Legacy sediment: A conceptual model and perspective on the role of dams

The term ‘legacy’ usually refers to past human activities that have long since ceased, but contemporary human activities can potentially leave future legacies. While legacy sediment is often defined as having upstream anthropogenic origins, such as from land clearing, infrastructure such as flood control structures and dams can trap sediment, whether naturally or anthropogenically derived, that can persist for centuries or more, thus leaving a legacy of past sedimentation. Dam removal is becoming more common, and accounting of sediment caused by the impoundment is an important component of planning a removal and its impacts. We provide four examples to explore mechanisms of legacy sediment generation: (1) land use changes with no infrastructure or dams, (2) a review of non-dam infrastructure that abstracts water and substantially reduces sediment transport, (3) a case study from the longest-duration geomorphic monitoring (2007 to 2023) of a short (<10 m) dam removed in 2008 and (4) a review of a well-studied tall (>10 m) dam removal. When dams are removed, fluvial processes typically erode sediment in and directly adjacent to newly formed or opened stream channels over short timescales, but inaccessible deposits remain as legacy sediment. We demonstrate that overbank flooding and sedimentation upstream of run-of-river dams can create unrecognised deposits outside of the modern reservoir pool. We further outline how certain dam and other infrastructure characteristics, dam removal style and sediment character can generate persistent legacy sediment and sometimes legacy landforms outside active stream channels where it is unlikely to be excavated by natural geomorphic processes except in the most extreme events. With the increase in dam building and dam removal in various parts of the world, legacy sediment deposits are becoming ubiquitous, with important consequences for river and land management.