Abstract
Studies presenting long-term observations of the recruitment and mobility of large wood in mountain watercourses are scarce, but they can considerably contribute to the knowledge of river/riparian forest interactions and the assessment of flood hazard resulting from wood mobility during floods. Widespread dieback of riparian forest along the headwater course of Kamienica Stream in the Polish Carpathians, caused by bark beetle infestation of spruce trees, has raised concerns about potential increases of large wood recruitment to the stream and of the flood hazard to downstream valley reaches. In October 2009, 429 trees growing along three sections of the stream were tagged with numbered metal plates and monitored over 10 years to determine the timing and causes of their delivery to the channel and the lengths of their displacement during individual flood events. Moreover, in 2012 the mode of location of wood deposits and a degree of wood decay were determined in the second- to fourth-order stream reaches. The monitoring of tagged trees indicated that trees were recruited to the channel during highintensity meteorological and hydrological events, mostly as a result of bank erosion during floods or windthrow. With 22% of tagged trees recruited to the channel during 10 years, the rate of turnover of the riparian trees was estimated at 45 years. As the riparian area is overgrown with trees with ages up to ~160 years, the rate evidences substantial intensification of large wood recruitment to the channel in the recent period. Results of large wood inventory and the 10-year-long monitoring of tagged trees indicated variable mobility of large wood along the upper course of the stream. Wood mobility was negligible in the second-order stream reach, very small in the third-order reach, and greater, but still limited in the fourth-order reach. Wood is transported longer distances only during major floods. However, the advanced state of decay of most pieces leads to their disintegration during floods, precluding distant transport. Thus, large wood retained in the upper stream course does not constitute an important flood hazard to downstream, inhabited valley reaches.
Highlights
Fallen trees and their fragments are an important component of fluvial environment, and their presence in river channels has been recorded in scientific literature since the 1830s (Lyell 1837)
This study aims: (i) to determine the rates and the main factors of wood recruitment to the investigated mountain stream, (ii) to determine the dynamics of large wood in the stream based on the long-term monitoring of tagged trees and the inventory of in-channel wood, and (iii) to infer about the flood hazard to downstream reaches caused by the large wood retained in the stream segment within the national park
The present study considers those characteristics of wood deposits, which are informative about wood dynamics, i.e. the mode of location in the stream and the degree of wood decay
Summary
Fallen trees and their fragments are an important component of fluvial environment, and their presence in river channels has been recorded in scientific literature since the 1830s (Lyell 1837). They are commonly known as large wood, which is generally defined as wood pieces with a length >1 m and a diameter >0.1 m. Logs spanning a channel influence fluvial processes by forming dams and steps that facilitate dissipation of flow energy, reduce sediment transport and increase channel stability (Gurnell et al 2002). Recent studies documented significant geomorphic functions of wood pieces smaller than the standard large wood (with a length ≥0.5 m and a diameter ≥0.1 m, or a length ≥1 m and a diameter ≥0.05 m), that can be very abundant in Carpathian headwater streams (Galia et al 2018a)
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