Nava Kanniyar

Nava Kannigais - nine maidens representing the nine rivers –

1. Ganga, 
2. Yamuna, 
3. Saraswati, 
4. Kaveri, 
5. Godavari, 
6. Narmada, 
7. Krishna, 
8. Tungabhadra and 
9. Sarayu 

arrive to bathe in the Mahamagham tank once every 12 years during mahamaham.

http://sarayutoayodhya.blogspot.in/2017/01/sree-kasi-viswanathar-temple-kumbakonam.html