Last Two Sections of the Waste Lands

"Death By Water" -

There are relatively no allusions in this section. Why? It is short and straight forward. Why?

"What the Thunder Said"

Refers to a Hindu text: The Upanishad.
Other allusions in this section: Bible - New Testament (Matthew, Mark, John). Holy Grail Legend, Shakespeare and the Roman General Coriolanus.

Return to the Desert. The Falling of Cities. The Drying up of Rivers. The lack of rebirth?

This is a HARD Section and yet it ends the poem. What is going on here. What are the connections to the other sections?