[en] Concentrations of ¹³⁷Cs in water were monitored from 1987 to 1991 in the Dora Baltea river (northwest Italy), a river characterized by a catchment area located in a mountainous area which is largely covered with snow and ice in winter. A strong seasonal variation of water contamination was observed. Increased water concentrations in summer were attributed to caesium deposition and accumulation on snow-covered surfaces in winter and a delayed release in summer during ice and snow melting. An attempt to relate the observed cyclical variations with estimated transfer rates from the catchment has also been performed. (Author)