Soil erosion and organic carbon export by wet snow avalanches

Abstract

Abstract. Many mountain belts sustain prolonged snow cover for parts of the year, although enquiries into rates of erosion in these landscapes have focused almost exclusively on the snow-free periods. This raises the question of whether annual snow cover contributes significantly to modulating rates of erosion in high-relief terrain. In this context, the sudden release of snow avalanches is a frequent and potentially relevant process, judging from the physical damage to subalpine forest ecosystems, and the amount of debris contained in avalanche deposits. To quantitatively constrain this visual impression and to expand the sparse existing literature, we sampled sediment concentrations of n = 28 river-spanning snow-avalanche deposits (snow bridges) in the eastern Swiss Alps, and infer an orders-of-magnitude variability in specific fine sediment and organic carbon yields (1.8 to 830 t km−2 yr−1, and 0.04 to 131 t C km−2 yr−1, respectively). A Monte Carlo simulation demonstrates that, with a minimum of free parameters, such variability is inherent to the geometric scaling used for computing specific yields. Moreover, the widely applied method of linearly extrapolating plot-scale sample data may be prone to substantial under- or over-estimates. A comparison of our inferred yields with previously published work demonstrates the relevance of wet snow avalanches as prominent agents of soil erosion and transporters of biogeochemical constituents to mountain rivers. Given that a number of snow bridges persisted below the insulating debris cover well into the summer months, snow-avalanche deposits also contribute to regulating in-channel sediment and organic debris storage on seasonal timescales. Finally, our results underline the potential shortcomings of neglecting erosional processes in the winter and spring months in mountainous terrain subjected to prominent snow cover.

Dynamics of development and variability of surface degradation in the subalpine and alpine zones (an example from the Velká Fatra Mts., Slovakia)

“…These are erosive grooves that are found on less steep slopes where the accumulation of material mostly occurs [25]. From the first monitoring, their area had been slightly increasing.…”

“…Since it is the case of the surfaces with no signs of destruction in 2003, geomorphological processes disrupted the integrity of vegetation and obtained eroded soil particles while moving in the course of the following years. This fine-grained material was transported by gravitational processes (especially by water) and enriched the debris area [25]. The increase of nutrients caused the development of woody vegetation in particular [32].…”

“…These processes become severe especially in the warm half of the year, when their destructive forces demonstrate themselves in the form of splash erosion processes and concentrated surface run-off (coming from rainfall and melting snow [24]). The phenomena bound to the running water activity in conditions of cold climate are actually polygenetic forms of surface destruction [25]. They are created simultaneously by water from snowmelt, rainfall, solifluction, ground needle ice and by wind eventually.…”

“…Overgrowing lower-situated debris areas by woody vegetation refers to a longer period of their stability, which might suggest that they are not loaded with new material. In 2003, debris areas enlarged their areas, but only in places situated in the upper parts of the slope, which refers to the weaker carrying strength of transport forces within erosive chutes [25]. This particularly refers to chutes no.…”

“…Compared with 1961, the presence of vegetation (debris overgrowing) indicates the presence of a higher proportion of nutrients and soil components in the debris, which were shifted from the header of avalanche chutes with active soil degradation processes. The frost heaving caused by the needle ice in combination with processes of snow patch and water-gravitational erosion results in disrupting the cohesion of the soil and its subsequent transport through the chutes into the debris location [25].…”