Morphodynamics and heavy metal accumulation in an artificially built near-natural river (Inde, Germany)

Heavy metals in riverine sediments threaten fluvial ecosystems worldwide. Sediment cascades transporting the compounds downstream lead to pollution stress downstream from the original input source. In regulated rivers, artificially built reaches with re-activated morphodynamics and morphological structures may become typical sink areas for contaminants. Therefore, a thorough understanding of transport mechanisms and accumulation dynamics is the key to managing enhanced heavy metal concentrations in alluvial sediments. In this study, we investigate a morphologically diverse 12 km-long river reach of the Inde River in Germany, that was relocated in 2005 due to an open-pit lignite mine. As the Inde catchment is a former industrial hotspot (beginning in the Iron Age; golden age at the end of the nineteenth century), the floodplain sediments that accumulated since the construction of the Neue Inde are enriched in heavy metals. The structures and sediments of the relocation are recycled repeatedly, causing the mixing of contaminated and uncontaminated sediments. The Neue Inde is an important sediment trap, especially due to the river’s intense regulation immediately upstream. Thereby, the input of sediments enriched in heavy metals into the receiving waters, the Rur River, is limited. So far, it is unclear whether there is a threshold of sediment storage that, if exceeded, turns the Neue Inde from a sink to a source. The morphodynamical development of artificially built river reaches mainly depends on the restoration design in terms of initially built morphological structures and the width of the river corridor.