Poetry speaks to each individual in a different way. My perceptions are as follows:
The rainbow in the poem may symbolize the speaker's dreams which have come crashing around him in the throes of reality: perhaps he is referring to the "civilized" world of business or commerce. In any event, he seeks refuge in the wilderness. There he builds a "shack" on the edge of a lake, at the bottom of a majestic mountain.
In his new environment, he learns to live on and off the land (references to "porcupine belly").
His appreciation for this new place is seen in his enthusiasm to get out into the world at dawn each day, regardless of a bright day ("yellowed bright as wood-columbine") or one that is grey and overcast ("a fuzzed moth in a flannel of storm").
The speaker personifies the mountain as if it were a living entity: "the mountain was clearly alive," "sent messages whizzing down," "boomed proclamations" (note the use of onomatopoeia with whizzing and boomed), "spread out a white guard of goat," and "falling asleep on its feet."
The energy that the speaker notes exuding from the mountain has an enormous effect on him.
When the speaker turns his attention to the lake, he notes the osprey (a fish-eating bird of prey) "falling like valkyries" ("a valkyrie...is one of a host of female figures who decide who will die in battle") on the cut-throat trout that live in the waters there.
Then the speaker waits till the mists and fog ("night smoke rose from the boil of the sunset") appear to study the landscape once more. As darkness descends, the moon casts shadows and makes the trees (I assume) appear like "unknown totems." The night owls scold him, and the grand and gigantic cedars circle the lake, now more like a "swamp" (perhaps because of its color at night, or because by comparison to the mountain, it is not half so grand).
At this moment, the image comes to the speaker that as the mountain "sleeps" (personification), the winds are sharpening (personification) its peak into "an arrowhead poised." (This is wonderful imagery that is extended in the next section with reference to the "flint"—the usual material used to make the arrow's head.) The fact that the "flint" comes "singing into his heart" could be referring to a joyful entrance of this knowledge into his heart (personification). There is no pain associated with the arrow that enters his heart: the mountain, rather than harming him, paradoxically allows him to feel truly alive.
The speaker prepares himself for morning, barring himself in and waiting, I assume, for the onslaught of the unsurpassable beauty of the mountain to rise up from the darkness of night, to pierce his heart once again with its grandeur.
My assumption is that the rainbow is forgotten as the narrator infers that his heart has been forever captured and changed by this gorgeous location, at the foot of the mighty mountain; and although there is beauty in the lake and in the moon's reflection of light, it is the mountain that holds the speaker entranced. (The arrow may even be symbolic of Cupid's "weapon of choice;" once shot by the "arrow," love never fades from one's eyes for the object of their affection.)