Hypothesized feedbacks between climate and tectonics are mediated by the relationship between top... more Hypothesized feedbacks between climate and tectonics are mediated by the relationship between topography and long-term erosion rates. While many studies show monotonic relationships between channel steepness and erosion rates, the degree of nonlinearity in this relationship varies by landscape. Mechanistically explaining controls on this relationship in natural settings is critical because highly nonlinear relationships imply low sensitivity between climate and tectonics. To this end, we present a coordinated analysis of cosmogenic 10Be concentrations in river sands paired with topographic, hydroclimatic, and tectonic data for the Greater Caucasus Mountains where topography is invariant along-strike despite large gradients in modern precipitation and convergence rates. We show that spatial patterns in erosion rates largely reflect regional tectonics with little sensitivity to mean precipitation or runoff. The nonlinearity in the erosion rate – steepness relationship may arise from v...
The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil pro... more The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.
Abstract The Atacama Desert contains the driest regions on Earth, with significant rain occurring... more Abstract The Atacama Desert contains the driest regions on Earth, with significant rain occurring only a few times per century, based on sparse historical records. However, the frequency and magnitude of rainfall remains speculative. On March 24–26 of 2015, an unusual storm caused rainfall rates and quantities to exceed many historical records. Of interest is whether this storm was able to activate geomorphic processes whose impacts are evident on numerous landscape features. Here, the results of a reconnaissance from N to S transecting through the plant-free expanse of the Atacama Desert, between 22 and 26° S, are examined in relation to evidence of past runoff activity coupled with soil architecture and soil hydraulic properties. The results suggest the rain initiated some minor runoff processes on the upper hillslopes. However, the rainfall was too small to reactivate many features that appear to be driven by larger, less frequent storms. The field evidence suggests that larger scale rainfalls have occurred throughout the Quaternary, and that there are fossilized (or infrequently active) features in various stages of “repair” that provide evidence of rainfall re-occurrence. The landscapes largely escaped overland flow alteration due to the high infiltration rate capacity caused by the salt-rich soils, which we estimated to average 78 mm h−1 for hillslopes and 244 mm h−1 for alluvial soils, based on disc infiltrometer measurements. This gives a resilience, and potential rainfall threshold, to alteration by intensive rainfall events. Published paleoclimatic records coupled with evidence from soil examined at the arid/hyperarid periphery of the desert show evidence of a cessation of carbonate formation since ∼11 ka, a time of aridification similar to the drying of lakes and marshlands in the hyperarid region. Thus, the past fluvial alteration features are likely to be, at least partially, remnant Pleistocene features which have been largely unaffected by Holocene events, whose magnitudes were similar to that of 2015.
Since the Plio-Pleistocene, southward migration of shortening in the eastern part of the Greater ... more Since the Plio-Pleistocene, southward migration of shortening in the eastern part of the Greater Caucasus (GC) into the Kura foreland basin has progressively formed the Kura-Fold Thrust belt (KFTB) and Alazani piggyback basin, which separates the KFTB from the GC. Previous work argued for an eastward propagation of the KFTB, implying that the western portion in Georgia is the oldest, but this hypothesis was based on coarse geologic maps and speculative ages for units within the KFTB. Here we investigate this hypothesis and focus on the Gombori Range (GR), which defines NW edge of the belt. Previous work divided the sediments of northern flank of the range into three facies. The rock types in the older and middle facies suggest a GC source provenance, despite the modern drainage network in the NE GR, which is dominated by NE flowing rivers.Paleocurrent analyses of the alluvial conglomerates of the oldest and youngest syntectonic units indicate a switch from dominantly SW directed pal...
Rehydroxylation (RHX) dating was recently suggested as a simple, cheap, and accurate method for d... more Rehydroxylation (RHX) dating was recently suggested as a simple, cheap, and accurate method for dating ceramics. It depends on the constant rate of rehydroxylation (the slow reintroduction of OH) of clays after they are fired and dehydroxylated (purged of OH) during the production of pots, bricks, or other ceramics. The original firing of the ceramic artifact should set the dating clock to zero by driving all hydroxyls out of the clay chemical structure. To examine whether this assumption holds, especially for pot firings of short duration and low intensity, as those in small-scale traditional settings, we performed thermogravimetric analysis of clay samples of known mineralogy at temperatures and for durations reported from traditional sub-Saharan, American, and South Asian pottery firings. Results demonstrate that in the majority of samples, complete dehydroxylation (DHX) did not occur within, or even beyond, the conditions common in traditional firings. Consequently, between 0.01 and 1.5% of a sample's mass in residual OH may remain after firings analogous to those observed in the ethnographic record. Lack of complete DHX at the scales we have observed can result in the over-estimation of ceramic ages by decades to tens of thousands of years, depending largely on the age of the sample, and the amount of residual OH present. Thus, in many cases, a key assumption underlying current RHX dating methods is unlikely to have been met, introducing considerable error in dates.
Quantifying erosion rates in high mountain environments is challenging due to the stochastic and ... more Quantifying erosion rates in high mountain environments is challenging due to the stochastic and often catastrophic nature of the processes. Better constraints on these rates are critical to further our understanding of the couplings between tectonic and climatic forces and surface processes. In the great range of the Himalaya, quantifying erosion rates is especially important to help untangle the role
The emergence of coupled natural and human landscapes marked a transformative interval in the hum... more The emergence of coupled natural and human landscapes marked a transformative interval in the human past that set our species on the road to the urbanized, industrial world in which we live. This emergence enabled technologies and social institutions responsible for human-natural couplings in domains beyond rural, agricultural settings. The Mediterranean Landscape Dynamics Project (MedLand) is studying the interacting social and biophysical processes associated with these novel socioecological systems and their long-term consequences using a new form of 'experimental socioecology' made possible by recent advances in computation. We briefly describe the MedLand modeling laboratory, a hybrid simulation environment that couples models of smallholder farming and herding, landscape evolution, and vegetation change managed through an interaction model. We then review three examples of experimental socioecology carried out in this laboratory. These examples offer new insights for scaledependent thresholds in agropastoral productivity, long-term sustainability of alternative land-use strategies, and identifying signatures of human and climate-driven landscape dynamics. We conclude with an overview of new directions for this interdisciplinary research on Anthropocene human-earth systems, including: modeling more diverse decision-making strategies for land-use, developing more sophisticated models of vegetation dynamics and fire ecology, and generating digital proxy data for more robust model validation against the empirical record.
We use the hybrid modeling laboratory of the Mediterranean Landscape Dynamics (MedLanD) Project t... more We use the hybrid modeling laboratory of the Mediterranean Landscape Dynamics (MedLanD) Project to simulate barranco incision in eastern Spain under different scenarios of natural and human environmental change. We carry out a series of modeling experiments set in the Rio Penaguila valley of northern Alicante Province. The MedLanD Modeling Laboratory (MML) is able to realistically simulate gullying and incision in a multi-dimensional, spatially explicit virtual landscape. We first compare erosion modeled in wooded and denuded landscapes in the absence of human land-use. We then introduce simulated smallholder (e.g., prehistoric Neolithic) farmer/herders in six experiments, by varying community size (small, medium, large) and land management strategy (satisficing and maximizing). We compare the amount and location of erosion under natural and anthropogenic conditions. Natural (e.g., climatically induced) land-cover change produces a distinctly different signature of landscape evolution than does land-cover change produced by agropastoral land-use. Human land-use induces increased coupling between hillslopes and channels, resulting in increased downstream incision.
This chapter was originally published in the Treatise on Geomorphology, the copy attached is prov... more This chapter was originally published in the Treatise on Geomorphology, the copy attached is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use. This includes without limitation use in instruction at your institution, distribution to specific colleagues, and providing a copy to your institution's administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution's website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at:
ABSTRACT The Tertiary paleoclimate of the Atacama Desert is poorly known and widely debated. Post... more ABSTRACT The Tertiary paleoclimate of the Atacama Desert is poorly known and widely debated. Post-10 Ma climate has at least one early period of hyperaridity (Resh et al., 2006, Geology, 34:761), and multiple lines of evidences for a strong and nearly-continuous post-Pliocene hyper-aridification largely coincident with changes in Pacific sea surface temperatures (Amundson et al., in press). In 2010, we examined a sequence of pre-10 Ma gravelly, continental sediments exposed at the El Tesoro Mine, near Calama Chile. The gravel underlies a 10 Ma ignimbrite, and the geomorphic surfaces above this layer consist of largely incised fluvial surfaces of Miocene (?) to recent age. Several tuff samples from nearby the mine have been identified and are being processed for 40Ar/39Ar geochronological analyses. Here, we focus on the ~300 m sequence of gravels we examined that contained a sequence of 22 or more paleosols (22° 57' S, 69° 5' W). The paleosols were developed on an aggrading alluvial fan system from streams largely to the east, and lie above the mineralized gravels that host the copper ore body. The soils are extremely well preserved due to minimal burial and little aqueous diagenesis. The remarkable feature of the paleosols is that they represent oscillating soil forming conditions between chemical weathering/clay production (humid: Alfisols) to environments that facilitated the accumulation of pedogenic carbonate (arid to semi-arid: Aridisols). All carbonate bearing samples are being analyzed for stable C and O isotopes. All bulk samples are being analyzed for total geochemical composition, to yield an examination of chemical alteration. Samples from the two oldest paleosols in the sequence (Calcids or Calciarids) average δ13C and δ18O values (VPDB) of ca. -3.1 and -4.2 respectively. Presently, the local region is a sulfate-, rather than carbonate-, forming soil environment. At elevations exceeding ~2500 m, carbonates begin to appear. Based on analyses of nearby sites made by Quade, et al. (2007, Geochimica, 71:3772), soils with similar carbonate isotope compositions are found about 1000 m higher in elevation (however the modern soils lack the degree of carbonate accumulation found in the paleosols). Given that post Miocene uplift in this area has been estimated to be on the order of 1 km (Rech et al., 2006, Geology, 34:761), the paleosol sequence suggests that climatic conditions at 1200 m or less, pre-10 Ma, oscillated from arid to humid conditions with enough moisture to periodically strip soils of salts and form clay. The El Tesoro soils provide a unique perspective relative to existing paleosol records from the same time period. Paleosols from the eastern Andes (Quade et al., 2007, Geochimica, 71:3772; Garzione et al., 2008, Science, 320:1304) reveal a decrease in O isotope ratios (due to the elevation effect of precipitation in that region) and an increase in C isotope ratios of soil carbonates, due to increasing aridity.
Dams greatly influence water and sediment discharge regimes and can have significant impacts on c... more Dams greatly influence water and sediment discharge regimes and can have significant impacts on channel morphology and sediment storage. By using the short-lived fallout radionuclide 7 Be (t 1/2 = 53.4 days) as a tracer of fine (~0.25-2 mm) bed material load transport, we capture the sedimentological and geomorphic impacts of the Union Village Dam, located on the Ompompanoosuc River, in eastern Vermont, USA. We measured 7 Be activities in approximately monthly samples from streambed sediments in a regulated stream and an unregulated control stream. In the regulated stream our sampling spanned an array of management conditions during the annual transition from flood control in the winter and early spring to run-of-the-river operation from late spring to autumn. Because sediment stored behind the dam during the winter quickly became depleted in 7 Be activity, it became possible to track this plug of "dead" sediment as it moved downstream. Measured average sediment transport velocities (30-80 m day-1) exceed those typically reported for bulk bed-load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in the bed sand fraction. Results also show that the length scale of the downstream impact of this dam management (winter pool and summer run-of-the river with minimal sediment trapping efficiency) on sediment transport can be short (~1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from point bars, tributaries and other downstream sources. The benthic community structure indicates significantly greater abundance of caddisflies downstream of the dam due primarily to a lack of bed disturbance following impoundment.
Hypothesized feedbacks between climate and tectonics are mediated by the relationship between top... more Hypothesized feedbacks between climate and tectonics are mediated by the relationship between topography and long-term erosion rates. While many studies show monotonic relationships between channel steepness and erosion rates, the degree of nonlinearity in this relationship varies by landscape. Mechanistically explaining controls on this relationship in natural settings is critical because highly nonlinear relationships imply low sensitivity between climate and tectonics. To this end, we present a coordinated analysis of cosmogenic 10Be concentrations in river sands paired with topographic, hydroclimatic, and tectonic data for the Greater Caucasus Mountains where topography is invariant along-strike despite large gradients in modern precipitation and convergence rates. We show that spatial patterns in erosion rates largely reflect regional tectonics with little sensitivity to mean precipitation or runoff. The nonlinearity in the erosion rate – steepness relationship may arise from v...
The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil pro... more The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.
Abstract The Atacama Desert contains the driest regions on Earth, with significant rain occurring... more Abstract The Atacama Desert contains the driest regions on Earth, with significant rain occurring only a few times per century, based on sparse historical records. However, the frequency and magnitude of rainfall remains speculative. On March 24–26 of 2015, an unusual storm caused rainfall rates and quantities to exceed many historical records. Of interest is whether this storm was able to activate geomorphic processes whose impacts are evident on numerous landscape features. Here, the results of a reconnaissance from N to S transecting through the plant-free expanse of the Atacama Desert, between 22 and 26° S, are examined in relation to evidence of past runoff activity coupled with soil architecture and soil hydraulic properties. The results suggest the rain initiated some minor runoff processes on the upper hillslopes. However, the rainfall was too small to reactivate many features that appear to be driven by larger, less frequent storms. The field evidence suggests that larger scale rainfalls have occurred throughout the Quaternary, and that there are fossilized (or infrequently active) features in various stages of “repair” that provide evidence of rainfall re-occurrence. The landscapes largely escaped overland flow alteration due to the high infiltration rate capacity caused by the salt-rich soils, which we estimated to average 78 mm h−1 for hillslopes and 244 mm h−1 for alluvial soils, based on disc infiltrometer measurements. This gives a resilience, and potential rainfall threshold, to alteration by intensive rainfall events. Published paleoclimatic records coupled with evidence from soil examined at the arid/hyperarid periphery of the desert show evidence of a cessation of carbonate formation since ∼11 ka, a time of aridification similar to the drying of lakes and marshlands in the hyperarid region. Thus, the past fluvial alteration features are likely to be, at least partially, remnant Pleistocene features which have been largely unaffected by Holocene events, whose magnitudes were similar to that of 2015.
Since the Plio-Pleistocene, southward migration of shortening in the eastern part of the Greater ... more Since the Plio-Pleistocene, southward migration of shortening in the eastern part of the Greater Caucasus (GC) into the Kura foreland basin has progressively formed the Kura-Fold Thrust belt (KFTB) and Alazani piggyback basin, which separates the KFTB from the GC. Previous work argued for an eastward propagation of the KFTB, implying that the western portion in Georgia is the oldest, but this hypothesis was based on coarse geologic maps and speculative ages for units within the KFTB. Here we investigate this hypothesis and focus on the Gombori Range (GR), which defines NW edge of the belt. Previous work divided the sediments of northern flank of the range into three facies. The rock types in the older and middle facies suggest a GC source provenance, despite the modern drainage network in the NE GR, which is dominated by NE flowing rivers.Paleocurrent analyses of the alluvial conglomerates of the oldest and youngest syntectonic units indicate a switch from dominantly SW directed pal...
Rehydroxylation (RHX) dating was recently suggested as a simple, cheap, and accurate method for d... more Rehydroxylation (RHX) dating was recently suggested as a simple, cheap, and accurate method for dating ceramics. It depends on the constant rate of rehydroxylation (the slow reintroduction of OH) of clays after they are fired and dehydroxylated (purged of OH) during the production of pots, bricks, or other ceramics. The original firing of the ceramic artifact should set the dating clock to zero by driving all hydroxyls out of the clay chemical structure. To examine whether this assumption holds, especially for pot firings of short duration and low intensity, as those in small-scale traditional settings, we performed thermogravimetric analysis of clay samples of known mineralogy at temperatures and for durations reported from traditional sub-Saharan, American, and South Asian pottery firings. Results demonstrate that in the majority of samples, complete dehydroxylation (DHX) did not occur within, or even beyond, the conditions common in traditional firings. Consequently, between 0.01 and 1.5% of a sample's mass in residual OH may remain after firings analogous to those observed in the ethnographic record. Lack of complete DHX at the scales we have observed can result in the over-estimation of ceramic ages by decades to tens of thousands of years, depending largely on the age of the sample, and the amount of residual OH present. Thus, in many cases, a key assumption underlying current RHX dating methods is unlikely to have been met, introducing considerable error in dates.
Quantifying erosion rates in high mountain environments is challenging due to the stochastic and ... more Quantifying erosion rates in high mountain environments is challenging due to the stochastic and often catastrophic nature of the processes. Better constraints on these rates are critical to further our understanding of the couplings between tectonic and climatic forces and surface processes. In the great range of the Himalaya, quantifying erosion rates is especially important to help untangle the role
The emergence of coupled natural and human landscapes marked a transformative interval in the hum... more The emergence of coupled natural and human landscapes marked a transformative interval in the human past that set our species on the road to the urbanized, industrial world in which we live. This emergence enabled technologies and social institutions responsible for human-natural couplings in domains beyond rural, agricultural settings. The Mediterranean Landscape Dynamics Project (MedLand) is studying the interacting social and biophysical processes associated with these novel socioecological systems and their long-term consequences using a new form of 'experimental socioecology' made possible by recent advances in computation. We briefly describe the MedLand modeling laboratory, a hybrid simulation environment that couples models of smallholder farming and herding, landscape evolution, and vegetation change managed through an interaction model. We then review three examples of experimental socioecology carried out in this laboratory. These examples offer new insights for scaledependent thresholds in agropastoral productivity, long-term sustainability of alternative land-use strategies, and identifying signatures of human and climate-driven landscape dynamics. We conclude with an overview of new directions for this interdisciplinary research on Anthropocene human-earth systems, including: modeling more diverse decision-making strategies for land-use, developing more sophisticated models of vegetation dynamics and fire ecology, and generating digital proxy data for more robust model validation against the empirical record.
We use the hybrid modeling laboratory of the Mediterranean Landscape Dynamics (MedLanD) Project t... more We use the hybrid modeling laboratory of the Mediterranean Landscape Dynamics (MedLanD) Project to simulate barranco incision in eastern Spain under different scenarios of natural and human environmental change. We carry out a series of modeling experiments set in the Rio Penaguila valley of northern Alicante Province. The MedLanD Modeling Laboratory (MML) is able to realistically simulate gullying and incision in a multi-dimensional, spatially explicit virtual landscape. We first compare erosion modeled in wooded and denuded landscapes in the absence of human land-use. We then introduce simulated smallholder (e.g., prehistoric Neolithic) farmer/herders in six experiments, by varying community size (small, medium, large) and land management strategy (satisficing and maximizing). We compare the amount and location of erosion under natural and anthropogenic conditions. Natural (e.g., climatically induced) land-cover change produces a distinctly different signature of landscape evolution than does land-cover change produced by agropastoral land-use. Human land-use induces increased coupling between hillslopes and channels, resulting in increased downstream incision.
This chapter was originally published in the Treatise on Geomorphology, the copy attached is prov... more This chapter was originally published in the Treatise on Geomorphology, the copy attached is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use. This includes without limitation use in instruction at your institution, distribution to specific colleagues, and providing a copy to your institution's administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution's website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at:
ABSTRACT The Tertiary paleoclimate of the Atacama Desert is poorly known and widely debated. Post... more ABSTRACT The Tertiary paleoclimate of the Atacama Desert is poorly known and widely debated. Post-10 Ma climate has at least one early period of hyperaridity (Resh et al., 2006, Geology, 34:761), and multiple lines of evidences for a strong and nearly-continuous post-Pliocene hyper-aridification largely coincident with changes in Pacific sea surface temperatures (Amundson et al., in press). In 2010, we examined a sequence of pre-10 Ma gravelly, continental sediments exposed at the El Tesoro Mine, near Calama Chile. The gravel underlies a 10 Ma ignimbrite, and the geomorphic surfaces above this layer consist of largely incised fluvial surfaces of Miocene (?) to recent age. Several tuff samples from nearby the mine have been identified and are being processed for 40Ar/39Ar geochronological analyses. Here, we focus on the ~300 m sequence of gravels we examined that contained a sequence of 22 or more paleosols (22° 57' S, 69° 5' W). The paleosols were developed on an aggrading alluvial fan system from streams largely to the east, and lie above the mineralized gravels that host the copper ore body. The soils are extremely well preserved due to minimal burial and little aqueous diagenesis. The remarkable feature of the paleosols is that they represent oscillating soil forming conditions between chemical weathering/clay production (humid: Alfisols) to environments that facilitated the accumulation of pedogenic carbonate (arid to semi-arid: Aridisols). All carbonate bearing samples are being analyzed for stable C and O isotopes. All bulk samples are being analyzed for total geochemical composition, to yield an examination of chemical alteration. Samples from the two oldest paleosols in the sequence (Calcids or Calciarids) average δ13C and δ18O values (VPDB) of ca. -3.1 and -4.2 respectively. Presently, the local region is a sulfate-, rather than carbonate-, forming soil environment. At elevations exceeding ~2500 m, carbonates begin to appear. Based on analyses of nearby sites made by Quade, et al. (2007, Geochimica, 71:3772), soils with similar carbonate isotope compositions are found about 1000 m higher in elevation (however the modern soils lack the degree of carbonate accumulation found in the paleosols). Given that post Miocene uplift in this area has been estimated to be on the order of 1 km (Rech et al., 2006, Geology, 34:761), the paleosol sequence suggests that climatic conditions at 1200 m or less, pre-10 Ma, oscillated from arid to humid conditions with enough moisture to periodically strip soils of salts and form clay. The El Tesoro soils provide a unique perspective relative to existing paleosol records from the same time period. Paleosols from the eastern Andes (Quade et al., 2007, Geochimica, 71:3772; Garzione et al., 2008, Science, 320:1304) reveal a decrease in O isotope ratios (due to the elevation effect of precipitation in that region) and an increase in C isotope ratios of soil carbonates, due to increasing aridity.
Dams greatly influence water and sediment discharge regimes and can have significant impacts on c... more Dams greatly influence water and sediment discharge regimes and can have significant impacts on channel morphology and sediment storage. By using the short-lived fallout radionuclide 7 Be (t 1/2 = 53.4 days) as a tracer of fine (~0.25-2 mm) bed material load transport, we capture the sedimentological and geomorphic impacts of the Union Village Dam, located on the Ompompanoosuc River, in eastern Vermont, USA. We measured 7 Be activities in approximately monthly samples from streambed sediments in a regulated stream and an unregulated control stream. In the regulated stream our sampling spanned an array of management conditions during the annual transition from flood control in the winter and early spring to run-of-the-river operation from late spring to autumn. Because sediment stored behind the dam during the winter quickly became depleted in 7 Be activity, it became possible to track this plug of "dead" sediment as it moved downstream. Measured average sediment transport velocities (30-80 m day-1) exceed those typically reported for bulk bed-load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in the bed sand fraction. Results also show that the length scale of the downstream impact of this dam management (winter pool and summer run-of-the river with minimal sediment trapping efficiency) on sediment transport can be short (~1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from point bars, tributaries and other downstream sources. The benthic community structure indicates significantly greater abundance of caddisflies downstream of the dam due primarily to a lack of bed disturbance following impoundment.
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