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Concentrations of in situ Be-10 measured in detrital fluvial sediment are frequently used to estimate long-term erosion rates of drainage basins. In many regions, basin-averaged erosion rates are positively correlated with basin average slope. The slope dependence of erosion allows model-based erosion rate estimation for unsampled basins and basins where human disturbance may have biased cosmogenic nuclide concentrations in sediment. Using samples collected from southeastern North America, we demonstrate an approach that explicitly considers the relationship between average basin slope and erosion rate. Because dams and reservoirs are ubiquitous on larger channels in the field area, we selected 36 undammed headwater subbasins (average area 10.6 km(2)) from which we collected river sand samples and measured Be-10 concentrations. We used these data to train a predictive model that relates average basin slope and Be-10-inferred erosion rate. Applying our model to 28 basins in the same region previously studied with Be-10, we find that the model successfully predicts erosion rates for basins of different sizes if they are undammed or if samples were collected >25 km downstream of dams. For samples collected closer to dams, measured erosion rates exceed modeled erosion rates for two-thirds of the samples. In three of four cases where paired samples were collected upstream of reservoirs and downstream of the impounding dam, Be-10 concentrations were lower downstream. This finding has implications for detrital cosmogenic studies, whether or not samples were collected directly downstream of dams, because dams obstruct most major rivers around the world, effectively trapping sediment that originated upstream.

Plain Language Summary Measuring the rate at which Earth eroded before humans arrived and changed the landscape is tricky business. Over the last two decades, geologists have used a collection of chemical and isotopic techniques, based on counting atoms rare in nature, to estimate how quickly parts of the landscape erode. They've collected and analyzed thousands of samples and now we know that some regions of the globe erode so rapidly that the landscape changes a few centimeters every hundred years whereas other landscape are so stable, only a hair's width of rock will erode per millennium. The problem is that dams can get in the way. Working in the heavily dammed southeastern United States, we show that samples collected within 25 kilometers of dams often lead to inaccurate results, overestimating long term erosion rates. We conclude that recent, human impacts on the landscape are likely even more damaging than previously thought, stripping soil and farm land at an unsustainable rate.