Prayag is the last of the five Prayags where the Alaknanda meets the Bhagirathi river. This confluence of rivers is a very religious and holy place for Hindus. People go here to have a darshan of this Sangam, worship here and at the end offer prasad and take bath.
In this way, there are total 5 Prayags in Uttarakhand where two rivers meet.
In the photo you can see that the color of both the rivers is completely different. The green one is Bhagirathi and the left one is Alaknanda which comes from Badrinath.
Har Har Gange
हर हर गंगे
