Pyrénées-Orientales consists of three river valleys in the Pyrenees mountain range -from north to south, those of the Agly, Têt and Tech - and the eastern Plain of Roussillon into which they converge. Most of the population and agricultural production are concentrated in the plain, with only 30% of the area. There is one water reservoir at Lac de Matemale. There is also a lake, Casteilla.