Back to pristine levels: a meta-analysis of suspended sediment transport in large German river channels

Abstract. Suspended sediment is an integral part of riverine transport and functioning that has been strongly altered during the Anthropocene due to the overwhelming human pressure on soils, sediments and the water cycle. Understanding the controls of changing suspended sediment in rivers is therefore vital for effective management strategies. Here we present results from a trend analysis of suspended sediments covering 62 monitoring stations along the German waterways with more than 440 000 water samples taken between 1990 and 2010. Based on daily monitoring of suspended sediment concentration (SSC), we found significant declines of mean annual SSC and annual suspended sediment loads at 49 of 62 monitoring stations between 1990 and 2010. On average SSC declines by -0.92 mg l^-1yr^-1. At some stations decreases during the 20 years represent up to 50 % of the long-term average SSC. Significant decreases of SSC are associated with declining SSL loads. The contemporary suspended sediment loads of the Rhine at the German-Dutch border approaches the natural base level of ~1 Mt yr^-1, which was achieved by the Rhine during the mid-Holocene when the suspended sediment load was adjusted to the Holocene climatic conditions and before the onset of increased loads due to human induced land use changes in the Rhine catchment. At this point we can only speculate regarding potential reasons for a decline in sediment supply to larger rivers. We argue that changes in soil erosion within the catchments and/or the sediment connectivity in upstream headwaters, e.g. due to the construction of small rainwater retention basins, are the major reason for declining SSC in the studied river channels.