During the 6-year study period, sediment data were collected on one or more of 

 these watersheds and included the effects of natural landslides within the drainages. 

 For the study period, the average sedimentation rate on the undisturbed watersheds 

 (weighted for drainage area) was 0.07 ton/mi . 2 /day . This rate is not unreasonable for 

 undisturbed forested lands in the Rocky Mountains ; for example, Leaf (1966) reports 

 average sediment production of 0.02 ton/mi. 2 /day for watersheds in Colorado. 



Using this average sediment production rate of 0.07 ton/mi. 2 /day for undisturbed 

 lands, we calculated that about 10.9 cubic feet of sediment would have been collected 

 in the three sediment dams from the area disturbed by roads during the 6-year study 

 period if the roads hadn't been built. Comparing this to actual sediment production 

 from road erosion for the 6-year study period, we find that sediment yields increased 

 approximately 770 times following road construction (220 times because of accelerated 

 surface erosion plus 550 times because of accelerated mass erosion) ! 



Time Trends in Surface Erosion 



The average annual sediment production from surface erosion on roads is an informa- 

 tive way of evaluating time trends (table 4) . Note that about 84 percent of the total 

 sediment for the 6-year study period was produced during the first year after construc- 

 tion. By the end of the second year, the total sediment production had risen to over 

 93 percent. 



A histogram of the average sediment production data for individual measurement 

 periods is an even more enlightening way of evaluating time trends (fig. 6) . A second 

 y axis on the right side of the figure indicates how many times sediment production 

 from roads exceeds that from similar lands that are undisturbed. Note that sediment 

 production during the first time period after construction averaged 109 tons per day 

 per square mile of road--about 1,560 times greater than sediment production from simi- 

 lar lands that are undisturbed. Note also that this high initial rate decreases 

 rapidly during subsequent measurement periods. As noted previously, sediment produc- 

 tion rates resulting from surface erosion averaged about 220 times greater than the 

 rates for undisturbed lands during the 6-year study period. 



Table 4. --Sediment production due to surface erosion on roads by years 



after construction 



Year : 



Sediment 

 production 



: Percent 

 : of total 



: Accumulated 

 : percent 





Cubic feet 







1961-62 



2,021.5 



83.8 



83.8 



1962-63 



226.6 



9.4 



93.2 



1963-64 



14. 3 



0.6 



93.8 



1964-65 



54. 2 



2.2 



96.0 



1965-66 



55.4 



2.3 



98.3 



1966-67 



41.5 



1.7 



100.0 



Total 



2,413.5 



100.0 





s 



