Characterizing landscape‐scale erosion using ¹⁰Be in detrital fluvial sediment: Slope‐based sampling strategy detects the effect of widespread dams

Abstract

Concentrations of in situ 10Be 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 km2) from which we collected river sand samples and measured 10Be concentrations. We used these data to train a predictive model that relates average basin slope and 10Be‐inferred erosion rate. Applying our model to 28 basins in the same region previously studied with 10Be, 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, 10Be 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.

Spatial and temporal replicability of meteoric and in situ10Be concentrations in fluvial sediment

“…() for Tibet ( n = 1, 10 Be i ), Reusser et al . () for the eastern USA ( n = 1, 10 Be i ), Regard et al . () for Cameroon ( n = 3), Nelson et al .…”

“…Sichuan, China (n = 2, 10 Be i ), West et al (2014) for Sichuan, China (n = 9, 10 Be i ), Portenga et al (2015) for Bhutan (n = 1, 10 Be i ), Palumbo et al (2011) for Tibet (n = 1, 10 Be i ), Reusser et al (2017) for the eastern USA (n = 1, 10 Be i ), Regard et al (2016) for Cameroon (n = 3), Nelson et al (2014) for Greenland (n = 9), and Vance et al (2003) for Nepal (n = 1, 10 Be i ). 10 Be i data are shown in red, erosion rate data in blue and 10 Be m in purple.…”

Agricultural land use doubled sediment loads in western China’s rivers

“…For 14 out of 20 sites that we compared, the contemporary sediment yield is higher than the sediment generation rate ( Figure 3B). Of the six remaining sites with higher long term sediment generation rates (ratios range from 0.13 to 0.92), three drain the high, flat Tibetan Plateau and the sparsely populated and rapidly eroding gorges of the Three Rivers Region (specifically, the Salween and Mekong Rivers) and three are samples taken within 2 km downstream of dams (as expected from the analysis of Reusser et al, 2017).…”

“…To make this estimate, we calculated the effective elevation of each watershed using the approach of Portenga and Bierman (2011) (Table DR2). We did not adjust calculations for watersheds with dams, but recognize that this could result in overestimating sediment generation rates for samples taken in close proximity to dams if those dams effectively restrict or cut off sediment supply from upstreamsomething unknowable without extensive fieldwork and details of the dam operations (Reusser et al, 2017). We used the time-invariant scaling scheme of Lal (1991) and Stone (2000) and the global production rate of 10 Be i (Borchers et al, 2016).…”

“…Prior comparisons of contemporary sediment yield with long term sediment generation rates determined by 10 Be i measurements in detrital sediment come to one of four conclusions: 1) the landscape is in mass steady state (Matmon et al, 2003;Wittmann et al, 2011), i.e., has similar background rates of sediment generation and short-term sediment yield; 2) dams have reduced contemporary sediment yield (Syvitski et al, 2005) or raised apparent 10 Be i -derived sediment generation rates (Reusser et al, 2017); 3) stochastic, high-magnitude, low-frequency events not well captured by contemporary sediment yield records increase background sediment generation (Covault et al, 2013;Kirchner et al, 2001b;Schaller et al, 2001); or 4) land use change has increased contemporary sediment yield (Hewawasam et al, 2003;Reusser et al, 2015), at least in the smallest watersheds (Vanmaercke et al, 2015). With the exception of 10 data points (Chappell et al, 2006;Schmidt et al, 2011), these studies have not focused on China, despite China's large population and long history of intensive human land use.…”