The place is called Koodalur as the Devas joined Nandagamuni here and worshipped the Lord together( Koodal means meeting place in tamil).
It is also said that since Cauvery is coming together in this Kshetram, this Kshetram is called as "Sangama Kshetram".
The original temple was damaged in floods by cauvery. The temple we see today is constructed by Rani Mangamma when the Lord came in her dreams and asked her to renovate the temple.