Factors influencing the presence or absence of tributary-junction fans in the Iberian Range, Spain
Introduction
Mountains are high-energy environments that encourage sediment transfer from hillslopes and, hence, the potential for development of tributary-junction fans. Avalanche channels, mass movements and active ravines move large volumes of heterometric sediment, especially during intense rainstorm events. In steep ravines, a wide variety of sediment transfer processes occurs, ranging from streamflow to debris flow (Wells and Harvey, 1987, Blair, 1987, Gómez-Villar and García-Ruiz, 2000, Crosta and Frattini, 2004), with sediment accumulation especially occurring at the mouths of mountain torrents (Harvey, 1990, Harvey, 1997). Many studies have related the characteristics of alluvial fans (size and gradient, sedimentary structure) to those of the basin (size, sediment yield, plant cover, type of sediment transfer) (Bull, 1962, Hooke, 1968, Harvey, 1990, Gómez-Villar et al., 1994, Gómez-Villar, 1996, Sorriso-Valvo et al., 1998, Ritter et al., 2000, Harvey, 2002, Mills, 2002), confirming the complex relationships between geomorphic and hydrological processes on the hillslopes, sediment transfer and fan occurrence. Nevertheless, in mountain areas, alluvial fans do not develop at the mouths of all torrents. Their spatial distribution seems to be random, although it is, in fact, related to a number of environmental factors, particularly past and present sediment yield (Rachocki and Church, 1990, Mack and Leeder, 1999, Mather et al., 2000, Harvey, 2003, Harvey and Wells, 2003) and the characteristics of the depositional area. Thus, studies should address the factors that determine the development of alluvial fans and their absence at the mouth of many ravines. This will contribute to an understanding of the spatial organisation of landscapes and will help to evaluate future geomorphic hazards under scenarios of climate and land use change.
This paper analyses the factors that are related to the presence or absence of alluvial fans in a mountainous area: the Iberian Range, northern Spain. Two valleys were selected (the Oja and Najerilla valleys), both characterised by wide variations in lithology, altitude and plant cover variability, with the intention of explaining through such factors the presence or absence of tributary-junction fans. The factors analysed were both physical and human-related, including distinct morphological, geological and land-use characteristics, as well as features of the depositional setting.
Section snippets
Study area
The study was carried out in the basins of the Oja and Najerilla rivers, with both upland and lowland sectors (Fig. 1). The former corresponds to the Sierra de la Demanda, in the north-westernmost part of the Iberian Range, with smooth divides (old planation surfaces) at around 2000 m a.s.l. (Arnáez and García-Ruiz, 1990). The entrenchment of the fluvial network results in long, regularised hillslopes, with slopes around 25–30°, covered by screes that are still active close to the highest
Database
In the Oja and Najerilla valleys, all those sub-basins that drain directly into the main rivers and tributaries and have a minimum area of 0.2 km2 were selected, giving a total of 293 basins. Such a minimum size was required to allow visual aerial photo analysis to be used for the detection of alluvial fans at the basin mouths (Fig. 4). A database was built for each basin, containing information on several parameters that define the hydromorphological functioning of the basins. Three groups of
Results
Alluvial fans were detected in 165 of the 293 basins analysed. In the remaining 128 basins, the mouths of torrents flow directly into the main rivers.
Discussion and conclusions
This paper focuses on the analysis of factors influencing the presence or absence of tributary-junction fans. The results of statistical analysis suggest a complex concurrence of factors on the genesis of alluvial fans, which is in accordance with other authors (Harvey, 1988, Sorriso-Valvo et al., 1998, Crosta and Frattini, 2004). Two groups of factors were studied: those related to the internal characteristics of the basins and those related to the characteristics of the depositional area.
Acknowledgements
Support for this research was provided by the following projects: PIRIHEROS (REN2003-08678/HID) and CANOA (CGL2004-04919-C02-01), both funded by CICYT, the Spanish Ministry of Education and Science. The maps were made with the help of the Mapping Service at the University of León. The authors also acknowledge comments from Adrian M. Harvey and Andrew J. Plater, whose suggestions contributed very much to improving the final text.
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