Application of the Sequoia Method 



FOR Determining Cumulative Watershed Effects 



IN THE Flathead Basin 



By Don Potts^ 



A cumulative watershed effects risk assess- 

 ment procedure has been applied to the entire 

 Swan River watershed and to 30 smaller Flat- 

 head Basin watersheds. The Swan and the 30 

 smaller watersheds were selected for study by 

 the Flathead Basin Water Quality and Fisheries 

 Cooperative because of their high fisheries val- 

 ues and growing concerns over possible im- 

 pacts from forest management practices. 



The risk assessment model, Sequoia, was 

 developed by the hydrology, soil science and 

 forest management staff on the Sequoia Na- 

 tional Forest in 1980. It is basically an account- 

 ing system for areal disturbance. The various 

 forest management activities are assigned a 

 Runoff Coefficient that varies with the degree 

 of site compaction and soil exposure. The coef- 

 ficients range from a high of 1.0 for permanent 

 harvest system roads to a low of 0. 1 for cable 

 system partial cuts and low intensity fires (ten 

 percent soil exposure). (See Table H-1.) The 

 Runoff Coefficient times the area disturbed is 

 called the Cumulative Runoff Acreage (CRA, 

 or in other Region 5 methods, the Equivalent 

 Road Acreage). Various assumptions are made 

 about the actual areas disturbed by roads, trails, 

 skid systems, and landings. (See Table H-2.) 

 The procedure assumes that disturbance from 

 all timber harvest-related activities recovers 

 within ten years except for roads, trails, recrea- 

 tion, and administrative sites which never re- 

 cover. 



The basic premise of Sequoia is that soil 

 compaction and soil exposure effectively in- 



crease the drainage efficiency of a watershed, 

 thus increasing the magnitude of peak flows, 

 which in turn may cause destabilization of chan- 

 nels and deterioration of fisheries and water 

 quality. Based on research conducted in Oregon 

 and California, the procedure recommends a 

 Threshold of Concern (TOC) for watersheds 

 with "average sensitivity" when the Cumula- 

 tive Runoff Acreage reaches 12 percent of the 

 watershed. 



The Swan River watershed was partitioned 

 into 54 analysis units ranging in size from 

 roughly 1,400 to 23,000 acres. (See Figure H- 

 1.) Many of the analysis units, particularly 

 those at higher elevations, have boundaries 

 corresponding to actual watershed boundaries. 

 All forest management activities during the 

 past decade and all existing road information in 

 each of the units was obtained from the land 

 owners. Sequoia estimates of areal disturbance 

 ranged from a high of nearly 40 percent to a low 

 of zero. Thirteen of the analysis units had dis- 

 turbance greater than Sequoia's 12 percent 

 threshold of concern. Nearly 1 1 percent of the 

 Swan River watershed received some sort of 

 harvest treatment during the 1980s. This in- 

 volved over 750 miles of temporary or perma- 

 nent roads totalling a Cumulative Runoff Acre- 



'Dr. Don Potts is a Professor with the Univer- 

 sity of Montana's School of Forestry in Missoula, 

 Montana. 



Flathead Basin Cooperative Program Final Report 



Page 105 



