The confluence of the rivers Segre, Cinca and Ebro, which is in the north-east of the Iberian Peninsula, is commonly referred to as the Aiguabarreig.(literally, "mixing of the waters"). This is the biggest river confluence on the peninsula and probably also the most interesting from a biological point of view.

Administratively speaking, one of the riverbanks is in Catalonia and in the immediate area it is possible to find a wide range of river landforms, spurs and ridges (Massís de Montllober, Tossals de Montmeneu and Almatret). These are all included in the Pla d'Espais Naturals (PEIN - Plan for Natural Spaces) and the Xarxa Natura 2000 (Nature Network 2000), which also includes the municipal territories of Serós, Massalcoreig, La Granja d'Escarp and Almatret (in the local district of El Segrià - Spain) and La Pobla de Massaluca (which belongs to Terra Alta).

The west bank of the river lies in the comarca (local district) of Baix Cinca and includes the settlements of Vilella de Cinca, Saidí, Fraga, Torrent de Cinca and Mequinensa and a small area (Faió) on the banks of the river Matarranya: all of territories belong to the autonomous community of Aragon. To date, no specific protection policy has been applied on this bank of the river, but in September 2001, it was declared a Zona d'Especial Protecció d'Aus (ZEPA - Special Protection Area for Birds), with the title ZEPA Matarranya-Aiguabarreig. The central area of the Aiguabarreig runs from the southern part of Torrent de Cinca, includes the whole Mequinensa riverside, and extends as far as the confluence with the river Matarranya in the comarca of the same name. The ZEPA for the Matarranya water basin includes the municipalities of Faió, Nonasp, Favara de Matarranya and Maella.

From the point of view of scenery, the Aiguabarreig contains hundreds of metres of open waters, with many small islands and the largest forest on the banks of the river Ebro. In the area it is also possible to find large stands of reeds, wide beaches of pebbles, pools and meanders, small fresh-water and salt-water lakes, rice fields and rushing streams. This is also an area of transition between the steppe flora of the Monegros desert and Mediterranean flora, while there are also elements more characteristic of mountain areas. The interlaced connection between riversides and lakes and a dry, Mediterranean area with a mixture of vertical cliffs and plains full of fruit trees, has given the Aiguabarreig its exceptional biological wealth.

On account of these special characteristics, very different types of fauna can be found in the same area. Of these, birds form the main group, as there are colonies of kingfishers, a wide variety of birds of prey and even birds usually associated with desert habitats, including a vast assortment of endangered species and birds that are rare in Europe. There are also important populations of reptiles, amphibians and mammals, with a large number of bats, an abundant population of deer, and the much shier otters and wild goats. The endemic species are particularly notable amongst insects and water creatures, and along with the naiads, populations of autochthonous fish, such as the fraret and the rabosa de riu (Blenius fluviatilis) manage to subsist with some difficulty (due to the effects of damming and the introduction of the catfish.

The strictly fluvial section of the Aiguabarreig is a very important wet zone, which is particularly significant as a mating, hibernating and stopping off point for migrating fauna. Its location, in the middle of the river Ebro basin and its proximity to the Ebro delta are further advantages of this extraordinary intermediate area.