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

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 m
3
ha
-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.  
VI
RESUMEN 
La tesis presenta el análisis ya sea de volúmenes, dimensiones y características, 
como así también del impacto morfológico de los “Grandes detritos leñosos” 
(Large Woody Debris, LWD) en cursos de agua de la región Alpina italiana, de la 
Cordillera de los Andes meridionales chilenos y en Tierra del Fuego Argentina. 
Las seis cuencas italianas analizadas están situadas en los Alpes Dolomíticos 
(Alpes del sudeste) y forman parte de la cuenca del Cordevole, un tributario del 
río Piave que fluye en el mar Adriático. El área de drenaje comprende entre 2,2 y 
58,1 km
2
, y la geología consiste principalmente en piedra caliza y rocas 
sedimentarias que forman vertientes muy escarpadas. La precipitación media 
anual es de 1100 milímetros, concentrándose principalmente en primavera y 
otoño. Cubren las cuencas, bosques de coníferas fuertemente manejados. 
Los sitios estudiados en América Latina, son dos cuencas en la Cordillera de los 
Andes en Chile (ca. latitud 38°S), y una cuenca situada en la parte argentina de la 
isla de Tierra del Fuego, con pendiente y desembocadura al canal de Beagle, en la 
ciudad de Ushuaia (ca. latitud 55°S). Las cuencas chilenas (9,1 y 11,1 km
2
)
presentan una geología caracterizada por rocas piroclásticas y capas 
sedimentarias, clima templado con precipitaciones invernales que presentan un 
valor medio de 2200 mm/año; el área de estudio está cubierta con bosque nativo 
maduro. Por otro lado, el clima de la cuenca argentina de Buena Esperanza (12,9 
km
2
) es subantártico, frío y húmedo sin estación seca, con una precipitación que 
varía entre 530 milímetros a nivel del mar y 1300 milímetros en la parte superior. 
La cuenca hidrográfica está caracterizada del punto de vista geológico por rocas 
marinas sedimentarias, y presenta un bosque nativo maduro hasta la altitud de 550 
m s.n.m. 
Durante el estudio, los cauces fueron divididos en tramos uniformes y 
homogéneos; su gradiente, ancho de bankfull y profundidad de flujo fueron 
medidos en el campo. En total, 11.450 pedazos de detritos leñosos - individuales y 
en acumulaciones - fueron medidos y clasificados según una serie de 
características cualitativas tales como presencia/ausencia de raíces, orientación 
respecto al flujo y ubicación en el cauce. 
VII
La cantidad total de LWD medidos en los cauces italianos no es comparable a la 
mayoría de los datos publicados anteriormente en diversas regiones del mundo, 
pero sí lo son si se relacionan con otros cursos de agua de los Alpes. Ellos se 
caracterizan por cantidades relativamente bajas de LWD de dimensiones 
pequeñas, con un escaso impacto morfológico en el cauce, probablemente como 
consecuencia de las características de los bosques y de las prácticas de manejo de 
los cauces para la mitigación de inundaciones. Por otra parte los resultados 
demuestran que existen variaciones muy grandes en el volumen de LWD por 
unidad de área del cauce, incluso para cauces individuales, y que los aportes al 
colector son principalmente debidos a la inestabilidad de las laderas adyacentes a 
los canales. 
Por el contrario, los valores encontrados en las cuencas sudamericanas son más 
similares a los datos publicados relativos a diversas regiones del mundo, tales 
como la región del noroeste pacífico de Norteamérica. Las cuencas cubiertas por 
bosque nativo maduro en los Andes meridionales, en efecto, ofrecen grandes 
cantidades de madera y detritos leñosos que pueden ejercer un fuerte control en 
los procesos fluviales, aunque las diferencias importantes en abundancia de LWD 
y los volúmenes existen incluso entre cuencas adyacentes, debido a la historia del 
impacto de la acción antrópica y de otras causas naturales en las cuencas 
(incendios forestales). Los volúmenes masivos de LWD (es decir 1.000 m
3
ha
-1
) se 
pueden alcanzar en las cuencas disturbadas por incendios seguidos de 
movimientos de masa y por coladas detríticas; el transporte de los sedimentos 
parece tener menor importancia comparado a los aportes locales desde las laderas, 
dado que las características de los tramos no se correlacionan con los volúmenes 
de LWD en los mismos. 
La disipación potencial de la energía debida al LWD (log-steps y valley jams) es 
aproximadamente de ¼ de la pérdida total de nivel en cursos de agua ricos de 
detritos leñoso y el sedimento almacenado detrás de estas estructuras puede ser 
del mismo orden de magnitud la producción anual de sedimento de la cuenca. 
Una diferencia asociada con la latitud ocurre en los diámetros de LWD, 
probablemente por el crecimiento más lento de los árboles en el clima frío de 
Tierra del Fuego. Esta cuenca ofrece una mayor cantidad de elementos, 
VIII
probablemente porque pedazos más finos son más fáciles de romperse durante las 
inundaciones. Finalmente, se puede concluir que los LWD representan una 
componente de los cursos de agua de montaña, donde la dinámica, abundancia e 
influencia geomorfológica son muy difíciles de evaluar y de modelar. Esta 
dificultad es aún mayor que en el caso de la evaluación de los procesos relativos 
al transporte de sedimentos. Los procesos locales de instabilidad, como derrumbes 
y coladas detríticas, contribuyen a hacer más caótico el modelo de presencia de 
LWD a lo largo de los cursos de agua que están firmemente confinados a las 
laderas adyacentes, tales como los cauces de montaña. 
IX
CHAPTER 1.   WOODY DEBRIS IN STREAM CHANNELS
Once being characteristic of all drainage networks in forested basins worldwide 
(Montgomery et al., 2003), in-channel large woody debris (LWD, i.e. pieces of 
wood at least 10 cm-large and 1 m-long) is now abundant only in relatively 
undisturbed mountain streams. In fact, the massive deforestation that has been 
pursued in the lower part of watersheds for centuries, and now particularly active 
in the developing countries, has mostly left forests in the upper part of the basins 
where conditions for stable human activities such as agriculture and industry are 
less favorable. In addition, “cleaning” larger, lowland channels from woody 
debris has long been carried out to reduce flood risk, improve navigation, and 
because LWD represents an available source of fuel for local people (Williams, 
2000; Montgomery et al., 2003). This is evident in Europe, where large lowland 
rivers – most often braided and meandering systems – have been regulated, 
channelized and cleared of woody debris for millennia and their surrounding 
floodplain forests extensively eliminated. 
In fact until very recently, organic debris was perceived mainly as an obstacle 
to navigation and as an hazard during floods for its potential damage to structures 
such as bridges (Diehl, 1997) and also for its additional resistance to flow that 
increases inundation frequency and duration (Shields and Gippel, 1995; Darby 
and Thorne, 1995; Dudley et al., 1998; Shields et al., 2001). However, ecologists 
and biologists have shown, quoting Montgomery et al. (2003), that “a good river 
for humans is not a good river for fish”, and demonstrated that woody debris is 
crucial for stream and river corridor ecology as a whole (see for example Harmon 
et al., 1986; Bisson et al., 1987; Gippel et al., 1994; Gurnell et al., 1995; Bilby and 
Bisson, 1998; Sundbaum and Naeslund, 1998; Haden et al., 1999; MacNally et al., 
2001; Keim et al., 2002). These ecological evidences led – at least in some 
countries – to a sharp shift from the “removal philosophy” to the opposite practice 
of adding natural or engineered woody debris into the channel to enhance fish 
populations (Abbe et al., 1997; Hildebrand et al., 1997; Zika and Peter, 2002; 
Barwick et al., 2004; Lacey and Millar, 2004; Shields et al., 2004). Furthermore, it 
has been recognized that the positive ecological role of WD is due to a large 
1
extent to its relevant geomorphic effects on channel form and processes that 
overall make the stream spatially more “diverse” at several scales. Large pieces of 
wood resting within channels, in fact, have strong consequences on river 
hydraulics, sediment transport, channel morphology, and river ecology, as 
summarised by Gurnell et al. (2002), Montgomery et al. (2003) and Montgomery 
and Piegay (2003). In particular, the morphology of mountain streams running 
through old-growth forests and their potential for sediment storage are largely 
controlled by LWD, which cause a forced morphology to establish (e.g. log-steps, 
log-jams and valley jams with the associated forced pools) as defined by 
Montgomery and Buffington (1997). 
Essential part of stream ecosystems and morphology in forested basins, woody 
debris (WD), his quantity, dynamics and effects, have been scarcely considered by 
researchers until the ‘80s, although with noteworthy exceptions (Lyell, 1837; 
Hack and Goodlett, 1960; Zimmerman et al., 1967; Heede, 1972; Beschta, 1979; 
Keller and Swanson, 1979; Keller and Tally, 1979). The scientific research on 
WD started in the North American rivers, particularly in the western regions, 
where pristine or semi-natural conditions were – and still are in some cases – easy 
to find, and large volumes of wood were present forming huge, stable rafts 
(Triska, 1984; Sedell and Frogatt, 1984). Only some research on this argoument 
has been carried out in Europe (see the summary reported by Comiti et al., 2006), 
while very few papers on the subject have been published from the other 
continents (e.g. Jacobson et al., 1999, for Africa; Baillie and Davies, 2002 and 
Webb and Erskine, 2003, for Oceania). Among the parts of the world lacking 
studies on LWD, the humid temperate and tropical regions of the Latin American 
continent are characterized by thousands of streams flowing through both pristine 
and managed forests featuring very large trees, where consequently LWD is 
expected to affect significantly stream morphology, however no investigations 
have been performed so far in such a huge area. The native forests are becoming 
progressively less abundant in the Southern Andes and neighbouring piedmont 
areas (within the territories of Chile and Argentina) due to deforestation for 
agricultural production and for the more profitable industrial plantations of fast-
growing species, mostly Eucalyptus spp., Douglas fir (Pseudotsuga menziesii 
2
(Mirb.) Franco) and Monterey Pine (Pinus radiata D. Don). In particular, the 
Chilean native temperate rainforest – the so-called Valdivian forest, very similar 
to the Pacific Northwest forests along the Coastal Range – has been largely 
eliminated in the piedmont valleys of central and southern Chile. 
Nevertheless in some location of South America portions of native, old-growth 
forest dominated by southern beech (Nothofagus spp.) and araucaria (Araucaria
araucana (Mol.) Koch) are still present within National Parks and Reserves. 
These locations thereby provide excellent investigation areas for analysing the 
influence of large woody debris on the morphology and dynamics of mountain 
rivers in an almost pristine environment other than North American basins, this is 
especially due to the forest trees growing in the southern Andes differ 
considerably – in terms of tree size, shape and growth habit – from the well-
studied basins covered with old-growth coniferous forest typical of the U.S. 
Pacific Northwest. 
In Europe, investigations of WD in large rivers include just some gravel-bed 
rivers in the French Rhône basin (Piegay and Gurnell, 1997; Piegay et al., 1999; 
Moulin and Piegay, 2004) and the Tagliamento River in the Eastern Italian Alps 
(Gurnell et al., 2000a, 2000b; Van der Nat et al., 2003). Surprisingly, few 
publications have also concerned European smaller streams (Gerhard and Reich, 
2000, and Kail, 2003, in Germany; Piegay and Gurnell, 1997, Gurnell and Sweet, 
1998, and Jeffries et al., 2003, in UK) and investigations on WD in high-gradient 
(>3%) mountain rivers of the Alps are limited to non-English technical reports 
and dissertations (Rickenmann, 1997; Degetto, 2000). 
Practically all the basins in the Alps have been impacted by humans since 
ancient times, but over the centuries the reasons have changed. Before World War 
II, forests were regularly and heavily harvested mainly for timber and firewood 
production, and in some cases eliminated altogether to create pastures for 
livestock production and agriculture. High-gradient rivers have been most likely 
affected in their sediment transport regime and morphology through the practice 
of splash-damming (i.e. artificial dam-break flows for the transport of timber logs 
downstream), forest harvesting causing the reduction of woody debris dimensions, 
and the removal of in-channel woody debris for fuel pruposes. However, with the 
3