The river begins at the
Grand Anaicut at the eastern end of
Srirangam Island, where it branches off from the Kaveri. After diverging from the Kaveri, the Vennar flows east. Northwest of
Thennankudi, at the Thenperambur dam, the Vennar splits into a northern and southern branch. The northern branch becomes the
Vettar River, while the southern branch continues east as the Vennar. The river flows to the north of
Thanjavur and
Ammapettai before turning to the southeast towards
Needamangalam. Northwest of Needamangalam, there is another dam across the river, and the river splits again, into three branches. The
Pamaniyar and
Koraiyar Rivers begin as the two southern branches created by this divergence, while the Vennar continues through the northern branch. After this divergence, the Vennar continues flowing southeast, and splits once again south of
Koradacheri, with one branch initially flowing steadily southeast and later more to the south, while the other branch initially flows more steeply southeast and later flows steadily east. Both branches eventually rejoin in Nagapattinam district. After the convergence, the Vennar flows east again, before turning to the northeast and emptying into the
Bay of Bengal south of
Velankanni. ==History and Etymology==