One of the tributaries of the Krishna River in western part of Maharashtra is the Koyna River. The river is just hundred metres wide and flows slowly. The river is famous mainly because of the Koyna Dam, which is built across the river. It is the largest hydroelectric project in the state. The dam is situated in Koyna Nagar, which is in the Western Ghats. The fifty kilometers long Shivaji Sagar Lake is also formed by the river.

Rich in algae and aquatic plant life, the water of the river has an olive green shade during the dry months and a bluish brown shade during the monsoon months. The river merges with the Krishna River at Karad in Satara district of Maharastra.