REASSESSING THE ROLE OF MILLDAMS IN PIEDMONT FLOODPLAIN
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.