The South Indian River referred to in the crossword solution is the Cauvery, which is a 7-letter word. The Cauvery is a major river in South India that originates in the Western Ghats and flows eastward into the Bay of Bengal. It is the longest river in South India and is an important source of water for the states of Karnataka, Tamil Nadu, and Kerala. The Cauvery is also an important source of irrigation and hydroelectric power for the region.