LEGACY SEDIMENT AND A PERSPECTIVE ON THE ROLE OF DAMS

Human-accelerated sedimentation and the creation of legacy sediment can disrupt natural sediment regimes in landscapes and create myriad issues in ecosystems. While legacy sediment is often described 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. Dam removal is becoming more common, and monitoring of erosion and downstream transport of sediment stored within the impoundment is sometimes conducted to better understand impacts after a dam is removed. We present a case study from a site on the Souhegan River, a fourth order stream in New Hampshire with a 250+ year dam history, the last of which was removed in 2008. Only 15% of the sand impounded in the former reservoir remains, as calculated from pre- and post-removal cross section surveys (Pearson, 2010; Pearson et al., 2011; Collins et al., 2017); however, by employing several field and GIS techniques, we verified that current landforms within the valley upstream of the former dam site are composed of legacy sediment, at least doubling the initial volume of dam-impounded sediment estimated prior to dam removal. We demonstrate that overbank sedimentation occurred adjacent to the reservoir, as evidenced by flood chute deposition at higher elevations than the former dam crest. In streams with small dams and stream flow over their crests, overbank deposition could be significant and widespread where valley bottom geometry has sufficient accommodation space. We suspect that this sediment deposited outside the active impoundment is most likely to persist following dam removal, therefore becoming legacy sediment. We call for more accurate accounting of the areal extent of sedimentation associated with different types of infrastructure, and for the consideration of the role of dams - and dam removal - in the creation of legacy sediment regardless of upstream land use change or sediment origins.