Echoes in the sediment: reconstructing the Senne through molluscs and caddisflies

The Senne River has shaped the landscape and development of Brussels for centuries. However, its course and environmental conditions have been continuously altered by human interventions. This study compares freshwater mollusc and caddisfly (Trichoptera) remains from two archaeological sites, one Gallo-Roman (1st-3rd century AD) and one medieval (10th-16th century AD), to reconstruct changes in river flow, sedimentation, and water quality over time. At the Gallo-Roman site, a previously unknown river meander of the Senne river was uncovered, initially characterized by fast-flowing water, as indicated by coarse sand and gravel deposits. Over time, sedimentation patterns show a shift to finer materials, such as silt and organic muds, suggesting a transition to slower-moving or stagnant water before the meander was ultimately cut off. The site is located within a rural environment. In contrast, the medieval site, located within the historic centre of Brussels, represents a heavily modified urban river environment. Mollusc remains indicate predominantly slow-moving water, with occasional stagnant conditions. The discovery of caddisfly larvae cases, including the confirmed presence of Brachycentrus subnubilus, suggests that despite extensive waste dumping, parts of the medieval Senne still maintained sufficient water quality for oxygen-dependent species. By combining data from these two sites, this study attempts to document the Senne’s hydrology, sedimentation, and aquatic ecosystems in a rural and urban environment. The findings describe the river’s natural environment alongside increasing human impact, from early riverbank modifications in Gallo-Roman times to the canalization and pollution of the medieval period.

EN, Abstract of an Oral Presentation or a Poster