Sapta Kanya.

Ganga, Yamuna, Sindhu, Kaveri, Godavari and saraswati were playing on the slopes of Vindhya mountains when a Gandharva by name Visvavasu happened to pass that way. He bowed to them in a reverential manner and continued on his errand. but the girls were girls and a dispute arose they decided to catch the culprit on his return journey and clear their doubt { To whom amongst them did he show his respects to} Lo Behold Visvasu was cornered in the evening on his return journey and was broached with their very important question. He of course cunning as ever left them in a greater confusion by saying it was meant for whoever was the greatest amongst them. the competition boiled down to ultimately between Ganga and Kaveri for they both laid claim to be the greater one. They decided to approach Brahma who narated to them the story of the origin of Ganga. 
Now Kaveri could not accept this and she did severe penance and Brahma appeared once again and as he was pleased he said now she had the equal status with Ganga. but Kaveri could not take this so she continued to want an ever greater status than Ganga so Brahma advised her to go to Sara kshetram sit under the arasa maram on the banks of sara puskarini and pray to perumal and her wish would surely be answered.
She did severe penance and on the day of full moon in the month of thai Maha vishnu manifested before her as a child. she held the child close to her and knew that it was the Lord and so desired he appear in his true swaroopa. the Lord is said to have appeared sitting on Garuda and accompanied by sridevi and Neela devi adorning the shanka and chakra. He also appeared in his dasa avtara before her.
Kaveri asked the Lord for three boons. one to continue to stay in sara kshetram, she requested moksa for every one and thirdly to be recognized as superior to Ganga.  this was also assured when the Lord came to reside in Srirangam. Kaveri was thus greater to Ganga. There is a small sannidhi for Kaveri on the banks of the sara tank under a peepul tree.