Most of the north border of the department is formed by the
Olimar Chico River and
Olimar Grande River. Most of the middle and north part of the department is crossed by many streams, flowing from northwest and south directions towards the northeast of the department, all being tributaries of Río Cebollati, which flows to the northeast and discharges into
Lake Merín, a natural border with
Brazil. These streams, from north to south, are: Arroyo Gutiérrez, Arroyo de los Molles, Arroyo de los Chanchos, Arroyo Polanco, Arroyo Barriga Negra, Arroyo Tapes Grande, Arroyo Marmarajá and finally Arroyo Aiguá, which forms most of the east border of the department. In the south of the department, flowing from east to west is
Santa Lucía River with its tributaries, Arroyo del Soldato, Arroyo Campanero Grande and Arroyo Verdún. Another stream in the south is Arroyo Solís Grande, which forms part of the natural border with
Canelones Department and flows south discharging into the
Río de la Plata near
Gregorio Aznárez. The south part of the
Cuchilla Grande hillrange crosses the department from the south to northwest. In the south and near Minas, there are two important tourist attractions, the hills of
Cerro Arequita and
Cerro Verdún (which is also a pilgrimage sanctuary). A bit further to the north is the hill Cerro Espuelitas. ==Demographics==