SUBSURFACE DRAINAGE ERODES FORESTED GRANITIC TERRANE

Abstract

Solution and landsliding, the dominant erosion processes in undisturbed forested mountainous watersheds, are both influenced by subsurface drainage. Biological processes that generate organic acids accelerate loss of dissolved solids by promoting the dissolution of primary minerals in granitic rock. These organic acids can also disperse the secondary minerals, kaolinite and ferric hydroxide. This accelerates erosion by subsurface drainage through suffusion and piping, particularly in well-drained, gap-graded soils. Solution, suffusion, and piping on steep slopes can promote conditions suitable for debris avalanches by forming weak, permeable zones. Consequently, subsurface drainage shapes topography over time, from the slow continual dissolution of rock to rapid episodic mass failures. Trees reduce the ratio of physical to chemical erosion while timber harvesting increases it.