rigorous, average temperatures can be up to 10 °C lower than in the valley. The two most important winds of Aragon are the cierzo and the bochorno or levant. The first is a cold and dry wind that crosses the Ebro Valley from northwest to southeast and that can become quite strong. The second is a warm wind, more irregular and smooth, coming from the south-east. The vegetation follows the oscillations of relief and climate. There is a great variety, both in natural vegetation and in crops. In the high areas there are forests (pines, firs, beech trees, oaks), bush and meadows, and in the central Ebro Valley, evergreen oak and juniper are the most common trees. Hydrography Most Aragonese rivers are tributaries of the Ebro River, which is the largest river in Spain and divides the community in two. Of the tributaries of the left bank of the river, the ones originating in the Pyrenees, the Aragón River stands out. Its headwaters are in Huesca, but it ends at the community of Navarre, the Gállego and the Cinca, which joins the Segre just before emptying into the Ebro at the height of Mequinenza. On the right bank, the Jalón, Huerva and Guadalope stand out. In the stream bed of the Ebro river, near the border with Catalonia, the Mequinenza Reservoir, of and a length of about 110 km; it is popularly known as the "Sea of Aragon". The small Pyrenean mountain lakes called ibones merit special mention. These lakes are very scenic, originating during the last glaciation, and are usually found above . The Autonomous Community lies within three hydrographic regions, the Ebro River, the Tagus River (which originates in the Sierra de Albarracín range), and the Júcar, which has as its main river in this community the Turia. Protected spaces In Aragon, protected natural spaces are managed through the Red Natural de Aragón, an entity created in 2004 to protect all elements with ecological, landscape and cultural value and at the same time coordinate and establish common standards that contribute to their conservation and sustainable use. In this entity are integrated national parks, natural parks, nature reserves, biosphere reserves and other protected natural areas that have been declared by the autonomous community, the Ramsar Convention or the Natura 2000. Within the protected areas is the only national park of Aragon: the Ordesa y Monte Perdido National Park, the