Since our brain is in our head, so the head is always a representation of knowledge. The purity and freeness of river are being compared with the freedom and the piousness that lies in the knowledge.
This is why Ganga is always shown flowing from the locks of Lord Shiva in the paintings.
The blue coloured body of Shiva shows his infiniteness. Infinity has no shape, size and no limit (obviously).
Hence Shiva was not a body, soul, or anything, but an infinite consciousness.
It is being believed that in order to make people understand, a representational or a physical manifestation of Shiva was done.
The entire universe or the world is Shiva only.
We will be back with some more exciting and mind-boggling details on Lord Shiva and his forms. Stay tuned for more updates. Thanks!