Source Risk Assessment 



increase of 8 percent. The Forest Supervisor 

 immediately imposed a 10-year timber harvest 

 moratorium on Forest Service lands. Industry 

 has cooperated, and there has been little activity 

 in the watershed during the past three years. 



A QjMULATivE Effec ts Risk In tdex Value 



An undisturbed watershed has a zero risk 

 index value. The risk index value for a managed 

 watershed depends on the areal extent of activi- 

 ties, the age of the treatments, and the type of 

 terrain on which they are located. The calcula- 

 tion of a cumulative effects risk index value is 

 straightforward and is outiined in the following 

 eight steps: 



Step 1: Determine the year/type/acreage of 

 past (or planned) forest management 

 activities. Permanent road disturbance 

 is calculated at 0.87 ha/km; temporary 

 road disturbance is calculated at.37 ha/ 

 km. Digitize this information for use in 

 the GIS if it has not already been done. 



Step 2: Determine the erosion classification 

 of the terrain on which the activity has 

 taken (or will take) place. This is gen- 

 erated by the GIS. 



Step 3: Determine slope category on which 

 the activity has taken (or will take) 

 place using GIS. 



Step 4: Obtain a risk value for each activity 

 from the Erosion-Impact Matrix that 

 has been developed. 



Step 5: Determine the areal percentage of 

 the watershed on which the activity 

 took place. 



Step 6: Use a recovery coefficient (RC — a 

 value between 1 and 0) to reduce the 

 risk associated with older activities. In 

 this application, we assumed a linear 

 10-year risk recovery for all activities 



except roads, which always maintain a 

 RC of 1. Thus, current year activities 

 have a RC of 1 , four- year-old activities 

 have a RC of 0.6, and so on. 



Step 7: Multiply the risk value times the 

 percentage of the watershed times the 

 RC to obtain the total risk for that 

 activity. 



Step 8: Sum all past and planned land-use 

 totals. 



Table I-l contains a summary of distur- 

 bance information, by year, for Howard Creek. 

 Harvest, site preparation and road data were 

 obtained directly from the three land owners in 

 the watershed. To avoid double accounting, 

 only the last entry onto a site was counted. 

 Usually this was for site preparation, which was 

 almost always dozer piUng of slash. If no site 

 preparation had been completed, then harvest- 

 ing was the only disturbance counted, and this 

 was almost always partial cutting with tractor 

 skidding. 



Note that in 1990, the cumulative risk index 

 value is 0.2284, but if the risk index value had 

 been calculated in 1986, the year the timber 

 harvest moratorium was called, it would have 

 been 0.3998. The differences between the 1990 

 and 1986 weighted risk columns in Table I-l 

 are due only to the effect of the recovery coef- 

 ficients. 



Thresholds? 



How much disturbance a watershed can 

 tolerate before reaching a level that produces 

 significant environmental damage is both tech- 

 nically and politically difficult to answer. In the 

 Forest Service's Region 5, a watershed's 

 "Threshold for Concern" is typically an areal 

 disturbance (actually, equivalent road acres) of 

 between 10 percent and 20 percent depending 



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Flathead Basin Cooperative Program Final Report 



