Mobilization of metal-contaminated sediment by ice-jam floods

A large river ice jam and associated flooding in February 1996 on the Black foot and Clark Fork Rivers of western Montana mobilized large amounts of fi ne-grained sediment. Metal concentrations in sediment downstream from a res ervoir containing large amounts of contaminated sediment were enriched in m etals after the ice jam, while open reaches above the reservoir were dilute d by ice-jam processes. This varied response shows the importance of unders tanding ice jam events to sediment and metal transport and suggests that ic e jams must be considered as important agents affecting sediment metal conc entration in rivers.