Geomorphic and stream flow influences on large wood dynamics and displacement lengths in high gradient mountain streams (Chile)

Abstract

Understanding large wood (LW; ≥1 m long and ≥10 cm in diameter) dynamics in rivers is critical for many disciplines including those assessing flood hazard and risk. However, our understanding of wood entrainment and deposition is still limited, mainly because of the lack of long‐term monitoring of wood‐related processes. The dataset presented here was obtained from more than 8 years of monitoring of 1,264 tagged wood pieces placed in 4 low‐order streams of the Chilean mountain ranges and was used to further our understanding of key factors controlling LW dynamics. We show that LW displacement lengths were longer during periods when peak‐flow water depths (Hmax) exceeded the bankfull stage (HBk) than in periods with Hmax ≤ HBk and that these differences were significantly higher for smaller wood pieces. LW length and length relative to channel dimensions were the main factors governing LW entrainment; LW displacement lengths were inversely related to the ratio of piece length to H15% (i.e., the level above which the flow remains for 15% of the time) and to the ratio of H15% to bankfull width. Unrooted logs and LW pieces located at the bankfull stage travelled significantly longer distances than logs with attached rootwads and those located in other positions within the bankfull channel. A few large logjams were broken during the period of observation, and in all occasions, LW from these broken logjams did not travel over longer distances than other pieces of LW moved in the same periods and in the same stream segments. Most importantly, our work reveals that LW dynamics tend to be concentrated within a few reaches in each stream and that reaches exhibiting high wood dynamics (extensive entrainment, deposition, or repositioning of LW) are significantly wider and less steep than less dynamic reaches.

Large wood load and transport in a flood‐free period within an inter‐dam reach: a decade of monitoring the Dyje River, Czech Republic

“…Likewise, we also recorded a significant annual wood exchange, but the total amount of wood underwent a significant change throughout the monitored period at the same time. Consistent with Tonon et al 2018and Iroumé et al (2018), we showed that mobilization and deposition of LW also occur during low-flow conditions below the bankfull stage. Downs and Simon (2001) documented accelerated LW input due to bank instability caused by knickpoint migration and channel incision.…”

Numerical Modeling of Instream Wood Transport, Deposition, and Accumulation in Braided Morphologies Under Unsteady Conditions: Sensitivity and High‐Resolution Quantitative Model Validation

“…The presence of roots or the size distribution of supplied wood are controlling wood motion and deposition as well. Logs with roots were less mobile (both in the flume and in the numerical simulations) and traveled significantly shorter distances than logs without roots in agreement with previous studies (Abbe & Montgomery, 1996;Braudrick & Grant, 2000;Cadol & Wohl, 2010;Iroumé et al, 2018;Welber et al, 2013). According to the observations made during the flume experiments, we observe that logs with roots aligned parallel to the flow with their rootwads located upstream were, in some cases, slowly pushed downstream by the drag force exerted on the rootwad, but that the rootwad prevented logs from rolling and, therefore, limited their mobility (Welber et al, 2013).…”

Reflections on the history of research on large wood in rivers

“…Studies of the geomorphic and ecological functions of large wood may come next (Montgomery et al, 2003). Ultimately, short-term studies may evolve into long-term programmes (e.g., Wohl and Goode, 2008;Iroumé et al, 2015Iroumé et al, , 2018aIroumé et al, , 2018b. Such studies can make important contributions in terms of direct observation of gradual processes and infrequent events, which are an important part of large wood dynamics.…”