In our Indian culture there is a legend attached to every ritual. Kanwar also has some interesting story behind this monsoon festival. Legend has it that at the time of Samudra Manthan (churning of the sea) both ambrosia (amrit ) and poison surfaced. While the Gods and Goddesses eagerly tasted amrit, no one wanted to have poison. Yet consuming poison was crucial as it would have caused enormous destruction had it touched the earth. It was then that Lord Shiva drank the poison. This generated tremendous heat inside Lord Shiva's body. It is to pacify this heat that Gangajal (water from the Ganga) is poured over Shivlingas, a process known as jalabhishek.
This is the reason why Kanwariyas fetch Gangajal from the Ganga in Haridwar, Gangotri or Gaumukh in Uttaranchal, return to their hometown to consecrate the lingams as a gesture of thanks giving to Lord Shiva.