EXTENT AND CAUSES OF SILTATION IN A HEADWATER STREAM BED: CATCHMENT SOIL EROSION IS LESS IMPORTANT THAN INTERNAL STREAM PROCESSES

The stream bed is a key habitat for many species, and its physicochemical properties govern important ecosystem functions. Stream bed colmation resulting from catchment erosion is considered a core problem of stream headwaters in which many species depending on coarse substrates are in decline. Here, we identify the origin of fine sediment and compare the contribution of external (i.e. catchment erosion) versus internal (i.e. in-stream carbonate precipitation and internal biomass production) sources of siltation using a case study of a pre-alpine river, the Moosach. The stream bed was dominated (51%) by fine sediment of mainly silt size. The average fine sediment thickness of 58 cm and a net deposition rate of 1.32 kg m(-2) yr(-1) confirmed siltation problems. Mineral analyses and mass balancing showed that still only less than 1% of the erosion input into the stream was deposited on the stream bed. Internal temperature-dependent calcite precipitation also removed less than 1% of the calcium from the water column. Thus, by far, most of the fine material from erosion and precipitation was transported and did not contribute to the deposition of fines. Since the colmation problem is governed by the sink, it cannot be solved by exclusively addressing the sources (erosion, precipitation). Despite the well-founded linkage between terrestrial and aquatic systems, this study suggests that restoration of stream bed quality must also focus on in-stream processes and flow regime management, while erosion control remains essential in its own right. Copyright (C) 2017 John Wiley & Sons, Ltd.