I always wondered why Ives put it there. It's not necessarily a useful question to ask of most composers, but Ives, despite his technical innovations, seldom interested himself in purely technical matters. Music for him was largely a philosophic exercise, and listening to it becomes a moral discipline. Thus, the violence of the contrast between the first half of the piece and the latter half provokes a philosophic question, rather than a formal one. I finally got my answer when I heard Ives's later song of the same title, to his own words.
Ives wishes to be the river's companion.
In that passage, we hear the strain and earnestness in the American soul's journey. Ives took, naturally, a view of the sentimentalism of the 19th century different even from most of his contemporaries. For him, it represented not the cheap way out, but the divine seen through a glass darkly. He and a bunch of other commuters were standing at an el train stop when they heard the news of the sinking of the Lusitania . The movement opens with a distant choir intoning the Te Deum chant against the ambient sounds of the night.