HR: 0800h
AN: H51E-0407    [Abstracts]
TI: Characterizing Erosion from Forest Roads in Western Montana
AU: Sugden, B
EM: brian.sugden@plumcreek.com
AF: Plum Creek Timber Company, PO Box 1990, 500-12th Avenue West , Columbia Falls, MT 59912 United States
AU: * Woods, S
EM: swoods@forestry.umt.edu
AF: University of Montana, College of Forestry and Conservation, 32 Campus Drive, Missoula, MT 59812 United States
AB: Unpaved roads are a primary sediment source in forested watersheds. Validation of erosion models and improvements to road management require measurements of road erosion rates. Obtaining these measurements is challenging because physically representative road plots may produce large quantities of runoff and sediment. We measured sediment yields from twenty 0.05 ha unsurfaced road plots in Belt Supergroup and glacial till parent materials of western Montana. Runoff and sediment from each plot was routed into a 30 cm diameter culvert at the foot of the plot, which drained into a 0.8 m³ (1.1 m x 0.9 m x 0.8 m) tub. The tubs were emptied and the mass of sediment weighed at the beginning of the snow free period and periodically after storms. The trap efficiency (the percentage of the total sediment yield trapped by the tubs) was determined by fitting five tubs with tipping buckets that measured the outflow, and a splitter device that collected a sample of the outflow water and suspended sediment. The calculated trap efficiencies ranged from 21% to 84% with a mean of 56%. The percentage clay content of the road tread was a significant predictor of trap efficiency (R² = 0.85, p = 0.03), and this relationship was used to correct the measured sediment yields at the other plots. Annual sediment yields from the plots ranged from zero to 96.9 Mg ha^-1y^-1 over three years (2002-2004). Annual mean sediment yields ranged from 2.1 Mg ha^-1 in 2003 to 9.9 Mg ha^-1 in 2004 with an overall mean of 5.4 Mg ha^-1y^-1. These values are similar to sediment yields from older, lightly used, unpaved roads in basalt and sandstone in central Washington and in sedimentary parent materials on the Olympic Peninsula. However they are considerably less than sediment yields measured on roads in the granitic parent material of the Idaho Batholith and in other more erodible parent materials. Road erosion in western Montana is limited by low erodibility of the dominant parent materials and by limited summer rainfall. A regression model with road slope, time since last grading, roadbed gravel content, and precipitation as predictive variables explained 68% of the variability in sediment yield (F=28.2, P<0.0001).
DE: 1815 Erosion
DE: 1826 Geomorphology: hillslope (1625)
DE: 1834 Human impacts
DE: 1879 Watershed
SC: Hydrology [H]
MN: Fall Meeting 2005