Pedogenesis and erosion history in a high rainfall, mountainous drainage basin - Cropp River, New Zealand

Abstract

Cropp River drains a 28.5 km² drainage basin in the western Southern Alps. Extreme erosional modification of this formerly glaciated valley has occurred in response to high rainfall (10,800 mm a⁻¹) and rapid tectonic uplift (12 ± 2 mm a⁻¹). Studies of geomorphology, soil development and distribution, and contemporary forms of erosion provide the basis for interpreting the history of post-glacial erosion. During late Pleistocene (14 to 22,000 years BP) glacial advances Cropp basin was largely occupied by ice. Till dated at 10,250 ± 150 years BP, in the middle reaches of Cropp River, provides a maximum age for the post-glacial topography of the upper basin. U-shaped cirques are present at the heads, of the larger tributaries. Elsewhere the glacial valley form has been largely destroyed by fluvial and mass movement erosion. This has produced a dominantly erosional landscape with extremely steep slopes (40 to 70° common), intense dissection, and steep V-shaped stream channels. Soil development sequences, independently dated by dendrochronology and radiocarbon dating, were examined on both gently and steeply sloping topography. Soil development is rapid and progressively forms recent soils (<200 years), yellow-brown earths (500 to 1000 years), podzolised yellow-brown earths (1000 to 1500 years) and gley podzols (2500 to 10,000 years). All soils are strongly leached indicated by rapid soil acidification, and the dominance of exchangeable Al and H even in recent soils. The older soils (>1000 years) are both podzolised and gleyed. Eluvial-illuvial coefficients, calculated from total element analyses, indicated the losses and gains of major elements during soil development. Morphology of soils on steep slopes under both scrub-forest and grassland vegetation indicates most soils range in age from tens to a few thousand years. Soils are removed from steep slopes by episodic and progressive erosional processes, generally within 500 to 1000 years. Evidence of active erosion is widespread although largely concealed by dense vegetation cover. Natural revegetation of eroded sites is rapid despite low soil nutrient status. On steep slopes eroded sites have complete vegetation cover within 50 years, and within 500 to 1000 years have vegetation similar to uneroded areas. Obvious visual evidence of erosion is short-lived because of rapid revegetation and plant succession.