Erosional and chemical denudation rates in the Southwestern Idaho batholith

Abstract

AbstractAnnual erosional denudation (ED) and chemical denudation (CD) rates are determined on four forested watersheds located in the Idaho batholith, Idaho, U.S.A. Bedrock in the study area is a coarse‐grained quartz monzonite. Annual sediment yields are computed from sediment collected in settling ponds at the watershed mouths and corrected for trapping efficiency. Solution loads for denudation components (Na, K, Ca, Mg, SiO2, A12O3 and FeO) are computed using equations relating concentration to flow and by integration of mean daily flow values. Eleven years of water chemistry data and 19 to 23 years of sediment yield data are used in the denudation estimates. Strong relationships exist between CD, ED and annual water yield, so probability distributions of annual water yield are used in Monte Carlo simulations to establish frequency distributions of annual CD and ED rates.Mean annual total denudation for the four watersheds (CD + ED) equals 235 kg ha −1 yr−1 or 8‐9 Bubnoffs. The 11 ‐year mean ED rates exceed CD rates for three of the four watersheds, although it is more probable in any given year that CD will exceed ED for three of the watersheds. ED is temporally more variable than CD for all watersheds. Simulations of denudation over 1000‐year periods indicate that the frequency of occurrence of ED is skewed toward higher denudation values, and the modal ED rate is substantially lower than the mean value. CD rates are less skewed, with mean and mode in the same class. Ninety‐seven per cent of CD is composed of Na, Ca and SiO2 fluxes. Episodic high ED rates are related to high peak flows from snowmelt runoff associated with winters of large snow accumulation. Perturbations such as fire or road construction would be expected to accelerate ED more than CD.