Sediment and suspended particle interactions during low water flow in a small heterogeneous catchment

The particle transport during a stable dry weather period was investigated in a small, heterogeneous catchment. Particle colours and particle size distributions of surficial fine-grained sediments, suspended particles and riverbank samples were compared to the suspended particle loads and stream velocities in order to increase the understanding of the low flow conditions in the Olewiger Bach. All solid samples show significant, non linear spatial trends of colour. The maxima of the polynomial curves of bed sediments and of the suspended solids show a spatial lag of 500 m and 800 m respectively in comparison to the riverbanksʹ colour maximum. The major result of the investigation is the exclusion of tributaries or riverbank material as dominant suspended particle sources under dry weather conditions. A cross correlation analysis between the solid samples indicates that a highly mobile thin surficial sediment layer enriched in organic carbon is the most dominant source of suspended particles. The spatial patterns of the sediment and of the suspended particle trends can be explained by mixing processes of both solid fractions. Remobilisation processes, even though dominant in the upper basin, occur in the whole longitudinal section. However, local changes of sedimentation and remobilisation show a surprising small scale even in the lower basin where sedimentation predominates. In their temporal sequence the colours of suspended solids within single flood waves reflect the spatial variability in the catchment. Therefore, sedimentary particle colour can be used as an indicator in a ‘fingerprinting’ approach towards identifying active sediment sources.