Despite the great difficulties in quantifying all the environmental and human factors that affect sediment discharge by rivers into the sea, a group of Catalan scientists has compiled data to describe and evaluate the solid sediment discharge from nine river basins in Catalonia - the Ter, Foix, Gaia, Besós, Llobregat, Francolí, Tordera, Muga, and Fluvia. "This was a slow job, with its fair share of difficulties", Miquel Canals, one of the authors of the study and a senior professor in Marine Geology at the University of Barcelona (UB), tells SINC.
The article, which has been published recently in the Journal of Hydrology, shows that the impact of human activities, through dams, agriculture, and changes to soil use, is the main cause of sediment accumulation.
The Catalan river basins have "a steep descent, easily eroded rocks, considerable plant cover, and little precipitation". Camino Liquete, lead author of the study and a doctor at the UB, tells SINC that these basins "may have one of the lowest rates of water and sediment discharge of all the rivers flowing into the Mediterranean".
Global warming, a serious threat to rivers
Added to the human factors is the continuous increase in the average temperature of the river basins over the past 50 years. The new research, carried out as part of a European Commission project (SESAME) and the National Plan for R&D&I (PRODELTA and GRACCIE Consolider-Ingenio), has shown once again that global warming can have a negative impact on river discharge.
In 2003, the United Nations Environment Programme (UNEP) said that the amount of water being discharged into the Mediterranean Sea stood at only half the levels at the beginning of the 20th Century.
The extraction of sand and gravel from river beds has been forbidden for the past 20 years in Spain. However, rivers such as the Tordera and the Llobregat suffered major impacts as a result of these activities. In addition, a study carried out in 2005 by researchers from the University of Lleida showed that the Tordera would need another 420 years in order for it to recover the shape of its course prior to this extraction.
While the Ter and Llobregat, which are characterised by frequent flooding, are the rivers showing a "clear reduction" in water discharge, the scientists have also shown that the Foix, Gaia and Francolí discharge the smallest amounts of sediments into the Mediterranean, with all the problems this entails.
In the face of increasing demand for water in Catalonia, the regional governments have proposed and started to develop infrastructure based around the transferral of water from river basins outside the area, sea water desalination, and the reuse of treated water.
Liquete, Camino; Canals, Miquel; Ludwig, Wolfgang; Arnau, Pedro. "Sediment discharge of the rivers of Catalonia, NE Spain, and the influence of human impacts" Journal of Hydrology 366(1-4): 76-88. 15 de marzo de 2009.