This is where Apocalypse Now enters its final act, the confrontation with Kurtz. If you wanted to divide it into three acts, I guess that the first act would go from the beginning to after the aerial attack; the second act would be the journey upriver; and the third act would be from here to the end.
Initially, the mood is somber and ominous. We see lots of fires and skulls, then Willard destroys his dossier as the boat winds its way through tall trees and cliffs, reminding one of the passage from Heart of Darkness (the book) quoted in Hearts of Darkness (the movie):
Hugging the bank against the stream, crept the little begrimed steamboat, like a sluggish beetle crawling on the floor of a lofty portico.
In fact, from this point on, Apocalypse Now follows Heart of Darkness much more than it does the original screenplay. After the boat reaches what appears to be the end of the road (river) and drifts through a coterie of spooky, white-painted natives, a figure straight out of Conrad pops up out of the crowd:
I saw a white man under a hat like a cartwheel beckoning persistently with his whole arm…(he) began to shout, urging us to land. “We have been attacked,” screamed the manager. “I know — I know. It’s all right,” yelled back the other, as cheerful as you please. “Come along. It’s all right. I am glad.”