Mill Dams Research Articles

Geomorphic Response to the Removal of the Ward's Mill Dam on the Watauga River, North Carolina

abstract: Though economic situations of individual dams vary, current dam removal studies largely indicate that ecological benefits of river re-connectivity may outweigh the costs of maintaining aging infrastructure. However, there is a lack of comprehensive studies in certain regions of the United States, including Southern Appalachia. We used the removal of the 6-meter-high Ward's Mill Dam on the Watauga River in North Carolina to conduct a before-after/control-impact study to monitor geomorphic impacts to such removals in this understudied region with the objective of identifying changes in channel form, analyzing bed sediment characteristics, and quantifying rates of volumetric evacuation and deposition. To capture geomorphic change repeated pre- and post-removal cross-sectional surveys, longitudinal profiles, and in-situ particle size sampling was conducted in upstream and downstream reaches. Field collections the day after removal show significant deposition of gravel sized particles immediately downstream of the dam (≤200 m). Channel bed texture remains significantly finer across the 2 km downstream study reach 506 days following removal. Erosion rates of the impoundment during the deconstruction period were on average ~860 m3/day and decreased to 75.5 m3/day by day 74. Below average mean daily flows during this period were able to exhume ~62.9 percent of impounded sediment.

Geomorphic Response to the Removal of the Ward’s Mill Dam on the Watauga River, North Carolina

Though economic situations of individual dams vary, current dam removal studies largely indicate that ecological benefits of river re-connectivity may outweigh the costs of maintaining aging infrastructure. However, there is a lack of comprehensive studies in certain regions of the United States, including Southern Appalachia. We used the removal of the 6 m high Ward’s Mill Dam on the Watauga River in North Carolina to conduct a before-after/control-impact study to monitor geomorphic impacts to such removals in this understudied region with the objective of identifying changes in channel form, analyzing bed sediment characteristics, and quantifying rates of volumetric evacuation and deposition. To capture geomorphic change repeated pre- and post-removal cross- sectional surveys, longitudinal profiles, and in-situ particle size sampling was conducted in upstream and downstream reaches. Field collections the day after removal show significant deposition of gravel sized particles immediately downstream of the dam (≤200 m). Channel bed texture remains significantly finer across the 2 km downstream study reach 506 days following removal. Erosion rates of the impoundment during the deconstruction period were on average ~860 m3/day and decreased to 75.5 m3/day by day 74. Below average mean daily flows during this period were able to exhume ~62.9 percent of impounded sediment.

River Reach: Chicagoland, riverine reflections and settler harm-reduction poetry

Both case report and epic, ‘River Reach’ is an exploratory anamnesis that presents a metamodern embodied reflection of systems entanglement – exposing attunements to and longings for a place-based riverine kinship capable of addressing a national heritage of displacements, immigrations, and enslavement. To improve state compliance with the US Clean Water Act, Graue Mill dam on Salt Creek at Fullersburg Woods (Oak Brook, IL) will be removed in coming months for creek restoration and re-realignment, creating a point of collision and potential among pasts and futures. ‘River Reach’ is a hyperlocal example of a practical, approachable step toward decolonial poetry and community – examining the insularities and permeabilities of whiteness while encouraging personal, deliberate engagement in the preparatory work required for recalibrating national identity cooperatively. As a centering poem for settler harm reduction and an adjunctive tool for collective action, the piece invites participants into a mimetic rehabitation of relationship through nature. ‘River Reach’ may be performed collectively in collaboration with movement, audiovisual elements, and facilitated reader/audience participation.

Legacy sediment and stream incision in the North Carolina Piedmont: Fluvial evolution before and after mill dam construction

Streams in the southern Piedmont are consistently incised forming deep (∼ 1–3 m) channels within wide valley bottoms. Here, we use a variety of methods to determine the drivers of stream incision in the region. We mapped ∼ 140 historic dams throughout the region since the breaching of mill dams is a known driver of incision elsewhere. We examined stream banks at 20 sites previously dammed and 8 sites with no known dams. At each site, we measured channel depth, described sediments, and dated sedimentary charcoal via radiocarbon dating. We also examined historical aerial photographs and used modern LiDAR to create cross-sections in multiple locations. Our findings indicate that while dams were common throughout the area, they were typically built within the incised streams indicating that incision predates dam construction. Locally, incision appears to have been part of an aggradation-degradation sequence driven by a period of Euroamerican deforestation. Most dams in the region are reported to have been built before 1850 indicating relatively early incision in the region. The legacy sediments overlie Holocene sediments that are similar to those in anastomosing systems that remain in a few stream reaches today. As such, anastomosing systems may have been common in the region during the Holocene. Both legacy sedimentation and erosion appear to continue into the present with alluvial sedimentation, stream straightening, headward erosion, and now a new discharge regime driven by urbanization in the region. Broadly, our results suggest that streams in the southern Piedmont have been consistently impacted by humans since Euroamerican settlement.

Pre- and postsettlement depositional processes and environments of the 3rd- to 5th-order White Clay Creek watershed, Piedmont Province, Pennsylvania and Delaware, USA

We extend two hypotheses based on studies of 1st- to 3rd-order Piedmont watersheds of southeastern Pennsylvania, USA, by collecting data in a larger 3rd- to 5th-order watershed nearby. One hypothesis posits that presettlement river corridors were dominated by wetlands, and the other suggests that river valleys were filled by millpond sedimentation following European settlement. Both hypotheses support new river restoration practices, so their generality is important to assess. Ten lithofacies indicate depositional environments, while pedostratigraphic criteria and 14C dating define presettlement and postsettlement stratigraphic units. Basal gravels similar to modern stream bed sediments represent presettlement channels with active bedload transport. Wedge-shaped gravel deposits resembling modern bars further document presettlement bedload transport by channelized flows. Extensive presettlement and postsettlement units of massive, organic-poor, fine-grained sediment formed when overbank flows inundated floodplains. Peat deposits, exposed at a single site (but absent elsewhere), represent a presettlement wetland. Decimeter-thick, discontinuous, massive carbonaceous fine-grained sediments occasionally overlie basal gravels; these may represent localized wetlands adjacent to presettlement channels or hydraulic backwater environments. Laminated sand and mud accumulated behind one 3-m-high mill dam, but these millpond deposits are absent at other sites. Instead of being dominated by wetlands, presettlement river corridors are better described as a complex mosaic of riparian environments including older colluvial landforms, floodplains (some of which may have been seasonally inundated wetlands), primary (and possibly secondary) channels, and depending on geomorphic setting, either localized or valley-spanning wetlands. After European settlement, millponds were important locally, but their deposits represent a minor component of the stratigraphic record.

Legal Status of the Navigable Rivers of the Grand Duchy of Lithuania in the 16th–18th Centuries

The article examines the legal status of the navigable rivers of the Grand Duchy of Lithuania (further – GDL) in the 16th–18th centuries, based on Lithuanian Statutes, Sejm resolutions (constitutions), city privileges, other legal sources, and historiographical data. The problem of the content and usage of the term "navigable river" found in multilingual sources is examined. The study showed that this term contained two meanings: it indicated the physical characteristics of rivers and defined their legal status as a free path. In these rivers, it was forbidden to build any obstacles that make navigation difficult (fish traps, mill dams, or other barriers) and to collect new "unusual" customs duties. In the GDL’s law, the problem of shipping obstacles has been tried to solve in two ways: a) by specifying the obstacles to be removed physically and/or providing fines; b) by the mandatory installation of gates in fish traps and dams.

Mill dams impact microbiome structure and depth distribution in riparian sediments.

Damming has substantially fragmented and altered riverine ecosystems worldwide. Dams slow down streamflows, raise stream and groundwater levels, create anoxic or hypoxic hyporheic and riparian environments and result in deposition of fine sediments above dams. These sediments represent a good opportunity to study human legacies altering soil environments, for which we lack knowledge on microbial structure, depth distribution, and ecological function. Here, we compared high throughput sequencing of bacterial/ archaeal and fungal community structure (diversity and composition) and functional genes (i.e., nitrification and denitrification) at different depths (ranging from 0 to 4 m) in riparian sediments above breached and existing milldams in the Mid-Atlantic United States. We found significant location- and depth-dependent changes in microbial community structure. Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria, Chloroflexi, Acidobacteria, Planctomycetes, Thaumarchaeota, and Verrucomicrobia were the major prokaryotic components while Ascomycota, Basidiomycota, Chytridiomycota, Mortierellomycota, Mucoromycota, and Rozellomycota dominated fungal sequences retrieved from sediment samples. Ammonia oxidizing genes (amoA for AOA) were higher at the sediment surface but decreased sharply with depth. Besides top layers, denitrifying genes (nosZ) were also present at depth, indicating a higher denitrification potential in the deeper layers. However, these results contrasted with in situ denitrification enzyme assay (DEA) measurements, suggesting the presence of dormant microbes and/or other nitrogen processes in deep sediments that compete with denitrification. In addition to enhanced depth stratification, our results also highlighted that dam removal increased species richness, microbial diversity, and nitrification. Lateral and vertical spatial distributions of soil microbiomes (both prokaryotes and fungi) suggest that not only sediment stratification but also concurrent watershed conditions are important in explaining the depth profiles of microbial communities and functional genes in dammed rivers. The results also provide valuable information and guidance to stakeholders and restoration projects.

Joseph Smith Sr.’s China Adventure

Joseph Smith Sr.’s China Adventure

Movement, Survival, and Delays of Atlantic Salmon Smolts in the Piscataquis River, Maine, USA

AbstractMovement, delays, and survival of hatchery Atlantic Salmon Salmo salar smolts were evaluated through the Piscataquis River, a tributary of the Penobscot River in Maine, USA. We explored the effects of the river’s four dams (Guilford, Dover, Browns Mill, and Howland dams) from 2005 to 2019. During this period, the downstream‐most dam (Howland Dam) transitioned from full hydropower generation to seasonal turbine shutdowns and later was decommissioned with the construction of a nature‐like fish bypass in 2016. We estimated survival through open‐river reaches and at each dam using acoustic telemetry (n = 1,611). Dams decreased survival, with per‐river‐kilometer (rkm) apparent survival averages of 0.972, 0.951, and 0.990 for Guilford, Dover, and Browns Mill dams compared to a per‐rkm survival of 0.999 for open‐river reaches. Turbine shutdowns increased survival at Howland Dam (to around 0.95), which was further increased by the nature‐like fish bypass (0.99). We used radiotelemetry in 2019 (n = 75) and demonstrated that approximately one‐third of the fish used the bypass, while the remaining fish used alternative routes. Smolts successfully passing the three upstream dams had lower apparent survival through Howland Dam than smolts that were released upstream of Howland Dam. Although smolts passing Browns Mill Dam had high survival, the dam caused extended delays, with median delay times surpassing 48 h in most years. Most of the delays caused by Browns Mill Dam occurred after fish had passed the dam and may indicate a sublethal effect of passage. Overall, while survival through Howland Dam has improved, passage and delays at the three upstream dams in aggregate represent a critical impediment to the effective use of the high‐quality spawning habitat found upstream.

Decadal-scale evolution of the 2006 Suncook River avulsion, New Hampshire, USA

Avulsions are rare events that cause major realignments to fluvial systems and hold potential for significant societal consequences. We present a case study of an avulsion in a minimally-aggrading landscape that was influenced by long-term human modifications to a fluvial environment that was initially setup for an avulsion during regional Pleistocene deglaciation. The 2006 Suncook River avulsion in New Hampshire, USA, is a valuable example because minimal environmental management took place for a decade following the event, allowing natural processes to dominate initial fluvial responses. Geospatial data collected during this period allow for the evaluation of the short- and long-term morphological evolution of a river that experienced an avulsion. We review data from technical reports on the event and present a new synthesis of aerial imagery and topographic data highlighting key geomorphic features associated with the avulsion. These features include headcutting and widening associated with a 0.6-km long channel created by the avulsion. Downstream of the avulsion site, extensive channel filling, deposition of meter-scale sand sheets on floodplain surfaces, and development of a cutoff channel were major manifestations of the event. The initial setup for the avulsion can be traced to Pleistocene deglaciation, when glaciofluvial ridges deposited in the valley blocked a more direct route for the river and shifted the channel towards an area of shallow bedrock. Human modifications to the channel—including mill dam construction, floodplain narrowing, road development, and gravel mining—contributed to the avulsion setup by increasing superelevation and undermining the glaciofluvial deposits that had directed the river during the Holocene. The avulsion was triggered by a ~50-yr flood event following a 70-yr interval with no major floods but marked by continued human modification to the landscape. The unusual nature of the event combined with the wealth of available geospatial and hydrologic data make the Suncook River system an attractive target for future studies that can examine avulsion processes in greater detail.

Flood risk in past and future: A case study for the Pawtuxet River's record-breaking March 2010 flood event.

In March 2010, a sequence of three major rainfall events in New England (United States) led to a record-breaking flooding event in the Pawtuxet River Watershed with a peak flow discharge of about 500-year return period. After development of hydrological and hydraulic models, a number of factors that played important roles in the impact of this flooding and other extreme events including river structures (reservoirs, historical textile mill dams, and bridges) were investigated. These factors are currently omitted within risk assessments tools such as flood insurance rate maps. Some management strategies that should be considered for future flood risk mitigation were modeled and discussed. Furthermore, to better understand possible future risks in a warmer climate, another extreme flood event was simulated. The synthetic/hypothetical storm (Hurricane Rhody with two landfalls) was created based on the characteristics of the historical hurricanes that severely impacted this region in the past. It was shown that while the first landfall of this hurricane did not lead to significant flood risk, the second landfall could generate more rain and flooding equivalent to a 500-year event. Results and the methodology of this study can be used to better understand and assess future flood risk in similar watersheds.

Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River, Massachusetts, USA

AbstractCenturies‐long intensive land‐use change in the north‐eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest‐clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6‐m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17‐m‐tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium‐137 (137Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment as historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long‐term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source. © 2020 John Wiley & Sons, Ltd.

The use of hydropower potential of the rivers of Moscow basin

The purpose of this article is to study the changes in the ways of using the energy potential of the rivers of the Moscow River basin in the XVIII-XXI centuries. Analysis of the data of the General Survey suggests that in XVIII century the energy of flowing water was used for several kinds of mills: gristmills, saw mills and fulling mills. Energy of flowing water was applied for oil mills and blacksmith hammers. Analysis of literary data shows that the number of mills reduced at the end of XIX century. Only mills built on large rivers continued to work. At the turn of the 19th and 20th centuries small hydroelectric power stations were built instead of mill dams, and the modern hydropower facilities in Moscow basin are small stations of local meaning only. As a result, the maps showing the location of different kinds of mills in the Moscow river basin in XVIII century were composed for the first time. These maps are unique for this territory.

Forgotten Legacies: Understanding and Mitigating Historical Human Alterations of River Corridors

AbstractLegacies are persistent changes in natural systems resulting from human activities. Legacies that affect river ecosystems can result from human alterations outside of the river corridor, such as timber harvest or urbanization, or from alterations within the river corridor, including flow regulation, river engineering, and removal of large wood and beaver dams. Human alterations of river ecosystems have been occurring for thousands of years in some parts of the world and are now ubiquitous, yet both river scientists and the public may be unaware of the persistent effects of historical activities. Failure to recognize the legacy of historical activities that no longer occur can skew perceptions of river process and form and the natural range of variability in river ecosystems. Examples come from rivers of the Mid‐Atlantic Piedmont and the Pacific Northwest regions of the United States. Mid‐Atlantic Piedmont streams in which legacy sediment accumulated behind now‐abandoned mill dams experienced a complete transformation from wide, shallow, marshy valleys to sinuous rivers lined with tall cutbanks, but the existence and the cause of this river metamorphosis was not widely recognized until the first decade of the 21st century. Rivers of the Pacific Northwest from which large wood was removed have changed during the past century from spatially heterogeneous, multichannel systems closely connected to their floodplains via frequent channel avulsion and lateral migration to single‐thread channels with more homogeneous floodplains and less lateral connectivity. Again, this river metamorphosis has only been recognized within the past two decades. In each of these regional examples, river process and form have changed so substantially that the river ecosystems can be described as having assumed an alternative state. In these and many other examples, the alternative state provides lower levels of ecosystem services such as habitat, biodiversity, and attenuation of downstream fluxes of water, sediment, organic carbon, and nutrients. River management designed to enhance and restore these ecosystem services can be more effective if the continuing effects of these historical legacies are recognized. The grand scientific challenges resulting from historical human alterations of river ecosystems include the following: (1) to recognize the existence of a legacy that continues to affect river ecosystem process and form; (2) to understand the source of the legacy with respect to chronology, type, spatial extent, and intensity of human activities; (3) to understand the implications of the legacy regarding how river process and form and river ecosystem services have changed; and (4) to design management or restoration strategies that can mitigate the loss of river ecosystem services. In summary, the existence of forgotten legacies challenges river scientists to recognize the continuing effects of human activities that have long since ceased and also poses challenges for the application of scientific understanding to resource management. Societal expectations for attractive, simple, stable rivers are commonly at odds with scientific understanding of rivers as dynamic, spatially heterogeneous, nonlinear ecosystems. Knowledge of how human actions, including historical actions that have long since ceased, continue to alter river ecosystems can help to bridge the gap between societal and scientific perceptions of rivers.

Historical erosion and sedimentation in two small watersheds of the southern Blue Ridge Mountains, North Carolina, USA

Sediment bodies produced during historical periods of human land use, sometimes referred to as legacy sediment, may be found in various locations within drainage basins, and potentially remobilized by hydrogeomorphic processes accompanying land use change. The amounts and locations of stored legacy sediment can be significant factors in modern drainage basin function and should be accounted for when possible. In this study, late nineteenth-century erosion and sediment storage were investigated and used to construct approximate sediment budgets for two small Blue Ridge Mountain drainage basins in North Carolina (USA). Erosion was quantified using a distributed implementation of the Universal Soil Loss Equation (USLE), and calibrated on the separate bases of soil profile truncation data and recent published rates for long-term erosion in the region. Sediment yield information was reconstructed from pond sediments trapped behind a mill dam. Alluvial sediment storage was quantified using field studies of streambank and floodplain sediment profiles and digital elevation data. Colluvial storage was calculated as a residual in the sediment budget, and further evaluated using soil profile studies of footslope deposits, and soil survey maps. The proportions of erosion accounted for by the different budget terms in the most reliable budget are: 28% sediment yield (i.e., a sediment delivery ratio of 0.28), 69% colluvial storage, and 3% alluvial storage. Blue Ridge basins with low levels of ground disturbance erode like Piedmont basins at high levels of ground disturbance, primarily due to higher slope angles. Sediment delivery ratio is high relative to those given for much larger basins in the adjacent and more frequently studied Piedmont province, and generally in accordance with published sediment delivery curves that reflect higher hydrogeomorphic connectivities within smaller basins. Low values for alluvial storage in Blue Ridge basins may be explained by high sediment transport within steep channels. Colluvial storage values have high uncertainties because of the well known problem of error accumulation in residual budget terms. Field data on colluvial deposits reported here are not sufficient to correct for this problem, and in general, the acquisition of accurate field data on historical colluviation remains an important methodological issue in historical sediment budgeting.

Fish Community Response to a Small‐Stream Dam Removal in a Maine Coastal River Tributary

AbstractSedgeunkedunk Stream, a third‐order tributary to the Penobscot River in Maine, historically has supported several anadromous fishes including Atlantic Salmon Salmo salar, Alewife Alosa pseudoharengus, and Sea Lamprey Petromyzon marinus. Two small dams constructed in the 1800s reduced or eliminated spawning runs entirely. In 2009, efforts to restore marine–freshwater connectivity in the system culminated in removal of the lowermost dam (Mill Dam) providing access to 4.7 km of lotic habitat and unimpeded passage into the lentic habitat of Fields Pond. In anticipation of these barrier removals, we initiated a modified before‐after‐control‐impact study, and monitored stream fish assemblages in fixed treatment and reference sites. Electrofishing surveys were conducted twice yearly since 2007. Results indicated that density, biomass, and diversity of the fish assemblage increased at all treatment sites upstream of the 2009 dam removal. No distinct changes in these metrics occurred at reference sites. We documented recolonization and successful reproduction of Atlantic Salmon, Alewife, and Sea Lamprey in previously inaccessible upstream reaches. These results clearly demonstrate that dam removal has enhanced the fish assemblage by providing an undisrupted stream gradient linking a small headwater lake and tributary with a large coastal river, its estuary, and the Atlantic Ocean.

Fish community dynamics following dam removal in a fragmented agricultural stream

Habitat fragmentation impedes dispersal of aquatic fauna, and barrier removal is increasingly used to increase stream network connectivity and facilitate fish dispersal. Improved understanding of fish community response to barrier removal is needed, especially in fragmented agricultural streams where numerous antiquated dams are likely destined for removal. We examined post-removal responses in two distinct fish communities formerly separated by a small aging mill dam. The dam was removed midway through the 6 year study, enabling passage for downstream fishes affiliated with a connected reservoir into previously inaccessible habitat, thus creating the potential for taxonomic homogenization between upstream and downstream communities. Both communities changed substantially post-removal. Two previously excluded species (white sucker, yellow perch) established substantial populations upstream of the former dam, contributing to a doubling of total fish biomass. Meanwhile, numerical density of pre-existing upstream fishes declined. Downstream, largemouth bass density was inversely correlated with prey fish density throughout the study, while post-removal declines in bluegill density coincided with cooler water temperature and increased suspended and benthic fine sediment. Upstream and downstream fish communities became more similar post-removal, represented by a shift in Bray-Curtis index from 14 to 41 % similarity. Our findings emphasize that barrier removal in highly fragmented stream networks can facilitate the unintended and possibly undesirable spread of species into headwater streams, including dispersal of species from remaining reservoirs. We suggest that knowledge of dispersal patterns for key piscivore and competitor species in both the target system and neighboring systems may help predict community outcomes following barrier removal.

Long‐term storage and transport length scale of fine sediment: Analysis of a mercury release into a river

AbstractExcessive suspended sediment concentrations create important water quality problems, but scientists disagree on how to predict its movement through watersheds. Most models assume that fine‐grained sediment moves rapidly far downstream, without recognizing the importance of episodic, long‐term storage. Here a historic industrial release of mercury is interpreted as a decadal sediment tracer experiment, releasing sediment particles “tagged” with mercury that are deposited on floodplains. As expected, floodplain mercury inventories decrease exponentially downstream, with a characteristic decay length of 10 km (95% confidence interval: 5–25 km) that defines the distance suspended particles typically move downstream before entering storage. Floodplain mercury inventories are not significantly different above and below three colonial age mill dams (present at the time of mercury release but now breached), suggesting that these results reflect ongoing processes. Suspended sediment routing models that neglect long‐term storage, and the watershed management plans based on them, may need revision.

Tectonics, geomorphology and water mill location in Scotland, and the potential impacts of mill dam failure

In this paper we assess the ways in which the topography of glaciated northern Britain has affected the siting and operations of water mills, and compare those factors and mill locations for mills in unglaciated southern Britain. We then explore the impacts of these findings on the potential downstream impacts of mill dam failure.We used a GIS to plot the locations of all 1712 localities in Britain's Ordnance Survey Gazetteer that include “mill”, “milton” (‘milltown’) and “miln” in their name. We then examined the geomorphology of mill locations in two study areas, one in northeast Scotland (glaciated; 421 localities) and one in southern England (unglaciated; 438 localities), assessing (i) mill location within the drainage net, and (ii) the steepness of an adjacent stream within a radius of 500 m of the mill locality. The large majority of mills are located within the first 10 km of the drainage net in both study areas, presumably on relatively stable bedrock channels. The data for most of the mills in both study areas indicate that catchment areas of less than 200 km2 are sufficient to supply the water necessary for operation of a mill, but the higher rainfalls and runoff in Scotland (almost twice the values in the England study area) mean that mill dams in S England must have been higher and of higher capacity than those in NE Scotland. That finding is consistent with the results related to channel steepness, which show that mills in Scotland are associated with steeper channels than is the case in England. The generally greater channel steepness in Scotland (and the greater downstream extent of those steeper channels, as also confirmed by the data) reflect both the many glacially steepened bedrock channel reaches in Scotland and the steepening of Scotland's coastal bedrock channels as a result of glacio-isostatic rebound.The technical requirements of water mill operation favour situations where water can be delivered to the top of, or at least part-way up, the mill wheel. Scotland's steeper rivers and its higher rainfalls mean that Scotland's mills require smaller mill dams, if they are needed at all. It would therefore be expected that catastrophic or managed failure of mill dam walls in northern Britain would release lower volumes of trapped sediment to the downstream fluvial system. These lower volumes would in turn result in lower geomorphological impacts downstream of the dam, both in terms of changing channel patterns and burial of the bed. Such dam failure is a key current issue in geomorphology and one case study of a small failed mill dam in western Scotland confirms the minimal downstream impacts of that failure.

Legacy sediments and historic land use: Chemostratigraphic evidence for excess nutrient and heavy metal sources and remobilization

Major and trace element geochemistry of sediment cores of “legacy” deposits (i.e., sediments eroded from upland areas starting in Colonial times, reflecting intensive land use, that have altered pre-Colonial environments and are impairing modern environments) from behind former mill dams in different land-use sub-watersheds of the Yellow Breeches Creek (Cumberland County, Pennsylvania, USA) provide insight into the origins of elements with multiple natural and anthropogenic sources. In comparing a forested land-use watershed with an agriculturally affected one, we find similar elemental ratios with Al for pre-settlement and early legacy sediments, but increasing trends of nutrient elements and select trace metals through time in the agriculturally affected legacy deposit. Likely sources of excess P, Cu, and Pb over background are fertilizer, pesticides, and other soil amendments. Pb trends show smaller increases over time in the forested land-use subwatershed, mostly due to the input of leaded gasoline emissions to the atmosphere and ubiquitous fallout to all watersheds. The agriculturally affected subwatershed cores show significantly more Pb, suggesting multiple sources. The continued remobilization of legacy sediments from thousands of mill ponds in the eastern United States suggests that this source of nutrients and trace elements may add significantly to the degradation of downstream ecosystems such as the Chesapeake Bay.