Most of the border of the department with
Durazno Department in the north is formed by the
Yí River, with many of its tributaries flowing in the north part of the department. From east to west, these are the streams: Arroyo del Pescado, Arroyo Molles del Pescado, Arroyo Illescas, Arroyo Sauce, Arroyo Mansavillagra, Arroyo Timote, Arroyo Molles del Timote, Arroyo de Castro, Arroyo Sarandí and Arroyo Sauce de Villaneuva, which also forms the border near the city of
Durazno. The border then follows Arroyo Batoví a tributary of Arroyo Maciel, the later forming big part of the border of the department with
Flores Department to the west and is itself a tributary of Yí River. Further south, separating the department from
San José Department to the west, is the stream Arroyo de la Virgen, a tributary of
Santa Lucia River. The later forms the south border of the department, separating it from
Canelones Department, with its tributaries flowing through the south part of the department. From east to west, these are: Arroyo Chamaré and Arroyo Casurá, both of which are part of the east border of the department, Arroyo Chamizo, Arroyo de Arias and the Santa Lucia Chico River, with Arroyo de Pintadito as its tributary. A dam in the course of the Santa Lucia Chico River near
25 de Mayo and
Cardal forms the Paso de Severino Reservoir. Crossing the middle of the department from west to east and separating the streams of the north with those of the south, is the Cuchilla Grande Inferior hillrange, which joins the
Cuchilla Grande hillrange in the east of the department. ==Demographics==