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Geology in Sobrarbe

Fluvial landforms

In Canciás and Olsón, we can still observe the conglomerates that formed from the first sediments moved by the rivers from the new born Pyrenean mountains, especially during the Oligocene. Those conglomerates covered most of the region and protected ancient rocks from erosion for a long time. When compression forces decreased, rivers eroded and transported these materials to the Ebro Basin, which was open to the Mediterranean Sea during the Miocene. Finally, during the Pliocene, our river systems were formed as we know them today.

Steps along the Arazas River in the Ordesa and Monte Perdido National Park.

Rivers in Sobrarbe behave uniquely in Europe, due to their very irregular volumes, exceeding the average amount more than 100 times. There are many different causes, such as summer and autumn storms and winter rainfalls melting the snow.

Añisclo Canyon moulded by the Bellos River.

Seasonal changes in the volume of those rivers flowing from northern Sobrarbe (Cinca, Ara or Cinqueta Rivers), especially in their upper reaches, are influenced by the snow on the summits. Their maximum volume is reached at the end of spring, due to the thaw, and their minimum volume is reached in summer and winter.

Those rivers flowing from the south, in Old Sobrarbe, (Alcanadre, Isuala or Balcés and Vero Rivers), depend almost exclusively on typical Mediterranean rainfalls, more often in spring and autumn. The maximum volume is reached in spring, when the thaw and seasonal rainfalls converge. These seasonal floods are known as mayencos in Aragon. In the past, timber was transported down the rivers during this season, from the high valleys to the factories by the coast, by means of timber rafts, known as nabatas..

The fluvial heritage in our Geopark is rich and diverse. Among fluvial landforms, the Ordesa, Añisclo, Pineta, Barrosa and Otal Valleys stand out. Braided streams, with large gravel bars, are one of the best examples of fluvial landforms threatened in Europe by reservoir building and artificial river regulation. Our gorges and canyons are also known world-wide: Los Navarros, Escuaín, Añisclo, Las Devotas, Jánovas, Balcés, Vero, etc..

Most of the Sobrarbe region belongs to the Cinca Riverbasin, from its source to El Grado reservoir. It completely embraces the basins of its tributaries: Ara, Cinqueta, Yaga, Bellos and Susía rivers; and part of the basins of La Nata, Usía, Alcanadre, Balcés and Vero rivers..

Upper course of the Ara river in Bujaruelo.

The Cinca River flows from the Monte Perdido glacier, rushing through waterfalls down the glacial valley of Pineta, where it develops into a wide braided stream. Past Bielsa, it starts an uneven course through narrow gorges, such as the one at Las Devotas. Along the river, we find several electrical power stations which modify its course. We can also find a well developed terraced system in the middle course of the Cinca River, whose four different levels can be associated with the terraces formed by the Noguera Ribagorzana and Gállego Rivers.

The Ara Riverbasin is greatly valued for its continuity, since it is the longest river in the Pyrenees (67 km / 41.50 miles) and not interrupted by any dam. Its source lies in the south face of the Pic Meillon and it presents a great diversity in the geomorphology of the valley and its course with some open or embedded channels and braided or meandering ones.

The Ara River is unaltered by dams or reservoirs, unlike other Pyrenean rivers. To its environmentally-friendly and bioclimatic values, we can add its geomorphological quality, reflected in its sedimentary bars, cones and terraces along its course from Fiscal to Jánovas.

Braided course along the Ara River before it reaches the Jánovas Narrow Pass. The Ara River is the Pyrenean river which has the least artificially modified course.

The geomorphological quality of its course from Fiscal to Jánovas particularly stands out due to its natural and easy flow. It is a model for the hydromorphological assessment of the Directive 2000/60/CE. According to several assessments carried out by the Aragon Government and the Confederación Hidrográfica del Ebro (Irrigation Union Group of the Ebro River) (Ollero et al., 2006; Ollero et al., 2007), the hydromorphology of its course from Fiscal to Jánovas has been graded with an A.

In 2005 the Aragonese Government passed a Bill granting the Ara River valley beside Janovas status as a Location of Comunitary Interest (LIC). It covers 329 ha and the course from Fiscal to Jánovas, as well as the Guargas de Cajol Gorge. It connects with the LIC in the Broto Valley, stretching 160 ha and to the LIC in the Ara River, stretching 1530 ha.

The Cinca River behaves like an artificial lake in the Mediano reservoir. Its banks are an ideal place for fieldwork where we can study several small-scale processes of fluvial and coastal dynamics..