Volume and Characteristics of Woody Debris in Mountain Rivers of Italian Dolomites and Southern Andes

The thesis presents an analysis of amounts, characteristics and morphological impact of large woody debris (LWD) in mountain channels of the Italian Alps, Chilean Southern Andes and Tierra del Fuego Argentina.
The six Italian basins analyzed are located in the Dolomites (southeastern Alps) and lie within the Cordevole basin, a tributary of the Piave River flowing into the Adriatic sea. The drainage area range from 2.2 to 58.1 km2, and geology consists mainly of limestone and sedimentary rocks forming very steep hillslopes. Precipitation are 1100 mm annual on average and falls mainly in spring and autumn. Long-managed forests covering the basins are mostly represented by conifers like spruce and larch.
Latin American study sites are two basins of the temperate old-growth forest in the Andean Cordillera of Chile (ca. 38°S latitude), and one basin of the Argentinean part of “Tierra del Fuego” island with slope to Beagle Channel and outlet in Ushuaia city (ca. 55°S latitude). Chilean basins (9.1 and 11.1 km2) present a geology characterized by piroclastic rocks and sedimentary layers, and a climate temperate warm with winter precipitations that present an average value of 2200 mm/year. The climate of the Argentinean basin (12.9 km2) is subantartic, cold and humid without dry season, with a precipitation that range from 530 mm at sea level to 1300 mm in the upper part. The catchment is placed over sedimentary marine rocks, and present a old-growth forest till the altitude of 550 m a.s.l.
During the survey, channels were divided into uniform reaches and their mean gradient, bankfull width and flow depth were measured. Overall, 11,450 woody debris pieces – both single and jam-forming – were measured and classified with respect to several qualitative attributes such as presence/absence of rootwads, orientation to flow and position in the channel.
Woody debris amounts in Italian streams are low compared to most previous data published from different world regions, but match with other mountain rivers of the Alps. They are characterized on average by relatively low amounts of woody debris having small dimensions and carrying rather weak morphological impacts on the streambed, possibly as a consequence of forest characteristics and stream management practices for flood mitigation. Moreover results show that very large variations in the volume of woody debris per unit of streambed area occur even within single streams, and that inputs are mainly due to slope instabilities adjacent to the channels.
On the contrary, the values encountered in South American basins are more similar to previously data published for different world regions such the Pacific Northwest region of North America. Old-growth forested basins in the Southern Andes, in fact, feature large quantities of wood, which may exert a strong control on fluvial processes, even though major differences in LWD abundance and volumes exist even between “adjacent” basins, due to the basins’ disturbance history (fire). Massive LWD volumes (i.e.>1,000 m3ha-1) can be reached in basins disturbed by fires followed by mass movements and debris flows; debris transport seems to be of minor importance compared to local inputs from the slopes, since reach characteristics are poorly correlated with woody debris reach volumes.
Potential energy dissipation due to LWD (log-steps and valley jams) is about ¼ of the total elevation drop in wood-rich streams, and the sediment stored behind these structures may be of the same order of the annual sediment yield.
A marked latitudinal difference in LWD diameters is present, likely for the slower tree growth in the cold climate of the Tierra del Fuego. The Fueginan basin features a larger number of wood pieces, possibly because thinner elements are easier to break up during floods.
Finally, it can be concluded that woody debris represents a component of mountain rivers whose dynamics, abundance and geomorphic influence are very difficult both to assess and to model, even to a greater extent than sediments. Local instability processes such as landslides and debris flows contribute to render very chaotic the pattern of LWD presence along water courses that are tightly coupled to adjacent slopes such as mountain streams.