Effect of seasonal snow cover on suspended sediment runoff in a mountainous catchment

The effect of snow cover and snowmelt on suspended sediment (SS) runoff at a mountainous catchment in the temperate zone was investigated. The observation was carried out from March 1999 to May 2002 (3 years) at an experimental catchment with the area of 0.347 km2 ranging from 280 m to 618 m above mean sea level with average stream gradient of 1/4. It was concluded that the snow accumulation and ablation process in the catchment had a significant impact on SS runoff characteristics especially during the snow-melting period. It was estimated that more than 60% of the annual SS load flowed out during the snow-melting period because of the enhanced water discharge. The daily average SS flux during the snow-melting period was calculated to be 2.54 g s−1 km−2, corresponding to a 6–7 times greater SS flux than that during other periods. The C–Q relation analysis in each runoff event revealed a dynamic change in the SS runoff mechanism during the snow-melting period. Strong clockwise hysteresis in C–Q relation in the early stage of the snow-melting period may be explained by consumption of the bed load by the first flush followed by quick depletion of the material on the stream bed due to suppression of further SS supply by thick snow cover. Counterclockwise hysteresis in C–Q relation was observed only in the late stage of the snow-melting period. SS production at steep slopes beside the stream by avalanches and surface soil slides, induced by an abundance of meltwater, was considered to be one of the main causes of the counterclockwise loops. It would be hypothesized that exposure of other steep slopes or bluffs in the catchment apart from the stream and stream bank failure by repeating surges of the stream water stage could also be other possible SS sources causing counterclockwise hysteresis in the late snow-melting period.