Testing the Use of ²¹⁰Pb_ex to Study Sediment Connectivity in a Mediterranean Mountain Basin with Badlands

Abstract

Basin management demands a sound understanding of sediment dynamics, particularly in Mediterranean mountain catchments with badlands, which affect water bodies and freshwater ecosystems. Connectivity has emerged as a framework for understanding how sediments move between geomorphic zones. We analyse the feasibility of excess lead‐210 (210Pbex) to study sediment connectivity in a 4‐km2 Mediterranean mountain basin with badlands (Vallcebre research catchments, Eastern Pyrenees) by applying mass‐balance models for hypothesis generation and experimental testing. Badlands in the area are weathered by freezing during the winter and are eroded in summer by high‐intensity showers. The eroded sediments may remain deposited within the streams from months to years. Application of 210Pbex balance models in our basin suggests: (i) a saw‐tooth seasonal pattern of badland surface 210Pbex activities (increasing from October to May, and depleted in summer) and (ii) a downstream increase in sediment activity due to fallout lead‐210 accumulation in stream‐bed sediment deposits. Our experimental results (obtained during 2013 and 2014) showed low sediment 210Pbex concentrations, illustrating the fresh‐rock origin of sediments, but also hampering their study due to high measurement uncertainty (especially for sediments with d50 > 20 μm) and dependence on sampling methods. Suspended sediment 210Pbex activity reproduced the simulated seasonal activity patterns for the badland surfaces. However, sediment activity decreased downstream, suggesting that flushing of fine sediments by flooding prevents fallout lead‐210 accumulation on the stream bed. Overall, high sediment connectivity was revealed between the badlands, streams and outlet of the basin, as well as the fast transmission of 210Pbex by the finest sediment fraction. Copyright © 2017 John Wiley & Sons, Ltd.

What have we learnt about Mediterranean catchment hydrology? 30 years observing hydrological processes in the Vallcebre Research Catchments

“…Results indicated that fine suspended sediment 210 Pb ex activity follows the seasonal pattern of fallout accumulation during most of the year and erosion by summer storms, and that 210 Pb ex activity in fine suspended sediments decreased towards the catchment outlet. These results suggest high connectivity between the badlands, streams and catchment outlet for fine sediments, but much lower connectivity of sandy or coarser sediments eroded from badland surfaces and untagged by the fallout, as they mainly remain in the stream bed (Moreno de las Heras et al, 2017).…”

Structural and functional control of surface-patch to hillslope runoff and sediment connectivity in Mediterranean dry reclaimed slope systems

“…The spatial arrangement of surface features (e.g. vegetation cover, rills, gullies, channels and terraces) were shown to control the distribution of source and sink elements in these landscapes from a structuralconnectivity perspective, largely driving the production and transference of water and sediments across scales (Cammeraat, 2004;Turnbull et al, 2010;Merino-Martín et al, 2015;Marchamalo et al, 2016;Moreno-de-las-Heras et al, 2019). Structural connectivity can be highly dynamic over long time periods (e.g.…”

Long-term temporal structure of catchment sediment response to precipitation in a humid mountain badland area

“…For instance, in Rainato et al (2017) In that respect, sediment connectivity and sediment transfer from slopes to the main channel is an important factor that should be taken into account. Many different methods have been used and highly variable results have been published in the literature, showing that sediment may remain within the streams from months to years (Moreno-de las Heras et al, 2018;Gallart et al, 2013;Buendia et al, 2016;Llena et al, 2020). For example, Jantzi et al…”