Characteristics of suspended sediment and metal transport during ice breakup, Saint John River, Canada

- High loads of suspended sediment and particulate metals at breakup affect river water quality.
- Relevant data have been obtained for the first time in Saint John River.
- Suspended sediment comprises very fine primary particles; median size ~ 10 μm.
- Indirect evidence suggests that flocculation occurs during breakup.
- All but one of the studied metals exhibited strong particulate attachment.

During the spring freshet and the resulting breakup of river ice covers, the erosive capacity of the flow is enhanced not only by the rising discharge, but also by the highly dynamic waves that form upon the release of ice jams. Concentrations of suspended sediment and particulate contaminants, such as trace metals, increase sharply. To enhance the very limited knowledge on this issue, comprehensive data have been collected on the Saint John River (SJR). Herein, first-time data sets are presented on sediment and metal transport characteristics during the highly dynamic flow conditions of the ice breakup period. The median sediment particle size is nearly 10 μm and decreases slightly with suspended sediment concentration (SSC). Though moving ice prevented direct examination of flocculation processes during the breakup period, indirect evidence suggests that it does occur and may be more pronounced than during open-water conditions. Owing to high breakup flow velocities, bed deposition and entrapment of suspended sediment are likely minimal in the study reach, though more pronounced in the reservoirs of downstream control structures. In general, the trace metals examined in this study exhibited strong attachment to suspended sediment. At any but the lowest of SSCs, sorbed amounts were much larger than dissolved, with the sole exception of strontium. Particulate metal concentrations and partition coefficients occurring during ice breakup in the SJR were within known ranges from previous, open-water studies and did not exceed “probable effect” levels of Canadian guidelines. Changing climatic conditions could lead to increased amounts of particulate metals.