Assessment of channel changes in a Mediterranean ephemeral stream since the early twentieth century. The Rambla de Cervera, eastern Spain

Highlights

• Aerial photographs and GIS were used to quantify changes in stream corridor.

• The Rambla de Cervera channel losts width (68.5%) and surface area (45.7%).

• The incision processes occurred along the study reach at an average depth of 3.5 m.

• Land use changes and gravel mining are the causal factors of the corridor changes.

• Changes are forced by randomness in flood frequency.

Abstract

An analysis of morphological changes during the last six decades is presented for a 16.5-km reach of the Rambla de Cervera, a Mediterranean ephemeral stream located in eastern Spain. Channel changes were analysed through a range of techniques, specifically the analysis of aerial photographs with geographical information systems (GIS) and comparison of topographic surveys. The gravel channel underwent a general decline over the study period, losing width (68.5%) and surface area (45.7%) caused by the development of established islands frequently attached to the floodplain. These morphological changes exhibit an interesting temporal variability, with a maximum decrease of the gravel channel in the period 1946–1956 and another narrowing stage between 1977 and 1991. Two periods (1956–1977 and 1991–2006) also had mixed performance. In addition, incision processes occurred along the entire study reach at an average depth of 3.5 m. Natural and human-induced factors producing contradictory effects are considered responsible for changes in the Rambla de Cervera.

Introduction

Over the last two centuries, Mediterranean rivers have undergone complex adjustments. Flow and sediment supply have fluctuated through time, meaning that continuous adjustments have taken place through the erosion and deposition of sediment. Climatic changes and human activities have been associated with these adjustments, influencing channel discharge and sediment supply. Much discussion has focused on the effect of human activities and its relative importance compared with climatic impacts in fluvial systems, and several studies have analysed the links between morphological changes and anthropogenic activity in the region (Hooke, 2006, Gurnell et al., 2009). In these works, channel narrowing and channel incision occurring throughout the twentieth century have been interpreted as adjustments to the new environmental conditions.

The decreases in flow discharge and sediment load (Liébault and Piégay, 2001, Pont et al., 2009, González et al., 2010, Cadol et al., 2011, Garófano-Gómez et al., 2012) have been considered as the main causes of narrowing and incision in the Mediterranean rivers. These alterations of river basin conditions have been frequently caused by anthropogenic actions such as dam construction, reforestation, torrent control works, or river channelisation (Roux et al., 1989, Bravard et al., 1997, Rinaldi, 2003, Surian and Rinaldi, 2003, Surian and Cisotto, 2007, García-Ruíz and Lana-Renault, 2011). Gravel extraction has also had a profound impact on rivers in the region (Liébault and Piégay, 2002, Surian and Rinaldi, 2003, Rinaldi et al., 2005, Wishart et al., 2008, Surian et al., 2009), mainly upstream- and downstream-progressing river incision, lateral channel instability, and bed armouring. The resultant incision alters the frequency of floodplain inundation along the river courses, lowers valley floor water tables, and frequently leads to the destruction of bridges and channelisation structures (Batalla, 2003, Rovira et al., 2005).

The observed changes in channel morphology are also linked to changes in river vegetation encroachment. The relation between plants and physical processes affects conditions for island and floodplain evolution. Vegetation traps and stabilises sediments, organic matter, and propagules; and they modify the local sedimentary and morphological environment by driving the development of landforms. Consequently, channel narrowing is frequently associated with vegetation development. The role of vegetation as a trigger for planform change and recovery is not yet well known, but it seems to be essential in river changes induced by human action (Gurnell et al., 2009). Linkages between the colonisation of vegetation and river morphology and morphodynamics of humid gravel-bed rivers have been recently established (Tal and Paola, 2007, Zanoni et al., 2008, Bertoldi et al., 2011, Wyrick and Klingeman, 2011, Gurnell et al., 2012). The interaction between vegetation and fluvial processes in ephemeral rivers has received relatively less attention. Recent works have shown that the influence of vegetation on river changes is strongly dependent on the variations of the temporal sequence of flood events (Hooke and Mant, 2002, Sandercock et al., 2007).

In the Mediterranean region of Europe, researchers have attempted to establish a chronology of recent historical changes in river morphology, linked to human actions and climatic changes. In French rivers, channel narrowing in the first half of the twentieth century has been associated with decreased discharge and sediment supply at the end of the Little Ice Age. In contrast, channel reduction in the second half of the twentieth century is considered a human-induced fluvial adjustment (Bravard et al., 1997, Liébault and Piégay, 2002, Arnaud-Fassetta, 2003). In Italy, Surian et al. (2009) detected small width changes during the nineteenth century, but with no significant trend. From the end of the nineteenth century to the 1980s/1990s, channel narrowing and incision occurred, with particular intensity after the 1950s. Finally, during the last two decades, widening sedimentation and bed-level stabilisation predominate, although some river reaches are still narrowing.

In Spanish rivers, discussion on the relative influence of human activities and climatic fluctuations is present in current works on palaeohydrology and flood frequencies. Several studies have indicated an increased frequency of floods in the Mediterranean region over the past centuries, with a particular rise in the eighteenth and nineteenth centuries, which they attribute primarily to climatic fluctuations during or at the end of the Little Ice Age (e.g., Barriendos and Martin-Vide, 1998, Barriendos and Rodrigo, 2006, Benito et al., 2008, Glaser et al., 2010). López-Bermúdez et al. (2002) examined the occurrence of floods in ephemeral streams at the beginning of the twentieth century in the Mediterranean region, which they attribute primarily to deforestation. Additionally, important changes occurred in the second part of the twentieth century dealing mainly with land use changes (Beguería et al., 2006, López-Moreno et al., 2006, García-Ruíz, 2010, Gallart et al., 2011) and torrent control works (Boix-Fayos et al., 2007). The impact of gravel mining was particularly severe in the period 1950–1980, and it is a major contributor to river incision in Spanish rivers (Batalla, 2003, Martín-Vide et al., 2010). However, not enough studies are available to establish a common chronology or to identify regional contrasts among different basins and river conditions in Spain.

Moreover, in Mediterranean Europe, most of the research in this area has been developed in perennial rivers where the impact of flow on channel morphology is permanent. Less is known about river adjustments in ephemeral streams where there are long periods of stability and a higher dependence of morphological changes on extreme or flash-flood events.

For this reason, in this paper we focus the analysis on an ephemeral stream of the eastern region of Spain. This study case has been selected after considering several examples in the region, in most cases refused because of the extreme artificialisation of channel conditions for urban, agricultural, or flood control works. The aim of this paper is to provide additional information to a better understanding of the narrowing and incision processes in Mediterranean ephemeral streams. Our work quantifies morphological changes in the channel of the Rambla de Cervera over the last six decades. To achieve this goal, aerial photographs dating from 1946 to 2006 were analysed to make the following specific research contributions: (i) the identification and quantification of trends in the active corridor, bar and island changes; (ii) the measurement of spatiotemporal width changes and incision; and (iii) the elaboration of a conceptual cause–effect framework to define the major factors and the timing of processes affecting recent historical changes in the Rambla de Cervera.