Strandzha is criss-crossed by a dense network of deep ravines, streams and rivers. The rivers have a discharge maximum in winter, in November - December. With a total length of , the Veleka river is the longest and largest in Strandzha. Its upper course has a rocky bottom and many rapids. As the river flows through the karstic region of the mountain, its banks are steep with cliffs and precipices. After the village of Zvezdets, the river valley grows wider, the inclination decreases, the water flow slows and the river creates many meanders. In the lower course the river banks are flooded seasonally forming a wetland forest of alluvial longose groves. The Veleka is navigable in the first up from the river mouth.