Channel changes and floodplain management in the meandering middle Ebro River, Spain
Alfredo Ollero
Dept. of Geography and Land Management, University of Zaragoza, Spain
Geomorphology 117 (2010) 247–260
abstract
The 346 km of the middle Ebro River between Logroño and La Zaida is a free meandering channel in a wide floodplain. This reach contains a discontinuous riparian corridor, including valuable riparian forests and oxbow lakes. The Ebro has witnessed substantial changes in channel morphology, gravel bars, riparian vegetation and floodplain uses over the last 80 years. The growth in sinuosity, migrations and meander cutoffs have been frequent before 1981. Afterwards, bank protections and dykes have stabilized the channel. There has been a progressive and significant decrease of both the area covered by water and the gravel bars without plant colonization. As a result the width of the riparian corridor has been dramatically reduced for human use. The deceleration and near elimination of the free meander dynamics of the Ebro channel represent an important loss of natural heritage. Dams, land-use changes throughout the basin, and construction of flood defences that restrict the main channel have changed the river system behaviour, which urgently needs a management plan combining both improvement and risk reduction. The solution proposed is the creation of a “Fluvial Territory”.
Keywords: Fluvial geomorphology Channel changes Floodplain River management Land management Ebro River
1. Introduction
River courses exhibit dynamic equilibrium (Chorley and Kennedy, 1971), adjusting themselves continuously in time and space to fluctuations in discharge and sediments, which results in lateral and vertical mobility (Werritty, 1997). This mobility regulates and drives ecological dynamics that guarantee the richness and diversity of natural systems (Malavoi et al., 1998). Change is therefore a natural and a vital component of a functioning river system (Brierley and Fryirs, 2005) that human activities potentially can modify.
The objectives of this paper are to assess the recent dynamics of the channel and the changes in the floodplain in the middle Ebro River, Spain over the last 80 years, to explain the causes for the progressive reduction of such dynamics, and to present feasible river channel, riparian corridor and floodplain management solutions. This paper is also a contribution from the field of fluvial geomorphology to the current scientific and technical debate on selection of land management models with reference to dynamic rivers in the Spanish plains, using the middle Ebro River as an example. A technical commission has been created by the Confederación Hidrográfica del Ebro (Ebro River Basin Authority) to evaluate these issues
The middle course of the Ebro River (Fig. 1) has witnessed substantial changes in channel morphology, gravel bars, riparian vegetation and floodplain uses. Over the last two decades, the recent hydrogeomorphological dynamics in the Ebro River along with its tributaries and floodplains have been mapped and analyzed, the ecological condition of their channels and banks evaluated, flood risk situations assessed, and numerous management proposals presented to the relevant authorities. This research started on the middle course of the Ebro River (Ollero, 1992), but later also involved several of its tributaries.It accepts the free meandering reach of the Ebro River as the datum.
The changes observed (Ollero and Pellicer, 1991; Ollero, 1991, 1992, 1995; Ollero et al., 2004, 2006a; Cabezas et al., 2008) are due to anthropogenic alterations. Similar hydrological, geomorphological and ecological consequences have been assessed in other fluvial systems. Such modifications have also been studied on the tributaries of the Ebro (García-Ruiz et al., 2001; Beguería et al., 2003; Batalla, 2003; Batalla et al., 2004; Acín, 2004; Granado, 2004; Frutos et al., 2004; Ollero and Martín-Vide, 2005). Most of the cases studied dealt with transformation of braided channels into meandering ones and problems due to incision, human invasion processes on the riparian corridors, and significant reductions of the channel mobility over the last decades. A number of recent studies were on dams (Petts and Gurnell, 2005; Magilligan and Nislow, 2005; Graf, 2006), and one of the Binghampton Symposiums was on various human impacts on the fluvial system (James and Marcus, 2006; Chin, 2006; Gregory, 2006; Wohl, 2006; Simon and Rinaldi, 2006). Hooke (2006) has updated the study of human impacts on fluvial systems in the Mediterranean region, stressing that during the 20th century major changes were made directly through channelization, dam construction, and gravel extraction.
7. Conclusions
The Ebro River in its middle course is a large fluvial system of high interest. The presence of 346.5 km free meandering channel on a wide floodplain is the longest and most valuable free meandering channel on the Iberian Peninsula and one of the most important ones in the Europe. Such a river reach should be protected because of its intrinsic value. It is accompanied by aquatic and riparian ecosystems which are equally valuable, but affected by various type impacts that have worsened over the last decades.
The free meandering middle Ebro was, therefore, a good example of river dynamics. But it is much more than this, and the management of such an extensive and complex space represents a big challenge that has not been resolved yet.
The channel and the riverbanks of the freely meandering Ebro have been under threat: over the last 50 years, due to agricultural and urban development. The use of aerial photographs into a Geographic Information System has been very useful for the analysis of these changes. Since 1927, the river morphology has changed in 81 locations. The most frequent changes correspond to sinuosity increase, followed by downstream meander migration, combination of widening and migration, chute cut-offs and neck cut-offs. The amount of surface mobilized was almost 2000 ha in 30 years (1927– 57), whereas it has only been about 400 ha between 1957 and 1981. Since 1981, the channel has been stable.
A considerable and progressive decrease of the amount of surface occupied by the flooded channel and the gravel bars with no plant colonization has occurred on the floodplain. Riparian vegetation has decreased, very remarkably among the first colonizers. Width of the riparian corridor, the surface of which is currently less than 58% compared with the 1927 figure, has decreased significantly between 1957 and 1998. The massive construction of defences has stabilized the channel morphology and has consolidated the human invasion of the fluvial space
Mainly as a result of the defence works that have stabilized the channel, the fluvial dynamics of the free meandering Ebro in Aragón have been significantly reduced, almost eliminated. The key solution proposed is the recognition of a Fluvial Territory that is wide and continuous enough to guarantee the conservation and improvement of the river and its banks, the management of floodable areas, minimization of risk to the inhabitants, and sustainable development of the floodplain
The usefulness of the Fluvial Geomorphology in the diagnosis and solution of problems relate to the functionality of the river and the environment and territory management of fluvial systems. This is in the line of application of the Water Framework Directive.
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