Global climate controls on the denudation of glaciated mountain

Abstract

This paper presents semi-quantitative data concerning the pattern of erosion under present-day humid climatic conditions for a ca. 10-km2 large soil-mantled drainage basin located in the northern foothills of the Central Swiss Alps. The present-day pattern of erosion was presumably initiated at ca. 15 ka at the time when the last glacial period was terminated.Present-day erosional processes are characterized by fluvial dissection and hillslope mass failure, resulting in linear geomorphic features. As fluvial dissection proceeds, the rivers tend to adapt steady state longitudinal stream profiles. In this case, the stream profiles display power–law relationships between channel gradient and upstream size of the drainage basin. Our study shows that establishment of steady state longitudinal stream profiles requires a balanced mass flux between processes on hillslopes and in channels. This situation is interpreted for locations where the downslope flux of mass on hillslopes does not exceed the erosional potential and the transport capacity in channels. Consequently, the development of steady state drainage systems appears to be controlled by erosional processes in channels as they dictate the elevation of the base level. In contrast, systems that are at transient states of longitudinal stream profile development very often display non-balanced mass flux between processes in channels and on hillslopes. Specifically, low mechanical strength of the bedrock (e.g. glacial till, marls) results in a downslope flux of mass that exceeds the transport capacity and the erosional potential in the channel, causing the elevation of the channel floor to increase. In this case, processes on hillslopes are not necessarily coupled with erosion in channels and appear to control the evolution of the drainage basin for a limited time interval.Besides interpretations of controls on surface erosion, this paper also estimates post-glacial sediment-yield for the analyzed drainage basin. These estimates are achieved by determination of the difference between the elevation of the glacial and the present-day surface. The data show that ca. 30% of the analyzed drainage basin experiences present-day surface erosion. Furthermore, average post-glacial sediment yield measures ca. 4.6 m/ka if the post-glacial volume of sediment is related to the area that has experienced surface erosion, or ca. 1.4 m/ka if it is related to the whole drainage basin. Interestingly, the erosional efficiency appears to be significantly higher for systems that display steady state stream profiles than for transient rivers.