SEASONAL ANOXIA AND METAL MOBILITY IN THE LOWER COEUR D’ALENE RIVER BASIN

Heavy metal concentrations increase during seasonal anoxia in deep meander bends found throughout the Coeur d’Alene River Basin in Northern Idaho, USA. Historical mining waste and dam controlled summer pool levels affect both Lake Coeur d’Alene and the rivers within the watershed with high heavy metal concentrations, seasonal inundation, and reduced flow conditions during inundation. An anoxic hypolimnion develops in the deep meander bends of the Coeur d’Alene River due to seasonal inundation allowing liberation of metals from iron-manganese oxyhydroxides in riverbed sediments. The drawdown of Lake Coeur d’Alene each fall likely results in the migration of liberated metals from the Coeur d’ Alene River into the Lake and ultimately downstream to the Spokane and Columbia Rivers, which may negatively affect terrestrial and aquatic receptors downstream. This study assessed this possibility by monitoring chemical trends as well as the role of biological productivity at four sites on the Coeur d’Alene River from June to September 2018 in the Lower Coeur d’Alene River Basin.