River Tundja is the main water resourceof the Municipality, with a water basin of about 7 880 square km. Big natural riches represent the abundant ground waters in the alluvial silts of river Tundja and its affluents. Its waters are used mainly for irrigation and for that purpose are constructed numerous pumps and water stations with maximum water capacity between 50 and 100 litres per sec. The dam “Jrebchevo” is constructed on river Tundja, 25 km away from Sliven, with capacity of over 400 million cubic metres and is used mainly for irrigation.
Sliven is crossed by the rivers “Asenovska”, “Selishtka” and “Novoselska”. On the river valley of “Asenovska” is constructed dam “Asenovec” which is the main water resource of the city.
There is an abundance of thermal springs in the region of Sliven Municipality. The water is particularly valuable in the treatment of gastro-intestinal and hepato-biliary disorders, diseases of the skeletal and muscular system and the peripheral nervous system.