Decadal to Millennial scale erosion rates in the Nepal Himalayas

On a sub-millennial time scale the spatial distribution of erosion is controlled to first order by tectonics, relief, and possibly precipitation, and secondly by vegetation, lithology, temperature and human activity. The Himalayas form a very distinct orographic barrier with a pronounced rainfall gradient from the South to the North and have a very rugged terrain, causing highly dynamic surface processes and fast erosion rates. Thus, the Himalayas provide an ideal site of investigation to study erosion and constrain its controlling factors. In this contribution we present an integrated comparison of mean catchment erosion rates, calculated from in-situ produced ¹⁰Be cosmogenic isotope concentration in river sands (representative for millennial time scales) and suspended sediment measurements (integrating the annual to decadal time spans). We discuss erosion rates and patterns in the context of precipitation-landscape features of the studied catchments. The samples cover all major rivers, and several minor tributaries of the Narayani watershed (30,000 km²) in central Nepal. They represent all lithologies, topographic units and climate regimes across the Himalayan range. The erosion rates, both from cosmogenic nuclide analysis and suspended sediment measurements, range from 0.1 to 4 mm/yr. These agree well between the two methods and also with already published data for the major outlet stations at the Himalayan front. However, in the Middle and High Himalayas the cosmogenic erosion rates are significantly higher than those from suspended sediment measurements. While on the short term (intra-annual) a clear relation between precipitation and erosion can be observed, the cosmogenic erosion rates show no clear dependency with the basin wide precipitation pattern. Furthermore, no relation could be observed with the dominant lithological units and the degree of glaciation.
Our observations confirm the overall established relationship between erosion rates, relief and slope, showing clearly that topography exerts a predominant control on spatial erosion rates on the millennial timescale. However, we observe a different relationship between main stream basins (> 250 km²) and small tributary basins (< 250 km²). Small basins show in general lower erosion rates than larger basins for respectively the same topographic characteristics.