Does anybody know the cultural reason why the Indian Navy's submarines are all named Sindhu- as given in the following story?

 

Five of, of India’s thirteen conventionally-powered submarines, naval sources have told The Hindu, have exceeded their design life. The INS Sindhughosh, Sindhudhaj, the Sindhuraj, Sindhuvir and ill-fated Sindhuratna, all Russian-manufactured Kilo-class submarines, were all inducted between 1986 and 1988. The most recent of the Kilo-class fleet, the Sindhushastra, was puchased in 2000, preceded by the Sindhurakshak in 1997, and the Sindhuvijay was inducted in 1991.

Thanks

Regards,

Palaniappan