Tha Caia river starts on the São Mamede Hill, at an altitude of 1020 metres, and is a tributary of the right bank of the Guadiana river. The Caia Dam furnishes water to the towns of Elvas and Campo Maior. In this place of great natural beauty, there are bird species such as the little bittern, the common nightingale, the cetti's warbler and the great reed warbler.