PREDICTING DOWNSLOPE TRAVEL OF GRANITIC SEDIMENTS FROM FOREST ROADS IN IDAHO¹

Abstract

The length and volume of granitic sediment deposits were measured annually over a 4.5-year period below 6.6 km of forest roads constructed on headwater watersheds in the mountains of Idaho. Sediment deposits were identified by source of runoff, location of the deposit terminus, and various site factors including descriptors of hillside sediment storage capacity. Prediction equations were developed using linear regression for travel distance of sediment originating from fill slopes, rock drains, and culverts, with r2 values ranging from 0.70 for fill slopes to 0.91 for culverts.Cumulative volume of erosion and length of obstructions on the hillside were statistically significant variables in the equation for fills and rock drains; hillside gradient and runoff source area also appeared in the equation for culverts. Sensitivity analysis showed that the volume of erosion was the most sensitive variable in all cases. A dimensionless relationship relating the volume of sediment storage on slopes to sediment travel distance was also developed. This study makes it possible for land managers to assess the risk of sediment delivery to channels from alternative road designs, locations, and erosion control practices and provides a means to evaluate the cumulative effects of past road construction on sedimentation.(KEY TERMS: erosion; sedimentation; land use planning; forest hydrology; forest roads; granitic soil; sediment budgets; sediment delivery.)

Present Effects of Past Wildfires on the Diets of Stream Fish

“…Within 1 or 2 years after the fires, some of the study streams experienced a massive scouring, erosion, and redeposition of bed materials (Benda et al 2003), removing much of the belowground vegetative structures. Consequently, streams like these may be prone to substrate-scouring events at moderate levels of precipitation (Megahan 1983, Megahan andKetcheson 1996). In turn, this continued scouring could further delay riparian recovery.…”

“…The bedrock substrate in the basin is dominated by moderate to well-weathered coarse-grained quartz monzonite typical of much of the Idaho batholith. Soils are lithosols with weakly developed A horizons ranging from 5 to 25 cm thick (Megahan and Ketcheson 1996). These granitic, droughty soils have limited fertility and are highly erodible when vegetation is removed (McGarth et al 2001).…”

Spatial and temporal patterns in erosion from forest roads

“…From this data, there is no clear threshold of length and slope beyond which erosion will become unacceptable, so the decision must be based on other information. Megahan and Ketcheson [1996] show that runoff leaving the road prism at concentrated points, such as cross-drains, is more likely to deliver sediment to streams than diffuse drainage. Montgomery [1994] argues that concentration of drainage by roads is the primary reason for gully development and landsliding below roads.…”

“…Montgomery [1994] recommends limiting road segment length based on slope of the hillside to which the segment drains. Megahan and Ketcheson [1996] suggest spacing to limit delivery distance across hillslopes above streams.…”

An Interdisciplinary Erosion Mitigation Approach for Coral Reef Protection – A Case Study from the Eastern Caribbean

“…These BMPs reduce flow velocity and thus promote settling of sediment, and include methods such as hay bales, sediment traps, check dams, and settling ponds (Anderson, 1994). Road drainage improvements (Type II) may also be viewed as attempts to reduce the downstream transport of eroded sediment as adequate placement of road drainage structures also promotes reduced connectivity with downslope water bodies (Megahan and Ketcheson, 1996;Croke et al, 2005).…”