The role of log jams and exceptional flood events in mobilizing coarse particulate organic matter in a steep headwater stream

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 endmember in a four-end-member model of CPOM dynamics of steep mountain streams based on wood delivery and export.

Testing models of step formation against observations of channel steps in a steep mountain stream

“…The tests of wall angle (GI.2) and planform curvature (GI.3) support this conclusion: for steps that form at a low jamming ratio the channel narrows (GI.2), and the difference of channel width change above and below the threshold jamming ratio of 5 is statistically significant (Figure 9). This interpretation is in agreement with findings from previous studies in the Erlenbach Jochner et al, 2015). However, this applies only for sedimentary steps; Figure 11b).…”

“…Jochner et al, 2015). Although wood-forced steps make up a minor fraction of the step population (27%), their cumulative height is approximately the same as that of the more frequent sedimentary steps.…”

“…However, jamming does not explain the entire population of steps along the Erlenbach, since 46% of the steps have a jamming ratio above 5 (Figure 7). Large wood logs promote jamming in narrow but not in wider than average channel sections, and log length rather than step height may be the relevant parameter (e.g., Abbe and Montgomery, 2003;Jochner et al, 2015). Thus, jamming cannot be the only mechanism of step formation in the Erlenbach.…”

Wood export prediction at the watershed scale

“…Forested watersheds provide opportunities to study wood budgeting components (Benda and Sias, 2003) in locations around the world (Moulin and Piégay, 2004;Comiti et al, 2006;Andreoli et al, 2007;Latterell and Naiman, 2007;Seo et al, 2008;Warren and Kraft, 2008;Wohl and Goode, 2008;Benda and Bigelow, 2014;Schenk et al, 2014;Jochner et al, 2015;Lucia et al, 2015;Ruiz-Villanueva et al, 2016b;Steeb et al, 2017). Wood recruitment and related processes that store, transport, break down, and ultimately export wood pieces through watershed networks are key elements of stream complexity and ecosystem health (Keller and Swanson, 1979;Harmon et al, 1986;Gregory et al, 2003).…”

River logjams cause frequent large-scale forest die-off events in southwestern Amazonia

“…They can decrease streamflow to such an extent that it leads to bank erosion and overbank flow, influencing channel-floodplain interactions and shaping the evolution of floodplain topography (Montgomery and Abbe, 2006;Sear et al, 2010;Wohl, 2013). Logjams are also an important component of river ecosystems, as they influence the transfer of solutes, mineral sediment, and organic material within the river channel system and between the river and the floodplain (Coe et al, 2009;Gurnell et al, 2002;Jochner et al, 2015;Wohl and Beckman, 2014). Despite the fact that logjams can have a dramatic impact on fluvial dynamics and fluvial-floodplain exchanges, their systematic study is relatively recent and has mostly been carried out in temperate zones (Dixon, 2016;Ruiz-Villanueva et al, 2016;Wohl, 2017Wohl, , 2013.…”