REASSESSING THE ROLE OF MILLDAMS IN PIEDMONT FLOODPLAIN

Research on the role of historical milldams in transforming river systems has ignited debate in scientific and environmental policy forums, especially in the mid-Atlantic USA. It has been suggested that features commonly described as floodplains along Piedmont streams formed primarily as slackwater deposits upstream of historical milldams, and that these deposits are a dominant component of sediment budgets contributing to downstream water-quality problems. Additional research has demonstrated high rates of bank erosion from milldam deposits along some channels, yet others reveal little impact on basin-scale sediment dynamics. No investigation has demonstrated that milldams are required for floodplain formation.

We address concerns raised in this ongoing discussion and evaluate the importance of milldam deposits as sediment sources. Our sample quantified 45 years (1960 - 2005) of streambank remobilization along segments spanning a range of drainage areas (0.18 - 155 km²). It provided (i) the largest sample of milldam-impacted streams currently found in the literature, (ii) the widest range of drainage areas in a single study, (iii) the first quantitative response to the call for comparing streams with and without influence from mill dams, and (iv) the ability to contextualize results from previous research at different spatial scales. Our multi-scalar comparisons provided a greater understanding of the role of milldam deposits in stream networks, allowing inference about the spatial extent over which milldams measurably impact streams within the mid-Atlantic Piedmont.

The results indicate that milldams were sufficient, but not necessary for historical sediment accumulation. While remobilization rates for milldam deposits were higher (μ = 15%) than adjacent upstream or downstream streambanks, grouped comparisons of segments with and without milldams demonstrated no significant difference (p<0.05). Our results suggest that milldam deposits increase erosion at drainage areas 10 - 60 km² by augmenting sediment availability along otherwise supply-limited channels. Channel banks in close proximity to breached milldams may serve as hotspots of local erosion and deposition, but broader comparisons failed to demonstrate significantly greater erosion from milldam influenced valley deposits.