Reassessing the role of milldams in Piedmont floodplain development and remobilization

Abstract

Recent emphasis on milldams as a cause of colonial-era floodplain sediment deposits and source for modern remobilization has ignited debate in scientific and environmental policy forums, especially in the mid-Atlantic USA. Although milldams are regionally ubiquitous and visually striking, no investigation has demonstrated that they are a dominant driver of regional floodplain sediment accumulation and remobilization. Long-term analysis in Baltimore County, Maryland found that milldams were sufficient, but not necessary for historical sediment accumulation. We compared erosion from streams upstream and downstream of confirmed milldam locations, in paired segment comparisons with and without the influence of historical milldams, and pooled across 37 study segments. We found the scale of inquiry produced different interpretations regarding the role of milldams in regional erosion and sedimentation. Although local milldam sediment remobilization rates were 15% greater than those immediately downstream, neither paired nor pooled segment comparisons provided evidence that milldam deposits are contemporary dominant sources of sediment. Further comparison across drainage areas demonstrated that milldam deposits could raise local remobilization rates by augmenting sediment availability along supply-limited channels. We introduce a unifying conceptual model that explains apparently contradictory findings in the literature where, rather than the sole cause of legacy sediment deposits, some milldams are nonetheless responsible for local increases in sediment remobilization rates along portions of the stream network.