Elsevier

Geomorphology

Volume 81, Issues 3–4, 29 November 2006, Pages 252-264
Geomorphology

Factors influencing the presence or absence of tributary-junction fans in the Iberian Range, Spain

https://doi.org/10.1016/j.geomorph.2006.04.011Get rights and content

Abstract

This paper analyses the factors which influence the presence or absence of tributary-junction fans in the Iberian Range, northern Spain. Two valleys were selected, both characterised by wide variations in lithology, altitude, land use and plant cover. Two groups of factors were studied: those related to the internal characteristics of the drainage basins, which particularly control sediment generation; and those related to the characteristics of the depositional area which control accommodation space and main river power. Among the internal factors, the development of alluvial fans was related to: (i) the capacity of the basin to yield large volumes of sediment, (ii) the occurrence of intense human pressure until recent times, a good indicator of sediment yield, and (iii) the capacity of the basin to quickly increase discharge during rainstorms (discharge density and torrentiality). It is suggested that the areas that were intensively cultivated in the past, and have therefore been affected by intense erosion, have played a decisive role on the development of alluvial fans. This would imply that many of these alluvial fans have a relatively recent origin, perhaps related to the beginning of a widespread deforestation. The basins without alluvial fans are characterised by relatively steep hillslope gradients (that is, slopes that never were subjected to historical cultivation), low drainage densities and dense forest and shrub cover, mostly coinciding with high altitude basins composed of quartzite and shale bedrocks. Regarding the external factors, the shape, size and longitudinal gradient of the main river to which the fans are tributary are the most relevant conditioning factors determining the development of alluvial fans.

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.

References (43)

  • G. Crosta et al.

    Controls on modern alluvial fan processes in the Central Alps, Northern Italy

    Earth Surface Processes and Landforms

    (2004)
  • J.M. Cuadrat

    El clima

  • R. Fernández-Aldana et al.

    Mapa forestall de La Rioja

    (1989)
  • J.M. García-Ruiz

    Movimientos en masa en los conglomerados del contacto Depresión del Ebro/Sistema Ibérico en La Rioja

  • J.M. García-Ruiz et al.

    Historical geomorphic processes and human activities in the Central Spanish Pyrenees

    Mountain Research and Development

    (1998)
  • J.M. García-Ruiz et al.

    Aspectos dinámicos de un cauce fluvial en el contexto de su cuenca: el ejemplo del río Oja

    (1987)
  • J.M. García-Ruiz et al.

    Debris flows subsequent to forest fire in the Najerilla River valley (Iberan System, Spain)

    Pirineos

    (1988)
  • J.M. García-Ruiz et al.

    Sediment yield under different land-uses in the Spanish Pyrenees

    Mountain Research and Development

    (1995)
  • A. Gómez-Villar

    Distribución superficial del tamaño de los sedimentos en dos conos aluviales generados por una tormenta (Alto Valle del Najerilla)

    Cuadernos de Investigación Geográfica

    (1990)
  • A. Gómez-Villar

    Conos aluviales en pequeñas cuencas torrenciales de montaña

    (1996)
  • A. Gómez-Villar et al.

    Colonización vegetal y actividad geomorfológica en abanicos aluviales del Pirineo español

    Cuaternario y Geomorfología

    (1994)
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