The marshy, silted-up channel of the Late Glacial Bergstraßenneckar near Mannheim provided a rare source of clay in the sand- and gravel-dominated environment of the Rhine river. Based on satellite images, sedimentological, geophysical, and historical data, as well as optical dating, we identify a plot of sand-filled trenches where clay was mined around 1865 CE, which was only possible after groundwater lowering resulting from large-scale river regulation starting in the early 19^th century.