Understanding the geomorphic consequences of enhanced overland flow in mixed agricultural systems: sediment fingerprinting demonstrates the need for integrated upstream and downstream thinking

Abstract

Sediment fingerprinting approaches help improve our understanding of the on- and off-site impacts of soil erosion in the critical zone, including upstream and downstream ecosystem services and the wider implications for food, water and energy security. This study evaluates the impact of intensive agricultural land management practices on overland flow generation with attention to process controls on sediment production and delivery from both hillslope and in channel sources. This study applied a sediment fingerprinting approach in the Merriott Stream catchment in rural Somerset, UK. Major and minor element geochemistry were determined for sources under cultivated (n = 12) and grazing (n = 12) land management regimes plus channel bank erosion (n = 10) for a notably incised downstream drainage network. These were compared to both suspended sediment (n = 3) and channel bed sediment (n = 4) sampled along the stream network. Sediment sources were apportioned with the MixSIAR mixing model. The dominant contributor to suspended sediment captured in transit was cultivated land (ca. 60–90%). The dominant contributor to channel bed sediment in the lower catchment was channel bank erosion (ca. 50–75%). The former was linked to enhanced overland flow and increased erodibility of cultivated soils. The latter was linked to contemporary incision of the stream network driven by a more erosive hydrological regime (i.e. increased magnitude and frequency of runoff events) which was causing localized erosion and bank failure. These in-channel processes of incision and bank failure were exacerbated by a lack of vegetation on the banks due to winter die-back of the invasive weed Himalayan Balsam. This exposes the bank surface to direct fluvial scour under winter high flows and prevents the establishment of a perennial root structure from native plants that typically plays an important role in bank stability. This study has demonstrated the need to consider a holistic catchment-based approach to understanding and managing bank erosion and, in particular, it highlights the need for integrated upstream and downstream thinking where distal runoff processes lead to increased potential for downstream channel instability.