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Abstract
Abstract. Coarse particulate organic matter (CPOM) fulfills important functions in the physical and ecological system of a stream. CPOM delivery to and export from the stream has implications for the stream's morphology and sediment transport capacity as well as the energy budget and food availability. Export rates of CPOM from mountain catchments have been observed to strongly increase with rising discharge, but the mechanism leading to this strong relationship is unclear. Here, we show that log jams in the Erlenbach, a steep headwater stream in the Swiss Prealps, are an effective barrier for the transport of CPOM pieces, and thus become sites of storage of large quantities of material over time. Exceptional discharge events with return periods exceeding 20 years play a dual role in CPOM transport in the Erlenbach. First, they appear to destroy existing log jams, releasing the stored material (wood and sediment). Second, they intensify channel–hillslope coupling, thereby recruiting new logs to the channel, around which new jams can form. This allows for the formulation of a new, fully episodic end-member in a four-end-member model of CPOM dynamics of steep mountain streams based on wood delivery and export.
Highlights
Coarse particulate organic matter (CPOM) in streams is typically defined as organic material with a diameter larger than 1 mm, and encompasses a wide range of different types and sizes, from leaves and twigs to wood fragments and entire trees (Naiman and Sedell, 1979; Bilby and Likens, 1980)
We elucidate the role of exceptional events in log jam stability by employing dendrochronological analyses, and we propose a conceptual model of wood dynamics in headwater streams
Extreme discharge events play a key role in CPOM dynamics in steep mountains streams
Summary
Coarse particulate organic matter (CPOM) in streams is typically defined as organic material with a diameter larger than 1 mm, and encompasses a wide range of different types and sizes, from leaves and twigs to wood fragments and entire trees (Naiman and Sedell, 1979; Bilby and Likens, 1980). CPOM is an important component both of the physical and the ecological system of the stream. It affects stream morphology and alters channel roughness, and flow velocity and sediment transport (Bilby and Ward, 1989). LWD affects stream morphology, habitat and riverine carbon, and nutrient storage by forming log jams that act as barriers for sediment movement, and that are sites of energy dissipation (Wohl and Beckman, 2012; Beckman and Wohl, 2014). Log jams create sites with both lower and higher flow velocities than the average of the stream, and develop habitats for different life stages of both fish and invertebrates (Wohl, 2013)
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