UNRECOGNIZED ANTHROPOGENIC BASE-LEVEL CONTROLS on STREAMS FROM HISTORIC DAMS AND DAM BREACHING: QUESTIONING CAUSE AND EFFECT

Base-level forcing has well-known, profound effects on fluvial processes in tectonically active areas; here, base-level forcing from widespread damming and dam breaching is documented in the tectonically inactive, US mid-Atlantic region. Numerous late 17^th to early 20^th c. milldams trapped fine sediment in extensive backwater repositories at the same time that land clearing, mining, and agriculture caused substantial upland soil erosion. As aging milldams breached during storms or were removed, local drops in base level caused stream channel incision into reservoir sediment. From lidar topography and field mapping, we document a doubling of channel gradient for hundreds of km of stream length due to dam breaching. Thousands of milldams remain and many are breached or removed each year, continuing to affect local base levels.

Although post-dam breach entrenchment is decoupled from modern upland land use, incision and bank erosion often are attributed to modern urban processes (e.g. excess storm water). Even with no increase in storm water runoff, however, a breached dam will cause incision, channel steepening, stream bank erosion, and increased suspended sediment loads. For valleys impacted by milldams, conceptual models that link channel condition and sediment yield exclusively with current upland land use, sediment supply, and runoff are incomplete. After dam breaching, factors more important than upland land use for predicting stream channel erosion include: (1) number of years since dam breach; (2) proximity to the dam; (3) dam height; (4) volume of sediment stored behind the dam; and (5) frequency of events that weaken stream banks, such as freeze-thaw.

These documented effects of widespread anthropogenic base-level controls in valley bottoms raise important questions about causality in modern fluvial geomorphic processes, which in turn affects decisions regarding restoration and management. For example, we propose that restoring the naturally occurring riparian wetlands buried beneath historic millpond sediment, rather than adding hard structures to protect stream banks or planting riparian trees on elevated millpond terraces, might be a more effective and sustainable approach to increasing wetland biodiversity and improving riparian habitat and water quality.