In short, the author's tone is one of reverence towards Rainy Mountain as well as of the natural world and of the Kiowa tribe. This tone echoes throughout the entire novel. The issue with your question is that this story has many beginnings. Do you mean the first poem that precedes the entire book? Do you mean the prologue? Do you mean the introduction? Do you mean the beginning of the first major part called "The Setting Out"? I am assuming you mean the beginning of it all: the poem called "Headwaters." I believe it contains the imagery you are talking about.
Noon in the intermountain plain: / There is scant telling of the marsh-- / A log, hollow and weather-stained, / An insect at the mouth, and moss-- / Yet waters rise against the roots, / Stand brimming to the stalks. What moves? / What moves on this archaic force / Was wild and welling at the source.
There is sight imagery, sound imagery, touch imagery, and even taste imagery here. Sight has to do with the plain, the marsh, the log, the insect, the moss, the stalks, and the roots. Sound imagery is found in the "scant telling." Reference to the mouth connects sound imagery to taste imagery. Even the word "roots" can be considered a taste image. Touch imagery can be seen in the question, "What moves?" Further, the idea that something is "welling" can also be seen as a touch image.
In conclusion, tone is an author's attitude toward his or her subject. As you can see, all of these things amount to great reverence for the natural world that can encompass all five senses. This idea reverberates throughout Momaday's book.