Wood quantities and transport in the Avançon de Nant river, Switzerland.

By creating pools and retaining sediment and organic matter, instream wood provides habitats for a vast variety of different species. It creates a complex river bed and is essential for a healthy ecosystem (Wohl et al., 2019). However, during extreme weather conditions, floods can mobilize the wood and transport it, causing a hazard to downstream infrastructure. Therefore it is important better understand river wood dynamics, such as storage and transport regimes. These regimes are influences by individual log characteristics (e.g. shape, density and orientation), but also individual river weather, climate and geographical factors. In the last decade, an increasing amount of case studies have been performed, although still limited in amount of logs tracked in European rivers (Wyzga et al., 2017). In our current contribution, we deploy a tracking and monitoring system in an Alpine river in the canton of Vaud, Switzerland. The Avancon the Nant is located in the Vallon de Nant, a valley that has been protected since 1969 (Vittoz and Gmür, 2009), and can therefore be argued to have a close to natural wood regime.

Figure: Locations of instream wood in 2022 as compared to 2021. In grey, 3 special sections (wider sections and sections with multiple streams) of river are represented.

In the summer of 2021, 948 (0001 to 0948) pieces of instream wood were tagged with a unique number and 2 unique RFID tags. One year later, in another field campaign, the movement of the pieces was assessed (see figure). From the pieces that have been recovered (7% were lost), a total of 20 pieces were found to have moved with an average of 260 meters. These movements took place in specific sections, primarily in single-threaded narrow sections. The two lower special river sections (w1 and w2) were found to contain pieces with a larger diameters as compared to the other sections. As the tree density decreases when moving up the river, also the total volume of wood storage and the amount of pieces decreased. Furthermore, more pieces with a high degree of decat were found as compared to fresher pieces. This indicated that in recent years, less wood recruitment has taken place. 

REFERENCES  

Vittoz, P., & Gmür, P. 2009: Introduction aux Journées de la biodiversité dans le Vallon de Nant (Bex, Alpes vaudoises), Mémoire de la Société vaudoise des Sciences naturelles, 23, 3-20. 

Wohl, E., Kramer, N., Ruiz-Villanueva, V., Scott, D. N., Comiti, F., Gurnell, A. M., Piegay, H., Lininger, K. B., Jaeger, K. L., Walters, D. M., & Fausch, K. D. 2019: The natural wood regime in rivers, BioScience, 69, 259–273. 

Wyzga, B., Mikus, P., Zawiejska, J., Ruiz-Villanueva, V., Kaczka, R. J. & Czech, W. 2017: Log transport and deposition in incised, channelized, and multithread reaches of a wide mountain river: Tracking experiment during a 20-year flood, Geomorphology, 279, 98-111.