Erosion Processes on Steep Granitic Road Fills in Central Idaho
1978
Megahan, W. F.
A set of thirty 20.2-m² (1/200-acre) erosion plots was used to study erosion occurring on steep road fills constructed with granitic soil materials in the Idaho Batholith. Erosion data were collected for a 3½-year period from 1969 through 1972. Erosion on bare control plots averaged 3.4 metric tons/km² per day for water years 1970 through 1972. Erosion was reduced an average of 44% and 95% by tree planting and straw mulching, respectively. Daily erosion rates were consistently higher during summer periods than during snowfree winter periods, presumably because of greater rainfall energy during the summer. Dry creep accounted for at least 15% of the total annual erosion for the years sampled and was as high as 40% in 1971. The erodibility index was a poor predictor of erosion for rain periods. Observations in the area suggest that wind was an important erosion factor on the steep slope studied. The median particle size of eroded materials (D₅₀) tended to decrease throughout the summer and fall until mid-October when it abruptly increased. Soil crusting during the summer and soil freezing and thawing in the fall may have caused these seasonal trends.
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