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Tom Black, USDA Forest Service Rocky Mountain Research Station, tblack@fs.fed.us (Presenter)
Charles Luce, USDA Forest Service Rocky Mountain Research Station, cluce@fs.fed.us

 

Forest roads have been shown to cause water quality impairment in many environments Megahan and Kidd 1972, Croke and Hairsine 2006). The main impacts to water quality result from increases in fine sediment delivery, increased mass wasting risk, stream channel diversions and alterations to local hydrology. We used the Geomorphic Road Analysis and Inventory Package (GRAIP) to inventory and analyze forest roads in the TMDL listed Middle Fork of the Payette in Idaho. Field data was collection on 17,203 drainage locations on 583 miles of roads. The total amount of fine sediment from roads delivered to the Middle Fork Payette River and its tributaries was 1,691 Mg/yr, or 20% of total production. The average GRAIP predicted sediment delivery rate from roads is 2 Mg/km2 (5.5 ton/mi2). The sediment delivery was modeled to be 10% of the natural background stream sediment mass as predicted by BOISED (Reinig et al. 1991), with a range from 0 to 22%. 113 miles or 19% of all road length surveyed was hydrologically connected. Connection between the road and stream was strongly controlled by slope position and channel proximity. 28% of drains located within 100 meters of the channel were connected, while only 4% of those further upslope connected. 33% of sediment generated on these stream proximal roads was delivered, while only five percent delivered from the rest of the network. Of all sediment delivered, 90% occurred within 100 m of channels. 967 drainage locations (7%) delivered 90% of the total sediment.