Recently, widespread valley-bottom damming for water power was identified as a primary control on valley sedimentation in the mid-Atlantic US during the late seventeenth to early twentieth century. The timing of damming coincided with that of accelerated upland erosion during post-European settlement land-use change. In this paper, we examine the impact of local drops in base level on incision into historic reservoir sediment as thousands of ageing dams breach. Analysis of lidar and field data indicates that historic milldam building led to local base-level rises of 2–5 m (typical milldam height) and reduced valley slopes by half. Subsequent base-level fall with dam breaching led to an approximate doubling in slope, a significant base-level forcing. Case studies in forested, rural as well as agricultural and urban areas demonstrate that a breached dam can lead to stream incision, bank erosion and increased loads of suspended sediment, even with no change in land use. After dam breaching, key predictors of stream bank erosion include number of years since dam breach, proximity to a dam and dam height. One implication of this work is that conceptual models linking channel condition and sediment yield exclusively with modern upland land use are incomplete for valleys impacted by milldams. With no equivalent in the Holocene or late Pleistocene sedimentary record, modern incised stream-channel forms in the mid-Atlantic region represent a transient response to both base-level forcing and major changes in land use beginning centuries ago. Similar channel forms might also exist in other locales where historic milling was prevalent.

Bidirectional (hyporheic) exchange of water between stream channels and sediments benefits stream ecosystems, yet the effects of urbanization on such exchange are poorly understood. Exchange is controlled by a set of geomorphic parameters collectively defined as ''hyporheic potential,'' including sediment hydraulic conductivity (K), vertical undulations of the streambed (VC), and channel sinuosity (S). We measured these hyporheic potential metrics in 10 stream reaches with varying percent impervious surface (0-47 percent) in their contributing watersheds. We performed linear regression between hyporheic potential metrics and two metrics of urbanization: percent impervious in the watershed and average riparian forest buffer width. We found that most trends between hyporheic potential metrics and both urbanization metrics were noisy, with low r 2 . Furthermore, hyporheic potential varied as much among our streams as in non-urban studies, and K varied as much within stream reaches as between stream reaches. These results collectively indicate that the effects of urbanization on hyporheic potential in our streams were minimal and that urbanization may not heavily constrain hyporheic exchange. Nevertheless, we did find that S increased with impervious cover in the contributing watershed, consistent with higher urban storm flows leading to sinuosity adjustments to reduce channel slope. Urbanization may therefore enhance hyporheic potential under certain circumstances, and bears further study. Furthermore, there may be value in increasing the benefits of hyporheic exchange above existing levels through ''hyporheic enhancement.'' Because K varied more among streams than VC, which varied more than S, efforts to enhance hyporheic exchange may have greatest effect if they manipulate K.