SUSPENDED SEDIMENT DYNAMICS IN THE MORTERATSCH PRO-GLACIAL ZONE, BERNINA ALPS, SWITZERLAND

Suspended sediment concentration (SSC) in the Ova da Morteratsch, Switzerland, measured during July 2007 was closely associated with discharge (Q) and showed statistically significant relationships at the p < 0.001 level at the proximal and distal ends of the 600 m pro-glacial zone. SSC predicted from 10-minute turbidity records gives a much more detailed insight into SSC fluctuations and identified SSC peaks which do not coincide with discharge peaks. Net (proximal – distal) 10-minute suspended sediment loads (SSL) are predominantly positive (i.e. suspended sediment is being stored in the reach) for most of the 7–19 July 2007 record. Net (proximal – distal) SSLs correlate closely with discharge for the first part of the record (7–13 July) but from 14 to 19 July suspended sediment exhaustion is in evidence and discrete phases of negative net SSL (i.e. sediment flushing) are likely for up to six hours on three separate days which coincide with phases of high discharge and exhaustion of the glacial suspended sediment sources. Analysis of Q at the Berninabach–Pontresina gauging station (5 km downstream) for the past five years revealed that maximum monthly discharges capable of generating sediment flushing events occur in an average of four months each year. The study emphasises the rapid change in suspended sediment transport and yields with distance from the glacier snout and highlights the importance of measurements as close to the glacier snout as possible if data are to be representative of the glaciated land up-valley. A better understanding of the processes of sediment exchange and the colonisation and stabilisation of sediment stores by vegetation in such pro-glacial zones is essential if we are to improve predictions of the impacts of climate change on river sediment dynamics and the subsequent effects on aquatic ecology.