Publication

Transient landscapes are most suitable to investigate the response of catchment-wide erosional processes to changes in incision. This has been done in the Trub and Grosse Fontanne that are two adjacent stream systems in the Napf region of the Swiss Molasse. Despite their proximity, these basins have responded differently to base level fluctuations. Although the catchments have never been glaciated, the base level changes have been caused by ice damming during the LGM. The Trub basin is characterized by terraces and smooth, equilibrium stream profiles. In the Fontanne catchment, knickpoint migration has caused as much as 80 m of incision after LGM. The upper hillslopes within the incised region do not differ significantly from those which are upstream of the knickzone. Because of this, the Fontanne is interpreted as being decoupled, with the rapidly incising stream having little effect on the upper reaches of the basin. In contrast, the Trub is believed to represent a coupled hillslope stream system. This interpretation was tested using cosmogenic 10Be-based basin-averaged erosion rates. Erosion rates throughout the Trub basin are similar, 350±50 mm/ky, implying a fully coupled quasi-equilibrium system. The upper basins within the Fontanne watershed are comparable to each other and to the neighboring Trub catchments, 380±50 mm/ky, while erosion rates from the incised Fontanne regions are higher, 540±100 mm/ky. The divergence of rates in the Fontanne watershed shows that decoupling has indeed occurred, and that the upper hillslopes are not responding to the incision. A possible explanation for this is that the erosional response time of this basin is longer than the time elapsed since perturbation. Terrace formation in the Trub has been dated at 20 ka, and a mass balance approach suggests that incision of the Fontanne basin began 16 ka.

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Norton, K. P.; von Blanckenburg, F.; Schlunegger, F.; Kubik, P. W. (2006): Cosmogenic Nuclide-Based Investigation of Spatial Erosion and Hillslope Coupling in the Geomorphically Transient Foreland of the Swiss Alps. AGU 2006 Fall Meeting (San Francisco, USA 2006).